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technical article
Characteristics of sensors and transmitters (5)
2011-7-28 10:58:36

压力变送器 7. Pressure transmitter
Pressure transmitters are also called differential transmitters. It consists of load cell sensors, module circuits, display heads, case and process connections. It can convert the received pressure signals such as gas and liquid into standard current and voltage signals, and supply them to secondary instruments such as indicator alarm, recorder, and regulator for measurement, instruction and process adjustment.
   
The measurement principle of the pressure transmitter is: the process pressure and the reference pressure are applied to both ends of the integrated silicon pressure sensitive element, and the differential pressure deforms the silicon wafer (the displacement is small, only μm level), so that semiconductors are used on the silicon wafer. The fully dynamic Wheatstone bridge made by technology outputs a mV level voltage signal proportional to the pressure when 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. During 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 can offset temperature drift with each other.

The amplified signal is converted into a corresponding current signal by voltage-current conversion, and then subjected to non-linear correction, and finally a standard current-voltage signal corresponding to a linear relationship with the input pressure is generated.
    
Pressure transmitters can be divided into general pressure transmitters (0.001MPa ~ 20MP3) and micro differential pressure transmitters (0 ~ 30kPa) according to the pressure measurement range.
 
浮简式液位变送器 a. Floating simple liquid level transmitter  
The float type liquid level transmitter is a magnetic float ball changed to a float, which is designed according to Archimedes' buoyancy principle. Float level transmitters use tiny metal film strain sensing technology to measure liquid level, boundary or density. It can perform routine setting operations through field buttons during work.

浮球式液位变送器 b. Float level transmitter
Float-type liquid level transmitter is composed of magnetic float, measuring tube, signal unit, electronic unit, junction box and mounting parts.
Generally, the magnetic floating ball has a specific gravity of less than 0.5, and can float on the liquid surface and move up and down along the measuring tube.
The catheter is equipped with a measuring element, which can convert the measured liquid level signal into a resistance signal proportional to the liquid level change under the action of external magnetism, 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, and corrosion resistance. The circuit contains a constant current feedback circuit and an internal protection circuit, which can output a maximum current of not more than 28mA, so it can reliably protect the power supply and make The secondary meter is not damaged.
静压或液位变送器 c. Static pressure or liquid level transmitter
The transmitter uses the measuring principle of hydrostatic pressure. It generally uses a silicon pressure pressure sensor to convert the measured pressure into an electrical signal, which is then amplified by an amplifying circuit and compensated by a compensation circuit, and finally outputted in a 4-20 mA or 0-10 mA current mode.

电容式物位变送器 9. Capacitive level transmitter
Capacitive level transmitter is suitable for industrial enterprises to measure and control the production process during the production process. It is mainly used for long-range continuous measurement and indication of liquid level or powdery solid level of conductive and non-conductive media.
   
Capacitive liquid level transmitter is composed of capacitive sensor and electronic module circuit. It is based on two-wire 4-20mA constant current output. After conversion, it can be output in three-wire or four-wire mode. 5V, 0-5V, 0-10mA and other standard signals.
Capacitive sensors consist of insulated electrodes and a cylindrical metal container with a measuring medium.
When the material level rises, the dielectric constant of non-conductive materials is significantly smaller than the dielectric constant of air, so the capacitance changes with the height of the material.
The module circuit of the transmitter is composed of units such as reference source, pulse width modulation, conversion, constant current amplification, feedback and current limiting.     The advantages of using the pulse width modulation principle for measurement are low frequency, good RF interference to surrounding elements, good stability, good linearity, and no obvious temperature drift.