Among all kinds of instruments, the transmitter is the most widely used and universal,and the transmitters are generally divided into pressure transmitters and differential pressure transmitters.Transmitters are commonly used to measure pressure, differential pressure, vacuum, level, flow and density. The transmitter has two-wire system and four-wire system,two-wire system transmitters more;There are intelligent and non-intelligent, intelligent transmitters gradually more; There are pneumatic and electric, electric transmitters in the majority;In addition, according to the application,there are intrinsic safety type (intrinsic safety type) and explosion-proof type; according to the application under working conditions,the main types of transmitters are as follows:
※ Low (micro) pressure / low (micro) differential pressure transmitter;
※ Medium pressure / medium differential pressure transmitter;
※ High pressure / high differential pressure transmitter;
※ Absolute pressure/vacuum/negative pressure differential pressure transmitter;
※ High temperature / pressure, differential pressure transmitter;
※ Corrosion resistant / pressure, differential pressure transmitter;
※ Easily crystallized / pressure, differential pressure transmitter.
Transmitter selection is usually based on installation conditions, environmental conditions, instrument performance, economy, and media.In practical applications,it can be divided into direct measurement and indirect measurement;which are used for process measurement, process control and device interlocking.Common transmitters include ordinary pressure transmitter, differential pressure transmitter, single flange transmitter, double flange transmitter,insertion type flange transmitter, etc.
Pressure transmitters and differential pressure transmitters measure pressures and differential pressures (the difference between the two pressures) by name alone,but there are many quantities that can be indirectly measured.For example, pressure transmitter can measure the liquid level in the device besides measuring the pressure.When measuring the liquid level in an atmospheric pressure vessel, a pressure transmitter is required. When measuring the liquid level of the pressurized vessel,two pressure/differential pressure transmitters can be considered, that is, one lower limit of measurement, one upper limit of measurement, and their output signals are subtracted, and the liquid level can be measured.In this case, differential pressure transmitters are generally used. It can also be used to measure the density of a medium if the liquid level and pressure in the vessel are constant.The measurement range of the pressure transmitter can be made very wide, starting from absolute pressure 0 to more than 100 MPa (general condition).
In addition to measuring the differential pressure of the two measured pressures, the differential pressure transmitter can also measure the flow of the medium fluids with various throttling elements, and can directly measure the liquid level of the pressurized vessel and the liquid level of the atmospheric pressure vessel and pressure and negative pressure.
The structure of the pressure/differential pressure transmitter is divided into a common type and an isolated type.The measuring diaphragm capsule of the ordinary pressure/differential pressure transmitter is one, which directly senses the pressure or differential pressure of the measured medium; The measuring diaphragm capsule of isolated type receives a pressure of a stabilizing liquid (generally silicone oil) which is sealed between the two diaphragms and the diaphragm directly receiving the measured pressure is an outer diaphragm. The diaphragm of the original common type diaphragm capsule is an inner diaphragm.When the pressure signal is received on the outer diaphragm, the pressure of the outer diaphragm is transmitted to the ordinary diaphragm capsule by the transmission of the silicone oil, so that the pressure felt by the outer diaphragm can be measured.