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Load cell

Load cell

Load cell

For centuries the primary means of weighing anything was with scales that used either the extension of a spring or a balance that compared one load with another. In recent decades though, for most industrial and commercial uses, these methods have been supplanted by load cells when a heavy weight needs to be measured.

Load cells are used to measure weight. They are an integral part of our daily life. In your car or at the cheese counter in the supermarket – we encounter load cells everywhere. Of course they are usually not immediately recognizable, because they are hidden in the inner workings of instruments.

Load Cell Types  

Load cell designs can be distinguished according to the type of output signal generated (pneumatic, hydraulic, electric) or according to the way they detect weight (bending, shear, compression, tension, etc.)

Load cell types according to the type of output signal generated:

Hydraulic load cells 

Hydraulic load cells are force -balance devices, measuring weight as a change in pressure of the
internal filling fluid. In a rolling diaphragm type hydraulic force sensor, a load or force acting on
a loading head is transferred to a piston that in turn compresses a filling fluid confined within an
elastomeric diaphragm chamber.

As force increases, the pressure of the hydraulic fluid rises. This pressure can be locally indicated
or transmitted for remote indication or control. Output is linear and relatively unaffected by the
amount of the filling fluid or by its temperature.

If the load cells have been properly installed and calibrated, accuracy can be within 0.25% full
scale or better, acceptable for most process weighing applications. Because this sensor has no
electric components, it is ideal for use in hazardous areas.

Typical hydraulic load cell applications include tank, bin, and hopper weighing. For maximum
accuracy, the weight of the tank should be obtained by locating one force sensor at each point of support and summing their outputs.

Pneumatic load cells

Pneumatic load cells also operate on the force-balance principle. These devices use multiple
dampener chambers to provide higher accuracy than can a hydraulic device. In some designs,
the first dampener chamber is used as a tare weight chamber.

Pneumatic load cells are often used to measure relatively small weights in industries where
cleanliness and safety are of prime concern.

The advantages of this type of load cell include their being inherently explosion proof and insensitive to temperature variations. Additionally, they contain no fluids that might contaminate the process if the diaphragm ruptures. Disadvantages include relatively slow speed of response and the need for clean, dry, regulated air or nitrogen.

Strain-gauge load cell

Strain gauge load cells are a type of load cell where a strain gauge assembly is positioned inside
the load cell housing to convert the load acting on them into electrical signals. The weight on
the load cell is measured by the voltage fluctuation caused in the strain gauge when it
undergoes deformation.

The gauges themselves are bonded onto a beam or structural member that deforms when
weight is applied. Modern load cells have 4 strain gauges installed within them to increase the
measurement accuracy. Two of the gauges are usually in tension, and two in compression, and
are wired with compensation adjustments.

When there is no load on the load cell, the resistances of each strain gauge will be the same.
However, when under load, the resistance of the strain gauge varies, causing a change in output
voltage. The change in output voltage is measured and converted into readable values using a digital meter

Piezoresistive load cell

Similar in operation to strain gauges, piezoresistive force sensors generate a high level output
signal, making them ideal for simple weighing systems because they can be connected directly
to a readout meter. The availability of low cost linear amplifiers has diminished this advantage, however. An added drawback of piezoresistive devices is their nonlinear output.

Inductive and reluctance load cells

Both of these devices respond to the weight-proportional displacement of a ferromagnetic core.
One changes the inductance of a solenoid coil due to the movement of its iron core; the other changes the reluctance of a very small air gap.

Magnetostrictive load cells 

The operation of this force sensor is based on the change in permeability of ferromagnetic
materials under applied stress. It is built from a stack of laminations forming a load-bearing
column around a set of primary and secondary transformer windings. When a force is applied,
the stresses cause distortions in the flux pattern, generating an output signal proportional to
the applied load. This is a rugged sensor and continues to be used for force and weight measurement in rolling mills and strip mills.

Load cell types according to the way they detect weight:

There are indeed many different types of load cells available from different manufacturers. This
is necessary to meet the demands of the many applications that there are for load cells.
Luckily manufacturers and industry have kept nomenclature easy and the names of the load
cells do correspond to the forces they measure. However, there are different types of load cell
within each category, usually based on their manufacture. Alternative names are often based on the ‘shape’ of the load cell eg. S-beam, beam load cell and column load cell.

COMPRESSION LOAD CELLS  

A compression load cell is designed for the measurement of compression or ‘pushing’ forces
only. They are ideal for general weighing applications, particularly silo and vessel weighing and
are often incorporated into both simple and complex centre of gravity systems.

Products that fall under this category are also known as; compressive load cell, column load cell,
bending ring load cell, torsion ring load cell, pancake load cell, low profile load cell, annular load
cell, donut load cell, through hole load cell, force washer load cell, S beam load cell and Z beam load cell. Examples of compression load cells are images 1, 3, 6 and 7.

TENSION LOAD CELLS

A tension load cell is designed to measure tensile or ‘pulling’ loads. A typical use of a tension
load cell is for hanging scales and they are also popular for vessel weighing. In the laboratory,
they are standard for general force measurement applications.

Products that fall under this category are also known as; tensile load cells, S beam load cell, Z
beam load cell, tension link, load link, toggle load cell. Examples of tension load cells can be seen in images 3, 5, 7 and 9.

TENSION & COMPRESSION LOAD CELLS

As the name implies, these load cells are a combination of the two categories above, and they
are able to measure both compression forces and tensile loads. Typical applications include
component testing and weighing systems. These load cells come in all shapes and sizes and
latest developments include small, yet accurate load cells that can be used where space is
restricted.

Products that fall under this category are also known as universal load cells S beams, Z beams and load links. Images 3, 7 and 9 are examples of tension and compression load cells.

BEAM LOAD CELLS

Bending beam or shear beam load cells are probably the most commonly used load cells as they
are extremely versatile and can be used in a wide variety of applications. They are particularly
suited for incorporation into weighing instrumentation such as scales, dynamometers and
tensile testing machines.

Products that fall under this category are also known as; shear beam load cells, bending beam
load cells, force beam load cells, single point load cells, cantilever beam load cells, dual
cantilever load cells and isometric force beams. Image 8 is an example of beam load cells

LOAD MEASURING SHACKLES

A shackle is a U-shaped piece of metal with a clevis or bolt across the opening. They are
commonly used within rigging systems from the maritime industry to industrial cranes to the
entertainment industry where shackles are used for lighting or scenery systems. With Load
Measuring Shackles, a load pin is incorporated into the shackle design so the load can be
monitored. They are often available in both wired and non-wired (wireless) versions. Image 4 is an example of a load shackle.  

LOAD MEASURING PINS

Load measuring pins are designed for many diverse applications as direct replacements for
clevis or pivot pins. They have many advantages over other load sensors in that they do not
normally require any change to the mechanical structure being monitored. They are typically
used in rope, chain and brake anchors, sheaves, shackles, bearing blocks and pivots.

Products that fall under this category are also known as; load pin, shackle load pin and cevis pin. An example of a load measuring pin can be seen in image 2. 

LOAD MONITORING LINKS

Load Monitoring Links are designed, as the name suggests, as a link between a shackle and the
object to be measured. They are commonly used in lifting and weighing applications,
particularly in harsh environments. The load can either be displayed on the link itself or on a
separate indicator.

Both wired and telemetry load links are available for greater versatility, and they have the
additional advantage of being simple to install with holes that are matched to standard shackle
sizes.

Products that fall under this category are also known as; link load cell, tensile link load cell, plate load cell, load measuring link and toggle load cell. Image 5 is an example of a load link. 

Wireless Technology

There are many situations where the use of traditional cabled load cells would limit distance or
movement, or where, for safety reasons, a greater distance is needed between the user and the
load. They are far more convenient to use, as they remove problems associated with long
lengths of trailing cable that are easily damaged and prone to snagging. And it’s not just the
logistics of the applications that add to the advantages of radio telemetry. Considerable savings
can also be made, as there are no cable costs or expenses for associated wiring installation, that
may in turn involve additional building modifications or maintenance issues.  

Strain gauge load cell basics

A load cell is a device that converts a force (mass multiplied by gravity) to an electrical signal. This is commonly done through either the piezo-electric effect or with strain gauges. Piezo materials are those which output a small electric signal as they are compressed. While piezo crystals are the best known, there are other similar materials that do the same, such as piezoceramics.

Capacitive load cells work on the principle of change of capacitance which is the ability of a
system to hold a certain amount of charge when a voltage is applied to it. For common parallel
plate capacitors, the capacitance is directly proportional to the amount of overlap of the plates
and the dielectric between the plates and inversely proportional to the gap between the plates.

Resistive load cells work on the principle of piezo-resistivity. When a load/force/stress is applied
to the sensor, it changes its resistance. This change in resistance leads to a change in output
voltage when an input voltage is applied.

The simplest type of load cell is a bending beam with a strain gauge. A typical bending
configuration is the ‘S’ beam load cell. In profile it looks like the letter ‘S’, and has four strain
gauges fixed to the horizontal sections. When a load is applied vertically downwards on the top
of the ‘S’ the bending puts two gauges in compression and the other two in tension. Connecting
these gauges in a Wheatstone Bridge arrangement enables the small changes in resistance to produce a measurable electrical signal.  

A strain gauge is an electrical device made from a material whose resistance changes with
strain, usually manifested as deformation. These are used in load cells designed to deflect in
response to a load. Most load cells are designed with a beam configuration that bends under
load, although some use the expansion in cross-section resulting from longitudinal or axial
compression. These generally give a less linear output than the bending configurations, making
calibration a consideration.

For a strain gauge load cell to give useful measurements the load must be applied in the
direction of operation. Side loads will result in inaccurate readings and may damage the device.
Piezo sensor systems are more robust in this regard but are less accurate overall. In addition,
the output from many piezo materials is quite temperature-dependent. 

Wheatstone Bridge Circuit

The four strain gauges are configured in a Wheatstone Bridge configuration with four separate
resistors connected as shown in what is called a Wheatstone Bridge Network.

An excitation voltage – usually 10V is applied to one set of corners and the voltage difference is
measured between the other two corners. At equilibrium with no applied load, the voltage
output is zero or very close to zero when the four resistors are closely matched in value. That is why it is referred to as a balanced bridge circuit. 

When the metallic member to which the strain gauges are attached is stressed by the
application of a force, the resulting strain – leads to a change in resistance in one (or more) of
the resistors. This change in resistance results in a change in output voltage. This small change
in output voltage (usually about 20 mVolt of total change in response to full load) can be
measured and digitized after careful amplification of the small milli-volt level signals to a higher
amplitude 0-5V or 0-10V signal.

These load cells have been in use for many decades now, and can provide very accurate
readings but require many tedious steps during the manufacturing process

There are various load cell designs in addition to bending beams. This includes for example:

● Load cells with column-shaped spring elements for high loads

● Hollow cylindrical load cells for very high loads

● Load cells with spring elements directly from the measuring bracket

● Ring torsion load cells

● Shear beam load cells

● Load cells with diaphragm spring elements.  

When to use a load cell?

A load cell measures mechanical force, mainly the weight of objects. Today, almost all electronic
weighing scales use load cells for the measurement of weight. They are widely used because of
the accuracy with which they can measure the weight. Load cells find their application in a
variety of fields that demand accuracy and precision.

The sensitivities of the load cells are divided into different denominations according to the
precision offered by that cell. The denominations, from less to more precise, are as follows:

D1 – C1 – C2 – C3 – C3MR – C4 – C5 – C6

At the lower end of the scale are D1 type cells, commonly used in the construction industry for
weighing concrete, sand, etc.

From type C3 onwards, these are cells for construction additives and industrial processes. The
most precise C3MR cells and the C5 and C6 cells are reserved for weighing and taring
high-precision tanks and scales.

Load Cell Selection

When evaluating load cells for an application, consideration should be given to the following:

● Measuring range

● Safe load limit (the maximum load that can be applied without causing a permanent shift in readings)

● Ultimate overload (the load that would break the load cell)

● Safe side load (the maximum lateral load the load cell can take without causing a permanent shift in readings)

Other potential issues to watch for are: the possibility of shock loading, off-center loading, andthe need for environmental protection. An example of shock loading would be when a load is dropped onto the load cell. Impact-absorbing materials can reduce the impact of such loads. Off-center loads will produce misleading results and can damage the load cell. Load cells intended for outdoor environments should be specified to meet appropriate IP and NEMA standards.  

Load Cell Specifications  

Measuring Range

Load cell measuring range or capacity of a load is the difference between the lowest and highest measurement the load cell can measure. The load cell measuring range differs based on the type of load cell that you choose. When choosing a load cell for your application, it is important to determine the load cell measuring range. Every load cell manufacturer will provide information about the division range in a specification sheet to help you determine the minimum and maximum weight that a load cell can measure.

Accuracy

Accuracy of a load cell is the difference between the measured value and the true value of the
test subject. When the measured value deviates too much from the true value, the load cell is
inaccurate. Load cell accuracy, however, is dependent on numerous factors such as
temperature, creep, hysteresis, repeatability. Other factors include fluctuations in excitation voltage, type of indicator used, and how load cells are installed.

Resolution

For a measuring device to show a change in output, it should first detect the change in its input. However, the ability of the measuring device to register that change depends on several factors. The resolution of a load cell is the smallest amount of force that can achieve a change in the output for a load cell. However, it is important to understand that selecting a load cell with a higher resolution doesn't mean better results. It only means that the load cell will be capable of displaying the smallest measurement value. For applications where small weight changes are not necessary to record, a load cell with higher resolution is of no value.

Accuracy vs Resolution

Load cell accuracy is the ability of the load cell to measure the force applied by an object to its
true value. It is also the difference between the actual output and the theoretical output of the
load cell. The resolution of a load cell is the smallest change in the input that causes a change in
output. In other words, the resolution is a degree to which the smallest change can be
theoretically detected. In the case of load cells, the resolution is the smallest increment that the
system can weigh. It is important to have a correct balance between accuracy and resolution
when selecting the load cell. A load cell with higher resolution doesn’t necessarily mean that it
will give accurate results. Similarly, a load cell with higher accuracy and lower resolution mean that you will not be able to record incremental changes in the weight.


Sensitivity

Load cell sensitivity is the change in output to the corresponding change in mechanical force
input. Load cells with high sensitivity can measure even the slightest change in the force. A
typical analog load cell will have a sensitivity rating in mV/V and is usually specified in the
specification sheet in 1 mV/V - 3 mV/V range. A load cell with higher sensitivity ratios is capable
of detecting the smallest change in loading conditions, resulting in an application with a faster response.

Zero Balance Error

Zero balance is the output of the load cell at a no-load condition. It is usually quoted as a
percentage of full scale and is also known as "zero offset". The zero balance error check ensures
whether the load cell has undergone a physical distortion due to overload, shock load, or metal
fatigue. To perform this test, the load cell must be in a “no-load” condition. This means all the
weight, including the dead weight, has to be removed. The signal leads are detached, and the
voltage across +/- signal is measured using a millivoltmeter. The resulting output must be less
than the manufacturer’s specification. Assuming a 10 volts excitation on a 3mV/V output load cell, the maximum signal for a 1% shift in zero balance is 0.3 millivolts.

Linearity

A load cell is designed to follow a linear relationship between the output voltage and the load
applied. However, that is an ideal case. In reality, due to several environmental and loading
factors, the output will deviate slightly and is represented through a non-linearity curve. This
curve represents the maximum deviation from a straight line starting from zero load to
maximum rate capacity. Non-linearity is also termed as linearity error and provides the load cell
weighing error over its entire operating range. It is also possible to linearize the output using compensation circuits and microprocessors

Frequency Response

The load cell frequency response is the ability of the load cell to accurately respond to dynamic
load changes. It is important to select a load cell with appropriate frequency response in
situations where the weight is either applied or changes at a rapid rate. It helps the observer in
optimizing the trade-off between load cell stability and response for a given frequency range.
Frequency response is often designated as “bandwidth” in load cell specifications. As a rule of
thumb, a load cell with a 10 times higher natural frequency than the highest frequency to be measured is generally selected.

Input Voltage

The load cell input voltage (excitation) is the voltage sent to the input terminals of the load cell.
Input voltage is necessary to enable the load cell to work because it is the source of the current
that flows through the Wheatstone Bridge inside the load cell. The maximum and
recommended excitation values are usually provided by the manufacturer, which should be strictly followed to ensure the best output results.

Excitation Voltage

To produce an output signal, load cells require an excitation voltage. The maximum and
recommended excitation voltage is often provided by the manufacturer. Maintaining these
values is critical to ensure the best output results. An excitation voltage, greater than the
maximum rated value, will increase the current flow and heat the strain gauge. It can even lead
to gauge failure. A lower excitation voltage than the manufacturer’s recommended value is
usually acceptable. However, for best results, stick to the recommended value.  

Life Span

Since load cells are made up of metals, the life of these measuring devices generally has a
longer life span. However, each load cell has a specific fatigue life, normally provided in the form
of cycle time. For example, if a load cell has a fatigue life of 100,000 times, the devices can be
loaded 100,000 times. If the loading occurs more than the specified cycles, the load cells may
not perform well as guaranteed in their specifications. Also, if the load cells undergo shock load,
or when the force applied is more than their rated capacity for a longer duration, the life of
these measuring devices gets shortened. With proper usage, maintenance, and protection, load
cells can last for years.

Orientation

Load cell orientation is about placing the weight in the right way onto the measurement setup.
When the load cell is not installed properly or if it is in an incorrect orientation, it would
produce incorrect readings. Most load cells come with an arrow on its housing that denotes the
direction of the load. When the load cell is installed correctly, the readings will be positive
values. It is also important to verify that the load cell terminals are properly connected. Refer to the manufacturer's color code guide to ensure that the load cell functions properly.  

Considerations before installation 

Types of Installations: In addition to typical installations of hydraulic, pneumatic, and strain
gage types of load cells, customers often ask about bending beam load cells, shear beams,
canister type, ring and pancake load cells, and button and washer type load cell installations.
Some other more advanced types of load cell installations for specific uses include helical, fiber optic, and piezo-resistive types of load cell installations.

Load Orientation: Service technicians find the most common cause of accuracy problems with
load measurements are incorrect load cell mounting which results in imprecise vertical loading
that creates extraneous force errors. The loads must act precisely in the direction of the load
cell.

Environment: Magnetic and electrical fields can sometimes create interference voltage within a
measuring circuit. To ensure protection from EMC, place the load cell, connection cabling, and
electronics in a shielded housing. Do not ground the indicator, load cell amplifier, and
transducer more than once.

Framework of Structure: Protect the measurement cable using steel conduits. Use shielded,
low-capacity measurement cables such as HBM cables. Avoid stray fields from motors, contact
switches, and transformers. Using a rigid design for the support structure of load cells in
compressive loading applications, preferred to pliable designs, to achieve even/balanced
lowering of all supports that also distribute tension, and providing an even contact surface.
Mounting load cells to the support structure, and rigid base plate, ensures even load transfer
from the base of the load cell to the support structure. This structure must also have the
capacity to support the forces corresponding with the load.

Today’s mechanical scales can weigh loads of all kinds, from pharmaceuticals to tanks and
shipping cars. Consistency of weight calculations and readings require the best weight balancing
mechanism designs engineered to sense force, proper calibration, and maintenance. Depending
on the output signals generated, we distinguish load cell designs according to weight detection,
such as tension, compression, bending or shear, for example. Strain gage load cells convert
acting loads into electrical signals. The change in pressure of internal filling fluid measures
weight using force balancing devices in hydraulic load cell designs. Higher accuracy
requirements can be achieved using multiple dampener chambers which also operate on the
force balance concept with pneumatic load cell engineering.  

Installing a Load Cell: Best Practices  

Each load cell installation is unique as there are several types. Consult a structural engineer
when your application requires very high accuracy, long-term stability, custom specifications, or
when using in a varied R&D environment.

In order to gain precise weighing results, be sure to use specified load applications for load cells.
Load cells have a specified load direction; do not apply side forces, bending or torsional
movements on load cells. Inappropriate loading applications will risk reducing the life of load
cells, plus distortion of correct measurement results.

Using a rigid design for the support structure of load cells in compressive loading applications is
preferred to pliable designs to achieve even lowering of all supports that also distributes
tension, and provides an even contact surface.

Mounting the load cells to the support structure and rigid base plate ensures even load transfer
from the base of the load cell to the support structure. This structure must also have the
capacity to support the forces corresponding with the load. Mounting aids may be needed for
compliance with load cell installation. Seek assistance from the design engineer to determine
the weighting of individual disturbance possibilities. Special considerations for weighing tanks,
thermal expansions, monitoring levels, and horizontal movements for certain tank shapes and
support structures are required to avoid measurement distortions. Your load cell support
structure may need end-stops to limit lateral deflection, and elastomeric bearings can also
regulate heat between the tank and load cell. Also if your load cell requires self-centering, the
design engineer may suggest a pendulum load cell that will automatically guide the super structure into its original position.

How to Wire a Load Cell?

One of the most common applications is acquiring data from a load cell or any full-bridge type
sensor such as a strain gauge bridge with an A/D board. It is also the least understood, and
many users make simple wiring errors, causing excessive noise, and in extreme cases, damage
to the sensor and instrument.

The first thing to remember when installing a load cell is that you must measure it with a
DIFFERENTIAL input type, and not a SINGLE ENDED input type.
First, determine if your A/D device (your PLC, meter or DAS) can be configured as a differential
input. Then, you must use a regulated power supply to provide excitation for your sensor.

If the power supply is noisy, or unregulated, then the sensor output will also be noisy or will
drift. Some A/D boards have built in regulated power supplies, however, you may not be able to
connect more than one or two sensors due to current limitations.

Plug-in boards usually provide a +5V and -5V connection, however, this is usually the computer’s
PC power supply which is not suitable for bridge sensor excitation.
The best thing to do is to purchase a separate highly regulated power supply like this one that
can handle the current for all of the sensors that need to be powered.

The following diagram will demonstrate the correct wiring configuration for a load cell to a
differential input. Keep in mind that if more than one load sensor is connected to the same
power supply, you only need to connect the ground screw to only ONE ground, otherwise, a
ground loop may be created causing additional noise.

Also, make sure that the power supply is FLOATING, meaning that it is not already connected to
another ground anywhere else.  

Load cell wire colors

It is important to know the wire color codes for a given load cell to ensure that they are properly
connected for accurate results. The color codes vary between manufacturers and the coding is
usually provided on the load cell’s calibration certificate. It is important to follow the color
coding as specified by the manufacturer. This will ensure proper wiring connections and avoid
erratic output.  

How to check if the load cell is working or not?  

You can check if the load cell is working or not by verifying two aspects of the load cell – load
cell resistance and the load cell voltage.

Load cell output resistance is measured between the positive and negative wires. The load cell
input resistance is measured between the positive excitation wire and the negative excitation
wire. In both cases, the value between the wires must be equal or similar to the datasheet
provided with the load cell.

To measure the load cell voltage, connect the two wires to the amplifier. Now measure the
response signal between the positive and negative signal wires while incrementing the load to see the corresponding signal increment.

Load cell verification procedure

Although load cells provide accurate measurement results, they are prone to errors due to
several environmental and loading conditions. Common faults found in a strain gauge load cell
are:

● Fluctuations in zero balance value

● Incorrect or no reading at all

● Unstable readings for a known weight

● Changes in readings when the weight location changes


To verify that the load cell is in good shape and to fix the above issues, several tests are
available. The basic verification procedure involves inspecting the mechanical supports, load cell
orientation, and the mounting surfaces for level alignment and cleanliness. Further, the cables
connected to the summing box should also be inspected for any wear and tear.  

Once this preliminary inspection is complete, you can check the load cell for faults by evaluating
the zero balance reading. To do this, follow the steps below:

1. Connect the load cell input and output terminals to an excitation/input voltage.
2. Measure the voltage for both the input and output terminals using millivoltmeter. Divide the obtained value by the excitation voltage to get the zero balance in mV/V.
3. Compare the obtained values with the ones provided in the manufacturer’s calibration certificate.

Similar tests are performed to check the insulation resistance and bridge integrity. If the values
obtained through these tests are erratic, chances are that there is an electrical component
failure, an internal short circuit, or possibly the strain gauge has failed.

Using Load Cells to Weigh Trucks, Trains, and Aircraft

In the airline industry, weight is money. Every additional pound requires more fuel to lift, so
making sure there’s enough gas in the tanks means knowing what the aircraft weighs. Weight
distribution is another factor. Too much at the back and the aircraft flies nose-up, altering the
angle of attack and consuming more fuel.

Weighing something as large as an aircraft—even a Boeing 747—is not particularly difficult. Theway it’s done is with load cells  

The Importance of Weight in Transportation

Aircraft, from two-seat civilian planes to the largest passenger and freight aircraft, are weighed
regularly for two reasons. First, it’s an FAA requirement that the operator knows the weight of
the aircraft, and as weight can change over time, periodic re-weighing is mandated. And second,
a pilot may wish to know the weight of cargo or passengers and luggage taken on-board to
determine both the fuel needed and the weight distribution (on small turboprop planes it’s not
uncommon for passengers to be moved to even out weight distribution).

Truck weighing is another big application of load cells. Heavy vehicles cause significant damage
to roadways and especially bridges, so states have limits on the maximum permissible load that
may be carried. Enforcement of these limits is performed at roadside weigh stations where all
trucks are required to stop for weighing.

Trains too need weighing. As with roads, excessive loads accelerate track wear, and as with
aircraft, uneven weight distribution can result in stability issues. Freight moved by rail is
sometimes priced on the basis of weight, making it essential to know the load in each rail car or
wagon.  

Truck Weighing Systems

Most weight stations use either piezo-based or strain gauge load cells. These are embedded into
the road surface and the load created by each measured axle. A recent innovation is so-called
Weigh-in-Motion (WIM) technology where the truck can be weighed accurately without
needing to stop. These systems use a combination of load cells and inductive loops that detect
vehicle presence. They are fast and accurate, and most importantly, eliminate the need for each
truck to stop to be weighed. This overcomes the problems of traffic backups experienced at
busy times, which often forces the temporary closure of the weigh station.

Requirements for the WIM systems for highway use are defined in ASTM E1318-02.  

Train Weighing Systems

As with trucks, systems are available for both static and WIM measurement. These can
determine individual axle loads, bogey loads and even the weight of an entire wagon or
locomotive. Load cells are used in these systems and have accuracies of ±1% or better.  

Aircraft Weighing Systems

Aircraft are weighed with platform scales incorporating load cells. Typically the aircraft is pulled forward so all the wheels are on platforms. The total weight is then the sum of the readings
from each platform. Distances and differences between platform readings are used to compute
weight distribution. 

Precise and Robust

Trucks, aircraft and trains all need periodic weighing. This is done using load cells. A load cell
employs either strain gauges where a measurable change in resistance occurs as the material is
deformed or piezo materials that produce electrical charge when under pressure. These signals are amplified and can yield readings accurate to ±1%. 

Specific Applications  

Gas Turbine Engine/Rocket: High accuracy measurements of the volumetric flow of gas through
the pipeline. The turbine rotor turns the rotor blades by gas flow into the meter, measuring gas
velocity. The rotor blades pass a pickup coil, generating an electrical signal pulse. The pulse is
equal to a specific volume of gas. Total volumetric flow is recorded by the number of pulses.
Expression of flow rate is measured in actual cubic feet or actual cubic meters, (ACF/AM3).

Engine Thrust Measurement: This built-to-order combination torque and thrust reactionary and
rotary load cell product is constructed in all stainless steel for long term reliability. Applications
in industrial environments include safe overload of 150% of capacity, ultimate overload at 300%
of capacity.

Bin Weighing: To simplify load cell installations, use tank and bin weighing systems with
capacities of up to 2500 pounds. Systems provide shock absorption for industrial installations
and help to prevent improper mounting, which can cause load cell damage. Make a complete
four point system with 4000 pound maximum capacity with heavy duty shear beam type load cells and tank and bin weighing system.

Process Control Systems: The wide range of process controllers include temperature
controllers, ramp and soak controllers, dual-zone controllers, and bench-top style process
controllers. Microprocessor based controllers come with various displays, wire RTDs, process
voltage and current. They can connect directly to the Ethernet network featuring an embedded
web server, downloadable data acquisition software.

High Load Fatigue Testing: Reduces time needed for troubleshooting scale systems, analyzes
conditions of strain gage-based load cells in scale and industrial applications. Test load cells
without disconnection, easy-to-read, clear screen messages. Provides essential data, like
possible distortions from overloads, metals fatigue or shock loading, and possible ground faults
or bridge resistance electrical problems.


Bridge Testing: Using a terminal block system with bridging and testing accessories across
different clamping technologies reduces inventory and logistics costs. A modular terminal block
design can be combined with different terminal block types or individually for application flexibility

Structural Design of Tank Weighing Systems  

Initial Observations

Some essential rules must be followed when installing load cells in tanks. For example, tanks are
frequently subject to weather conditions or effects related to production. When new upright
tanks are erected outside (silos, coal hoppers, etc.), applicable building regulations must be
observed for the relevant structures. Note that subsequently installed weighing devices may
also be considered as "significant changes" in terms of building regulations. The advice of a
structural engineer is recommended in these cases. Building regulations generally cite the "state
of the art" in terms of safety considerations.

The project engineer for a tank layout should also be informed about any special company specific rules. Tanks must frequently be secured so they cannot be picked up, even in areas covered by a roof, if the contents are hazardous and forklift trucks are in operation around the storage area

Load Distribution

An optimum arrangement of load cells for determining the weight of tanks is achieved when the
tank rests on three bearing points and a load cell is positioned on each support. This state is
referred to as statically determinate. The overall load should also be distributed as evenly as
possible over the three load cells. In the case of upright or suspended cylindrical tanks, the best
way to meet this requirement is if the three load cells are arranged at equal distances from the
vertical axis of the tank and are offset from each other by 120° on the same plane. Figure 1
defines the arrangement of bearing points for horizontal tanks.

If not all the supports in a system are equipped with load cells, an uneven distribution of the
support load is recommended. The supports with load cells should have greater loads than the
supports without load cells. Following this guideline can improve the overall accuracy of the
weighing device. When designing the system and selecting the load cells, it is preferable for all the load cells to be subject to loads of the same magnitude

If a tank is supported on four or more points, the bearing of the tank is statically redundant.
Load cells must be installed at all the bearing points in these cases. An even distribution of the
load on the individual transducers can only be achieved during assembly. The transducer loads
must be measured individually for this purpose. Then, if there are impermissible differences,
the height of the relevant load cells must be changed (with compensating shims, etc.). Load cells with loads that are too low are generally positioned diagonally from each other.

Center of Gravity of a Tank

Ideally, the center of gravity of a filled tank should not be any higher than the bearing points of
the tank – a feature that is frequently not implemented.

For reasons of stability, it is advantageous for the center of gravity to be lower than the bearing
points. The position of the center of gravity as a function of the filling level has a considerable
effect on the number of load cells that are used. If the filling is arranged symmetrically to the
load cells, it may be possible to set up a weighing device with one load cell, given that the
position of the center of gravity moves along a vertical line. If the center of gravity also moves
to the side as the mass of filling changes, all the supports must be equipped with load cells.
Rocker and fixed bearings should never be considered for applications of this type!

Figure 2 illustrates the necessity of using load cells on all bearing points if the position of the
center of gravity changes.  

Supply Connections on Tanks  

Tanks frequently require supply connections, for example, to supply and discharge contents and
for the electrical, hydraulic or pneumatic supply of additional units mounted on the tank.
These supply connections can lead to force shunts,
which manifest themselves as errors that affect the
measuring accuracy of the weighing device. Supply
connections must be flexible in the vertical
direction. Figures 3 to 7 show some examples of
suitable designs for supply connections. In any
case, these aspects should always be taken into
consideration for financial reasons in the design
and planning phase. If rigid pipes without a flexible
link are used, the tank should preferably be connected with the longest possible horizontal pipe
section. The pipe section should also have expansion compensation in the longitudinal direction (Figure 3)

The horizontal section of the pipe has a spring
effect in the vertical direction, which becomes
more yielding as the length increases. The
mechanical force exerted by the pipe in the form of
a pseudo load (tensile or compressive) on the load
cells becomes correspondingly small and is no

longer relevant for measuring accuracy.

Several pliable couplings can also be used instead
of a layout with one long pipe (Figure 4). Good results in terms of preventing force shunts are
obtained with hose connections made of readily malleable elastic materials. In this case, the
compatibility of the elastic materials with the filling
and/or cleaning materials must be checked (in the food
industry or pharmaceutical technology, for example).
Another possibility for reducing undesirable force shunts
caused by connecting pipes is using a layout with a pipe
elbow (Figure 5).  

In cases where a vertical pipe supply is required (i.e., in
the direction of the gravitational force to be measured) or
hose connections cannot be used, pipe connections with
compensators (such as metal bellows) have proven
effective (Figure 6). Strict installation tolerances must be
observed when installing these compensators. If a second
set of metal bellows is used and connected with the first
by a section of pipe, it is possible to compensate for
greater tolerances. Metal bellows are not permitted in
some cleaning-intensive areas of the food industry.  

The connecting branch shown in Figure 7 represents the
best solution in terms of reducing force shunts. An open
connecting branch prevents contact between the pipe
and the tank. This form cannot be used in closed systems,
such as in pressure tanks.
It should always be noted that the material in the
connecting lines is included as part of the weight. The
filling level of the supply and discharge lines that are
directly connected with the tank should therefore be
reproducible when the weight is measured. This means
the lines should be either always empty or always full
when measurements are taken.

                  

Pressurized Tanks

In closed plants, the pressure in the system can affect the weighing results. Especially in the
chemical industry, high positive pressures are required for some processes. On the other hand,
extraction plants for weighing powdered material generate negative pressures of 100–300
mbar. If the piping is connected to the tank vertically, as shown in Figures 5 and 6, a force is
exerted that directly affects the measurement results. The effect is equivalent to the product of
the force multiplied by the cross-sectional area of the piping. If pressure conditions during the
weighing process are constant, this amount can be taken into consideration (calculated) in the
measurement. A horizontal pipe layout is more suitable and preferable to a vertical pipe
connection in every case. In this case, the parasitic forces that arise are absorbed by the
Installation supports.  

Examples of Designs for Arranging and Installing Load Cells

Typical tank designs are represented in a stylized
manner by way of example. Design details and
references to problems are presented in greater
detail in the relevant sections. 

Upright Tanks

Installation with two fixed supports and one load cell is possible for weighing liquids and bulk goods in tanks with central filling. To provide the appropriate level of accuracy, the tank must have a symmetrical layout. The product weight must be spread evenly across the tank or hopper in order to receive the proper results and maintain the accuracy required. In all other cases, especially if greater degree of accuracy is required, an installation with preferably three or, in some circumstances, more load cells is necessary.  

Rigid Installation of a Load Cell

This simple design on a carrier with a rigidly installed load cell is not recommended. The design alone results
in problematic effects on the load cell. Due to deformations as the filling level changes, as well as vibrations and changes in temperature, effects on the load cells generally cannot be ruled out. Nevertheless, a few cases of this design may be found.  

Upright Tank with Two Fixed Bearings and One Load Cell with Compensation

This filling level measurement uses a load cell arranged in a cradle with two fixed bearings, which also serve to restrain horizontal movement of the tank. This cost-effective design keeps the load cell free of unacceptable effects. 

High Round Silo on Three or Four Load Cells

Exact levels are usually measured on three load cells. Although arrangements with four load cells are sometimes found in rectangularly symmetrical designs, this arrangement is generally not favored due to its static redundancy and higher price. On the other hand, they are easier to install in the structure. Self-centering elastomer bearings do not require any stay rods. They are usually combined with fixed stops instead. Additional stay rod guides are needed in the upper area for very high tanks. In the example in Figure 10, they are designed as stay rods with loose initial stress and locking. Fixed stops would continuously come in contact with the tank at this point whenever it unavoidably moved slightly out of its ideal position and the contact friction would lead to force shunts. Roller stops or cable guides are less frequently used alternatives.  

Round Silo on Three Weighing Modules

Three weighing modules with integrated stay rods that contact the circumference of the
structure tangentially will hold the tank horizontally stable with no need for any additional
measures. The anti-liftoff device, also located in the weighing module, prevents the tank from
tipping over. This eliminates several structural details in the outer construction. Typical weighing
modules for lower, medium and higher loads are also illustrated here by way of example. These
standardized elements simplify design and save considerably on construction expenses. On the
other hand, the design requires considerable care and overhead to ensure that contact surfaces
are parallel, heights are aligned, etc.  

Flanged Tanks on Weighing Modules

Flanged tanks, which are used quite frequently in practical applications, have an outer casing
that extends to the base and serves to ensure the overall stability of the arrangement.
Installation on load cells is not a simple matter. Figure 12 shows a design variant for weighing
these tanks with load cells. This suggestion is also relatively easy to implement in existing
systems. Braces are mounted or welded into the inside wall of the tank. The load is inserted
rigidly into the load cell. Load cell weighing modules should preferably be used in this case as
well, because they contain an anti-lift off device, etc. (not shown in Figure 12 to enhance
clarity). Raising the structure even slightly is enough to direct the entire weight force into the
load cells. The system must be sealed frequently. This is achieved through a circular sealing ring
that does not cause any force shunts due to its flexibility.  

Rectangular Hopper in a Filling Station on Four Load Cells

          

  

Harsh environments or unusual mechanical effects can affect the quality of your measurements. For example, severe vibrations due to shaker devices or mixers, the influence of a bulky or heavy product being added to the vessel, or the discharge of the contents in controlled amounts. In addition, there the temperature can cause the structure to expand and contract as the temperature changes throughout the production process. All of these real-time mechanical influences will twist and torque the load cells in multiple directions which results in non-reliable measurements or poor accuracy. In these situations, it is recommended to have additional support points and load cell modification which will ensure you do not lose any measurement integrity but rather add stability to your structure. This in the long term will result in reliable and accurate measurements in the end product, which will keep your customers happy. Many times it is recommended to place a stay rod or a sway bar built into the structure or as an additional support point attached to the structure. Load cells with elastomer bearings or installed in a vertical position like those in the example of pendulum load cells (see section 6.3) can also benefit from these support devices. An Applications Engineer can offer assistance and recommendations as to where to strategically
place these support points.  

Suspended Tanks

Centering problems can often be
eliminated or simplified on
suspended tanks with simple,
pliable, round tie rods. In addition to
providing protection against tipping,
which is always necessary, stay rods
are needed to prevent oscillating and
turning  


Suspension on Two or Three Load Cells

The overall simple design requires
several tangentially arranged stay rods. In cases
with reduced stress, their functions can be
assumed by a lower side pipe outlet.  

Centric Suspension on One Load Cell

A special restraint is essential in this arrangement to prevent oscillating and turning.  

Horizontal Tanks for Liquids

Horizontal tanks for liquids usually fulfill the condition that the center of gravity of the content
must move approximately along a vertical line as the mass of filling changes. An arrangement
with one load cell under one tank bracket and two fixed bearings under the other tank bracket
is therefore sufficient for relatively simple level measurements. The idealized tank rests with
half its weight on one self-centering pendulum load cell and on two fixed bearings. No further
restraint is required under normal circumstances. In the case of very long tanks, however, for
additional protection against tipping over due to lateral impacts on the tank, fixed stops can be
provided to limit lateral movements at both ends of the tank cradle that is resting on the load
cell.
In practical applications, however, the distribution symmetry of the contents is often disturbed
by a slight, deliberate inclination of the base line to one side and the outlet that is located there.
A self-centering arrangement of three load cells is the optimum solution for more exact
weighing, with fixed stops as the best way to implement horizontal restraints  

Level measurement using Load cell

The level liquid in the tank is proportional to the weight of the tank. As the level increases, the
pressure exerted on the strain gauge increases, This makes the strain gauge shrink and to
reduce resistance.

The weight of the fluid = Weight of vessel with fluid - weight of the vessel

Here Volume of the fluid = weight of fluid / density of the fluid

Level of the fluid = volume of fluid / surface area.

Weight of the vessel is known and the weight of the vessel with fluid is what we measured using
load cells. 

Which is Better: Three or Four Load Cells?

It is a commonly asked question: Is it better to use three or four load cells on a scale?
In theory, you can use any number of load points (load cells) to support a vessel. The actual
number used is dependent upon these factors:
• The vessel’s geometry (shape and number of supports).
• The vessel’s gross weight (both live and dead weight).
• The vessel’s structural strength.
• The environment in which the vessel is located.
• What is available structurally to provide a stable, load-bearing support.

• The characteristics of the material being weighed.

Three Load Cells

For short, upright cylindrical vessels in a compression installation, three load cells spaced at
120° intervals provide the most convenient support. Three-leg weighing systems balance like a
tripod, with load distribution being virtually automatic, and they only require minor balancing at
installation. You must install all of the load cells in the same plane within 3° of each other.
Cylindrical vessels suspended symmetrically in tension with three load cells provide the
advantage of equally distributing the load among the load cells. What is available structurally is
important in this situation.
A vessel in tension can be hung in a corner where there are two supporting structures at right
angles. All it requires is a 45° cross brace to provide support for the third load point. Of course,
the support beams must be sufficiently strong and stiff to support not only the fully loaded
vessel, but also other vessels that may be supported from the same structure, and any changes
in the structural load, such as an accumulation of snow, water or ice  

Four Load Cells

Square, rectangular, horizontal cylindrical, tall cylindrical vessels or those that require greater
stability should use at least four load cells: one in each corner. Vessels subject to fluid sloshing,
material agitation or mixing, violent internal chemical reactions, high winds or seismic effects
require greater stability to guard against tipping.
A four-leg weighing system adds structural strength but requires more care in the installation
process to balance the loading on the four legs. With this type of support system, it is necessary
to equalize, or level the base, to spread the load evenly among the four cells.
For scales with accuracy requirements equal to or greater than 0.1%, the base plate support
surfaces must be within 0.4° (0.08mm/100mm). If one cell is mounted on a lightweight
crossbeam that has a high deflection, it can sag and throw the load onto the two adjoining cells,
possibly overloading them.
It is a simple but critical process to shim the load cells or, if equipped, fine-tune the adjusting
bolts on the cell mounting hardware during installation to balance the four legs. Proper load
sharing should have a difference of only ±0.5 mV between load points.
To accomplish this, measure the DC mV signal between each of the load sensors, plus and minus
signal wires with handheld meters, or through the weighing instrumentation if that feature is
available.
Larger differences between load cells due to motors hanging off one side of the vessel or
excessive or low flexing piping should not exceed ±2.0 mV between the highest and lowest
reading. Four legs also offer a larger area to support the vessel, provide for equal load sharing
among the legs, and help to keep the vessel from tipping over.
Long horizontal tanks with saddles symmetrically positioned in from the ends should also use
four load points. If the material is self-leveling, if there are no partitions in the vessel, and low
accuracies of 0.5% or greater are acceptable, then using one or two load points at one end and
flexures at the other is satisfactory. The load fraction seen by the load cells must stay the same,
no matter what the level in the vessel.
Very large capacity or heavy vessels in excess of one million pounds require more than four load
cells. Because the wall thickness and supporting structure of the vessel increase as the number
of supports decreases, the vessel’s deadweight and, therefore, installation costs, increase
dramatically. These vessels usually are designed for six or eight load points. Generally, you
should not use any more than eight load cells. It becomes proportionally more difficult to get
even weight distribution, and therefore better accuracy, on vessels with more than four load
cells.  

Environmental effects on load cells

One special feature of load cells is that the environment in which they are used plays a decisive role – in a number of ways.  

Ambient temperatures

Every material changes with temperature, expanding in response to heat and contracting in
response to cold. Of course the same applies to load cells and their strain gauges. This also
changes the electrical resistance of the conductor. Yet load cells must measure the correct
weight everywhere in the world, regardless of the ambient temperature. To achieve this, a
sophisticated temperature compensation mechanism is built into some load cells.

Load cells must be able to withstand various effects. Consider a truck scale: These scales are
exposed to the elements: rain, dirt or heat – they have to be able to withstand outdoor ambient
conditions. And we are talking world-wide: A truck scale in Siberia, for example, is exposed to
different effects than one in South Africa. But they do have one thing in common: They must be
designed for environments with severe weather and must therefore be correspondingly rugged.

Application of force in other directions ("parasitic forces")

Depending on the technical environment in which a load cell is installed – for example in a
system for weighing containers or in a weighing cell under a conveyor belt – other loads in
addition to the weight may occur. "Parasitic forces" are forces acting on the load cell not only in
the desired principal direction, but also from the side, from below or in some other way. The
load cell was not developed for this purpose and the measurement results may be inaccurate or
simply wrong. Care must therefore be taken during installation to ensure that there are no parasitic forces, or as few as possible.  

How to Check the Load Cell with a Multimeter

Set the multimeter in DC millivolts and connect the output wires of the load cell to the
multimeter. Supply a voltage of 5V or 9V DC at the excitation leads and place a test weight on
the load cell. The multimeter will register a change in voltage measured across the load cell’s
output.  

How to Test a Load Cell

To test a load cell before putting it to use, you’ll need a multimeter and a voltage source. Measure the resistance of the input and output leads of the load cell by setting up the multimeter in Ohms. Compare the measurement values with the calibration certificate from the manufacturer to see if they closely match each other. Similarly, check the load cell for accuracy by measuring the millivolts signal from the input leads. With no force applied to the load cell,the value should be zero. Apply a calibrated dead weight as specified in the calibration certificate and compare the values again.  

How to Measure Load Cell Output

Load cell output is measured using a digital meter. The digital meter connects to the output of
the load cell. It converts the digital signals produced by the load cell into readable digital values.
You can also measure the output of a load cell using a multimeter. However, a multimeter will
delete the output voltage in millivolts, and will not convert it into force or weight.

How to Check Load Cell Resistance

A load cell test is performed using a digital multimeter. The digital multimeter is connected
between the positive signal wire and the negative signal wire of the load cell. The output
between them should be equal or to a value specified in the datasheet. This is the test for load
cell output resistance. Now check the signal between the positive excitation wire and the negative excitation wire. They should be equal. This is the test for input resistance. 

Load Cell Value Fluctuation

Load cell values can fluctuate due to several reasons. From physical damage such as shock
loading and overloading for a longer duration to environmental conditions such as temperature,
moisture, water ingress, or corrosion, the load cell is likely to produce erroneous output.
Measurement values will also fluctuate if the cables break or if there’s a short circuit. To check
what is causing fluctuations in the load cell value, perform a visual check to identify the fault
location. Perform a zero balance check to identify if the strain gauge has undergone permanent
deformation. An insulation resistance check can further help you identify if moisture is getting
inside the load cell. Additionally, a bridge resistance check will determine if there’s a short
circuit within the load cell. 

Load Cell Zero Drift

Zero drift is the condition where zero measurements of the load cell change randomly under no-load conditions. It can also happen when the apparatus is loaded, and this phenomenon is called Drift. Several reasons such as mechanical errors, fluctuation in excitation voltage, and temperature variations could cause drift. To troubleshoot the load cell for zero drift, it is important to inspect the entire system. 

Load Cell Negative Reading

Load cell negative reading occurs when the load cell is in an incorrect orientation. If the load cell
is upside down, it would produce negative readings under loading. There is usually an arrow on
the load cell that shows the direction of loading. A Load cell used to measure tension will not
reflect negative reading if installed upside down and will result in an erroneous reading.
However, if the load cell is installed correctly and the readings are still negative, verify the wire
connection according to the color code specified by the manufacturer. 

Load Cell Overload

Every load cell comes with rated capacity. Loading the load cells beyond the rated value
overloads the load cell. The telltale signs of load cell overload are inconsistent display reading,
reading not coming back to zero even after the load is removed, the dramatic change of zero
balance, etc. Shock overload is one of the most damaging among overloads. Here the weight on
the load changes to a significant degree in a very short period. Most load cells endure some
overload and this value is called Safe overload. Anything beyond that can lead to permanent damage. 

Which System to Choose?

As for which system is better, there is no correct answer. The precision can be equal on one,
three, four or more compression or tension load cell systems. The road to higher accuracy is
mostly in the care taken during system design and installation.
Taking into account level footings, balanced load points, absence of binding, flexible piping,
environmental characteristics such as wind and temperature, and the load’s center of gravity
within the footprint of the scale all make for a good scale installation.  

6th Sep 2021 Sadia Naseer

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