Introduction to the hottest instruments and meters

2022-07-31
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Introduction to instruments: how to select various transmitters

I. integrated temperature transmitter

integrated temperature transmitter is generally composed of temperature measuring probe (thermocouple or thermal resistance sensor) and two-wire solid-state electronic unit. The temperature measuring probe is directly installed in the junction box in the form of solid module to form an integrated transmitter. The integrated temperature transmitter is generally divided into two types: thermal resistance and thermocouple

thermal resistance temperature transmitter is composed of reference unit, calibration and adjustment conversion unit of r/v electronic tensile testing machine force measurement system, linear circuit, reverse connection protection, current limiting protection, v/i conversion unit, etc. After the signal of temperature measuring thermal resistance is converted and amplified, the nonlinear relationship between temperature and resistance is compensated by the linear circuit, and a 4 ~ 20mA constant current signal with linear relationship with the measured temperature is output through the v/i conversion circuit

thermocouple temperature transmitter is generally composed of reference source, cold end compensation, amplification unit, linearization processing, v/i conversion, couple breaking processing, reverse connection protection, current limiting protection and other circuit units. After the thermoelectric force generated by the thermocouple is compensated and amplified by the cold end, the nonlinear error between thermoelectric force and temperature is eliminated by the linear circuit, and finally amplified and converted into a 4 ~ 20mA current output signal. In order to prevent accidents caused by temperature control failure due to thermocouple wire breakage during thermocouple measurement, the transmitter is also equipped with a power-off protection circuit. When the thermocouple wire is broken or poorly connected, the transmitter will output the maximum value (28ma) to cut off the power supply of the instrument

the integrated temperature transmitter has the advantages of simple structure, lead saving, large output signal, anti-interference ability Table 3 carbon nanotube has strong conductivity and thermal conductivity, good linearity, simple display instrument, solid module anti-seismic and moisture-proof, reverse connection protection and current limiting protection, and reliable operation

the output of the integrated temperature transmitter is a unified 4 ~ 20mA signal; It can be matched with microcomputer system or other conventional instruments. It can also be made into explosion-proof or fire-proof measuring instruments as required by users

II. Pressure transmitter

pressure transmitter, also known as differential transmitter, is mainly composed of load cell sensor, module circuit, indicator, case and process connector. It can convert the received gas, liquid and other pressure signals into standard current and voltage signals to provide secondary instruments such as indicators, recorders and regulators for measurement, indication and process regulation

the measurement schematic diagram of pressure transmitter is shown in Figure 3. The measuring principle is: the process pressure and reference pressure act on both ends of the integrated silicon pressure sensor respectively, and the differential pressure causes the silicon wafer to deform (the displacement is very small, only μ M level), so that the full dynamic Wheatstone bridge made of semiconductor technology on silicon chip can output mv level voltage signal proportional to pressure driven by external current source. Due to the excellent strength of silicon material, the linearity and variation index of output signal are very high. During operation, the pressure transmitter converts the measured physical quantity into mv level voltage signal and sends it to the differential amplifier with high amplification factor and can offset the temperature drift. The amplified signal is converted into the corresponding current signal by voltage and current conversion, and then it is non-linear corrected. Finally, a standard current and voltage signal with a linear relationship with the input pressure is generated

pressure transmitter can be divided into general pressure transmitter (0.001mpa ~ 20m3) and micro differential pressure transmitter (0 ~ 30KPa) according to the pressure range

III. liquid level transmitter

1. Floating ball liquid level transmitter

floating ball liquid level transmitter consists of magnetic floating ball, measuring conduit, signal unit, electronic unit, junction box and mounting parts

generally, the specific gravity of the magnetic floating ball is less than 0.5, which can float above the liquid level and move up and down along the measuring tube. A measuring element is installed in the conduit, which can convert the measured liquid level signal into a signal proportional to the liquid level change under the action of external magnetic field, and strive to make the guide focus on the following aspects: resistance signal of, and convert the electronic unit into 4 ~ 20mA or other standard signal output. The transmitter is a modular circuit, which has the advantages of acid resistance, moisture resistance, shock resistance, corrosion resistance, etc. the circuit contains a constant current feedback circuit and an internal protection circuit, which can make the maximum output current not more than 28ma, so it can reliably protect the power supply and the secondary instrument from damage

2. Floating type liquid level transmitter

the pontoon type liquid level transmitter changes the magnetic floating ball into a pontoon, which is designed according to Archimedes' buoyancy principle. The float type liquid level transmitter is used to measure the liquid level, boundary or density by using the tiny metal film strain sensing technology. It can perform routine setting operation by pressing on-site keys during operation

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