Temperature Transmitter Temperature measurement using modern scientific thermometers and temperature scales goes back at least as far as the early 18th century, when Gabriel Fahrenheit adapted a thermometer (switching to mercury) and a scale both developed by Ole Christensen Røemer. Fahrenheit's scale is still in use, alongside the Celsius scale and the Kelvin scale. Many methods have been developed for measuring temperature. Most of these rely on measuring some physical property of a working material that varies with temperature. One of the most common devices for measuring temperature is the glass thermometer. This consists of a glass tube filled with mercury or some other liquid, which acts as the working fluid. Temperature increases cause the fluid to expand, so the temperature can be determined by measuring the volume of the fluid. Such thermometers are usually calibrated, so that one can read the temperature, simply by observing the level of the fluid in the thermometer. Another type of thermometer that is not really used much in practice, but is important from a theoretical standpoint is the gas thermometer.
How does an industrial Temperature Transmitter work?
There is some kind of temperature sensing element -- a thermocouple, thermistor, resistance thermal device, semiconductor temperature sensor, or other temperature transducer. The output of the temperature transducer will be some voltage proportional to temperature; either directly from the transduer, or through a converter circuit that outputs voltage. This voltage is applied to a voltage-to-current converter circuit, which translates the various voltage levels (dependant on temperature) to a current that generally ranges from a low of 4 mA to a high of 20 mA. 4 and 20 mA corresponds to the minimum and maximum temperatures that are to be measured. The voltage-to-current converter does not *source* the current, it *allows* that much current to flow in the loop. 4-20 mA is pretty standard, but there are other lesser-used standard loop current ranges. The reason it is 4-20 and not zero-20, is because if the minimum were zero, you could not tell if it was a low reading or an open loop (cut wire in the loop).
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