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The difference between pressure transmitter and temperature transmitter

In some cases, the user will confuse the pressure transmitter and the temperature transmitter. Although it looks similar, there is still a big difference in performance and structure. The following is a brief introduction to the pressure transmission. The difference between a transmitter and a temperature transmitter.

First, the integrated temperature transmitter
An integrated temperature transmitter typically consists of a temperature probe (thermocouple or RTD sensor) and a two-wire solid state electronic unit. The temperature probe is mounted directly in the junction box in the form of a solid module to form an integrated transmitter. Integrated temperature transmitters are generally classified into two types: thermal resistance and thermocouple type.

The thermal resistance temperature transmitter is composed of a reference unit, an R/V conversion unit, a linear circuit, a reverse connection protection, a current limiting protection, a V/I conversion unit, and the like. After the temperature-measuring thermistor signal is converted and amplified, the nonlinear relationship between the temperature and the resistance is compensated by the linear circuit, and a constant current signal of 4-20 mA which is linear with the measured temperature is output after the V/I conversion circuit.

The thermocouple temperature transmitter is generally composed of circuit elements such as reference source, cold junction compensation, amplification unit, linearization processing, V/I conversion, burn-off processing, reverse connection protection, and current limiting protection. It is to compensate the thermoelectric potential generated by the thermocouple through the cold end compensation, and then the non-linear error of the thermoelectric potential and the temperature is eliminated by the linear circuit, and finally amplified and converted into a 4-20 mA current output signal. In order to prevent accidents caused by temperature control failure due to galvanic breakage in the thermocouple measurement, the transmitter also has a power-off protection circuit. When the thermocouple breaks or the connection is poor, the transmitter will output the maximum value (28mA) to turn off the power.

The integrated temperature transmitter has the advantages of simple structure, saving lead wires, large output signal, strong anti-interference ability, good linearity, simple display instrument, anti-seismic and moisture-proof of solid modules, reverse connection protection and current limiting protection, and reliable operation.

The output of the integrated temperature transmitter is a uniform 4 to 20 mA signal; it can be used with a microcomputer system or other conventional instruments. It can also be made into explosion-proof or fire-proof measuring instruments.

Second, the pressure transmitter

The pressure transmitter is also called a differential transmitter, which is mainly composed of a load cell sensor, a module circuit, a display head, a case and a process connection. It can convert the received gas, liquid and other pressure signals into standard current and voltage signals to supply secondary instruments such as alarms, recorders and regulators for measurement, indication and process adjustment.

The measurement principle of the pressure transmitter is that the process pressure and the reference pressure respectively act on the two ends of the integrated silicon pressure sensitive element, and the differential pressure causes the silicon wafer to be deformed (the displacement is small, only μm level), so that the semiconductor on the silicon wafer is used. The fully dynamic Wheatstone bridge made by technology outputs an mV-level voltage signal proportional to the pressure driven by an external current source. Due to the excellent strength of the silicon material, the linearity and variation index of the output signal are high. In operation, the pressure transmitter converts the measured physical quantity into a mV-level voltage signal and sends it to a differential amplifier with a high amplification factor that cancels the temperature drift. The amplified signal is converted into a corresponding current signal by voltage and current conversion, and then subjected to nonlinear correction, and finally a standard current voltage signal linearly corresponding to the input pressure is generated.

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