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Special Types of Pressure Sensors

Special Types of Pressure Sensors

In all industries there are applications which require special care and needs. for pressure measurement applications sometimes these needs are measuring a very hot medium, or a very strong wave of water. Here we want to explain some special type of pressures sensors which are equipped and enhanced to work in such situation.

Dynamic Pressure Sensor 

Dynamic pressure measurements can be done through the use of piezoelectric pressure sensors. This high-quality and reliable dynamic pressure sensor with a piezoelectric sensing element provides a charge output and converts the measured pressure into an electrical signal. 

Dynamic pressure sensors are not applicable for measuring static pressure. because of high natural frequencies, piezoelectric pressure sensors can be employed for a variety of applications where dynamic pressures have to be measured with very fast pressure rise times of up to 1µs, and also they have the special ability to measure small pressure changes in acceleration, pressure, strain, temperature, or force at high static pressure levels. 

These piezoelectric sensors are a preferred choice for the measurement of dynamic pressures, small pressure pulsations at high static pressures, and Quasi-static pressure measurements in need of very small or high-temperature sensors in a variety of applications including:

  • Motorsport
  • Flight testing
  • Automotive
  • Inkjet printers
  • Biomedical measurements
  • Hydraulic & pneumatic systems
  • Blast and ballistics
  • Airbag components
  • Pyrotechnical devices
  • Energetic materials in closed vessels
  • Shock tubes
  • Shockwave physics
  • Industrial safety applications
  • Water hammer effects

Types of Dynamic Pressure Sensors 

There are two types of dynamic pressure sensors. Charge mode pressure sensors and ICP (Integrated Circuit Piezoelectric) voltage mode ­sensors. 

The first type generates a high­ impedance charge output. The voltage mode ­sensors are sensors with built-in microelectronic amplifiers that operate based on the conversion of the high­ impedance charge signal into a low impedance voltage output.

Piezoelectric Pressure Sensor Working Principle 

Piezoelectric sensors operate based on the principle of the piezoelectric effect

In piezoelectric sensors, quartz crystal is mainly used as a piezoelectric material that provides stable, repeatable operation. 

The quartz crystals are usually preloaded in rigid housings to create good linearity. When pressure is applied to a quartz crystal, a charge is developed across the crystal that is proportional to the applied pressure. 

These sensors are unsuitable for the measurement of static pressures but useful for dynamic pressure measurements because the electric signal generated by the quartz crystal decays rapidly. After applying pressure to a crystal, it is elastically deformed that this deformation creates a flow of electric charge. This electric signal can be measured as an indication of the applied pressure to the crystal. 

The output of Piezoelectric sensors for measuring dynamic pressure is often indicated in "relative" pressure units. These sensors are sensitive to temperature variations and need special cabling and amplification. 

These sensors are suitable for measuring rapidly changing pressures arising from blasts, explosions, pressure pulsations, or other sources of shock or vibration. where providing power to the sensor is impractical or impossible the crystal's self-generated voltage signal can be used that also provides high-speed responses.

Polarity 

When applying positive pressure to an ICP pressure sensor, the sensor generates a positive voltage. Charge output mode sensors are mainly used with external charge amplifiers that invert the signal. The polarity of the system output from a charging output sensor that is used with a charge amplifier generates the same output as the ICP sensor. The reverse polarity sensors are available as well.

High-Frequency Response 

Piezoelectric pressure sensors are usually created with either compression mode quartz crystals preloaded in a rigid housing or unconstrained tourmaline crystals. These constructions make the microsecond response times for the sensor and resonant frequencies in the hundreds of kilohertz, with minimal overshoot or ringing.

Low-Frequency Response 

The charge amplifier determines the low-frequency response of a charge mode pressure sensor. Based on the charge amplifier model used, the discharge time constant (DTC) of the amplifier that sets the low frequency response can be very long or very short. A longer DTC provides lower frequency measurements. A shorter DTC limits the low frequency response.

The discharge time constant and the low frequency response of ICP® pressure sensors are set by Internal resistance and capacitance values. The low frequency response analogous is set by The discharge time constant same as the action of a first-order R-C high pass filter. The DTC of the signal conditioner can influence the low frequency response of the overall system.

Flush Diaphragm Pressure Sensor 

A type of diaphragm that is completely open and directly connected to the pressure media is called a Flush Diaphragm. The Flush Diaphragm is mounted at or slightly behind the fluid surface. 

To connect the diaphragm flush pressure sensor, use a clamp and attach it to the flange around the outer diameter of the diaphragm. Also, threaded connection ports can be used that are large enough to fit the diaphragm at the tip. 

This type of transmitter is used to measure the pressure of:

  • High viscosity fluids such as plastics, paper pulp, bitumen, sewage, paint, and adhesives.

Diaphragm Materials & Design 

The materials used in the manufacture of flush diaphragms are based on the type of sensor technology and the liquid media to be measured. There are two different approaches: Using oil-filled elements and dry cell elements.

Oil-filled sensing elements have thin stainless steel isolation diaphragms. This type can be used in sanitary conditions and in high-temperature environments.

There are different types of mechanical designs for flush diaphragms. Threaded Style Process Connections and Clamped Style Process Connections are two types that will be explained in the following.

Threaded Style Process Connections are the most common form that has a threaded connection with a completely open diaphragm on the end. the surface area of the diaphragm is accommodated by a pressure connector that results in the use of larger diameter threads such as BSP 1 / 2 or 1 in mechanical design.

In clamped style, there is no gap between the thread which would allow the build-up of residues and bacteria. This characteristic makes it suitable for use in food and pharmaceutical industries where very high levels of cleanliness are necessary.

Applications for Flush Diaphragm Pressure Sensor 

Flush apertures are used in specialized applications where the media may block the connection of the enclosed process, increasing the volume will cause disruption to process performance, or in industries where a high level of hygiene is required. 

  • High Viscosity Liquids
  • Pharmaceutical
  • Food and Beverage
  • Biotech
  • Waste Water

Flush Diaphragm Pressure Sensor vs. Internal Diaphragm 

Pressure Sensor Internal diaphragm pressure connections are used in electronic pressure measuring instruments and are the industry standard for most gaseous and liquid media because it is inexpensive and easy to handle. 

This type of pressure connection is unsuitable for all media types because a medium enters the process port and is in contact with an internal diaphragm, which measures the pressure. but a flush diaphragm has two diaphragms. 

The medium is just in direct contact with the first diaphragm, which then acts on a transition fluid that retransmits pressure to the second diaphragm, so the media is not required to enter the pressure port. 

Due to this feature, flush diaphragms are a preferred choice for troublesome media and harsh environments like urea fertilizer production as well as ideal for hygienic applications due to residue-free cleaning.

SMART Pressure Sensor 

What is a SMART pressure sensor? 

The smart pressure sensor is an intelligent sensor operating based on the function of microprocessors for signal transmission that allows increasing its accuracy and expanding the list of its functions. 

It converts pressure into an electric signal, amplified, and passed to the built-in converter. The converter processes the measuring information on the fly and actively regulates measurements, but in conventional sensors, the signal is transferred to a central controller. 

The smart pressure sensor has an analog output as well as generates a digital communication signal based on the HART protocol that as a result atmospheric readings are compensated during digital communication. 

It is composed of a sensor or input circuitry, the microprocessor, memory, and a communication block. The smart pressure sensor can take zero and span readings and also provides frequency shift keying (FSK). 

This type of sensor is used for measuring absolute pressure, differential pressure, and process temperature. Also, the mass flow rate and volumetric flow rate of the process fluid can be computed.

Advantages of SMART Transmitters 

  • The smart pressure sensor has microprocessors and bi-directional communication remote re-calibration and re-ranging
  • Low maintenance costs
  • Improved Long term stability and reduced re-calibration frequency
  • Offer improved accuracy and reliability
  • Set for non-linearities and give linear output
  • The self-calibration capability allows the removal of zero drift and sensitivity drift errors
  • Wide measurement ranges
  • Automatic selection of the required range
  • Smart pressure sensor consists of multiple sensors
  • Analog to digital conversion
  • Secondary sensors for measuring and compensating for environmental disturbance

Explosion Proof Pressure Transmitter 

If pressure transmitters are used in hazardous zones with flammable gasses or dust such as petrochemical refineries and natural gas distribution pipelines, they need to be marked in compliance with the ATEX product guideline 94/9/EC.
Pressure transmitters with ATEX certification are ideal for use in a hazardous area that needs the use of instrumentation that prevents potential ignition sources of flammable gasses

An ATEX- approved pressure transmitter has a flameproof or explosion-proof enclosure where the transmitter’s electronic components should be installed inside a housing that is designed to contain the flame when it explodes or catches fire. 

ATEX needs the type label of the sensor to contain the ATEX marking, so normally this is labeled directly onto the sensor during production. 

In addition to the Notified Body and the approval number, also there is important information about the marking of pressure sensors for the user. 

For instance, after the approval number, there is an “X” that expresses an existing special condition that is necessary when using the device. 

This information is either provided in the approval document or in the corresponding manual. Especially, marking based on ATEX presents information on the possible areas that the device can be used and the type of protection used for the pressure sensor: 

Explosion group: :

  • I Mining industry, II other areas of use Device class
  • Use in zone 0/20 in the vicinity of von flammable gases (1G), dust (1D) 
  • Use in zone 1/21 in the vicinity of flammable gases (2G), dust (2D) 
  • Use in zone 2/22 in the vicinity of flammable gases (3G), dust (3D)  

However, there are devices in which only certain parts may be installed in other areas, for example, a pressure sensor with a 1 / 2 G mark may be installed in a tank with flammable content. The electronic part of the transmitter is located on the outer side of the tank and thus in Zone 1. 

25th May 2022 Saeed Abd

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