1, the main types of transmitters
Transmitters are the most widely used and most common, and transmitters are roughly divided into pressure transmitters and differential pressure transmitters. Transmitters are commonly used to measure pressure, differential pressure, vacuum, level, flow and density. There are two wire transmitters and four wire transmitters, especially two wire transmitters; There are intelligent and non-intelligent points, intelligent transmitters gradually more and more; There are pneumatic and electric points, electric transmitters are mostly; In addition, according to the application, there are intrinsically safe type and flameproof type; The main types of transmitters according to the application conditions
As follows:
Low (micro) pressure/low differential pressure transmitter; Medium/differential pressure transmitter; High pressure/high differential pressure transmitter; Absolute pressure/vacuum/negative pressure differential pressure transmitter; High temperature/pressure, differential pressure transmitter; Resistance to
Corrosion/pressure, differential pressure transmitters; Easy crystallization/pressure, differential pressure transmitter.
The selection of the transmitter is usually considered according to installation conditions, environmental conditions, instrument performance, economy and application media. The practical application is divided into direct measurement and indirect measurement; It is used for process measurement, process control and device interlocking. Common transmitters are ordinary pressure transmitter, differential pressure transmitter, single flange transmitter, double flange transmitter, insert flange transmitter and so on.
Pressure transmitters and differential pressure transmitters only nominally measure the pressure and the difference between two pressures, but there are many parameters that they measure indirectly. Such as pressure transmitter, in addition to measuring pressure, it can also measure the liquid level in the equipment. When measuring the liquid level in the atmospheric pressure vessel, a pressure transformer is required. When measuring the liquid level of the pressure vessel, two pressure changes can be used, that is, the measurement of the lower limit of one, the measurement of the upper limit of one, their output signal can be subtracted to measure the liquid level, generally choose a differential pressure transmitter. It can also be used to measure the density of the medium when the liquid level and pressure value in the container are unchanged. The measurement range of the pressure transmitter can be very wide, starting from absolute pressure 0 to 100MPa (in general).

2, pressure/differential pressure transmitter introduction
In addition to measuring the differential pressure value of the two measured pressures, the differential pressure transmitter can also measure the flow with various throttling elements, and can directly measure the liquid level of the pressure container and the liquid of the atmospheric pressure container
Bit as well as pressure and negative pressure.
2.1 Production
According to the structure of pressure and differential pressure transmitter, there are ordinary type and isolated type. The common type of measuring capsule is – one, which directly feels the pressure and differential pressure of the measured medium; The isolated type of measuring capsule receives the pressure of a stabilizing liquid (usually silicone oil), which is sealed between two diaphragms, and the diaphragm receiving the measured pressure is the outer diaphragm. The diaphragm of the original ordinary type diaphragm is the inner diaphragm and the outer diaphragm. When the pressure signal is received, it is transmitted by silicone oil intact
The pressure of the outer diaphragm was transferred to the ordinary capsule, and the pressure felt by the outer diaphragm was measured.
The isolated transmitter is mainly used for the special measured medium, such as the measured medium will crystallize after leaving the device, and the use of the ordinary transmitter needs to remove the medium, which will block the pressure guide tube and the diaphragm chamber so that it can not work normally, so the isolated type must be selected. The isolation type is usually flanged, that is, the device under test is opened and flanged so that the sensor diaphragm is part of the device wall after the transmitter is installed, so that it will not take out the medium under test, and generally will not cause crystallization blockage.
When the crystallization temperature of the measured medium is high, the diaphragm protruding structure can be selected, so that the sensing diaphragm can be inserted into the inside of the device, so that the induced medium temperature will not be reduced, so that the measurement is guaranteed, that is, the plug-in flange transmitter is selected. Isolated transmitters are available in teletype and – type. The remote transmission type is that the outer membrane box and the measuring membrane box are connected by a strengthened capillary tube, the general capillary tube is 35 meters, so that the outer membrane box is installed on the equipment, and the inner membrane box and the transmitter can be installed on the support for easy maintenance; The other form is the outer membrane box and the transmitter – one directly mounted on the equipment by the flange. For isolated pressure transmitters it can also be made threaded, that is, the outer cartridge or outer elastic element can be installed in front of the thread, as long as the device under test. The transmitter can be screwed directly onto the equipment by welding the internal screw boss, which is very convenient to install.
The production of isolated pressure/differential pressure transmitters is complex and the material requirements are high, so its price is usually 3 times that of the ordinary type.
2.2 Selection Principles
The selection of pressure/differential pressure transmitter is mainly based on: the nature of the measured medium shall be subject to the index, in order to save money, easy installation and maintenance as a reference. If the measured medium is high viscosity, easy crystallization and strong corrosion, isolation transmitter must be selected. In the selection of its medium to consider the corrosion of the diaphragm metal, must choose a good diaphragm material, otherwise a short time after use will be corroded the outer diaphragm, flange will also be corroded resulting in equipment and personal accidents, so the material selection is very important. The membrane box material of the transmitter is ordinary stainless steel, 304 stainless steel, 316L stainless steel, tantalum membrane box material, etc.
In the selection to consider the temperature of the measured medium, if the temperature is high – generally 200°C^ 400°C, to choose a high temperature type, otherwise silicone oil will produce vaporization expansion, so that the measurement is not accurate.
The working pressure level of the equipment should be considered in the selection, and the pressure level of the transmitter must be consistent with the application. From an economic point of view, the outer film box and the insert part of the material is more suitable, but the connection flange can be selected carbon steel, chrome plating, which will save a lot of money.
The best choice of isolated pressure transmitter is to use the threaded connection form, which saves money and is convenient to install.
For ordinary pressure and differential pressure transmitter selection, the corrosiveness of the measured medium should also be considered, but the temperature of the medium used can be ignored, because the ordinary type of pressure change is induced into the table, and the long-term working temperature is normal temperature, but the maintenance amount of the ordinary type is larger than that of the isolation type. The first is the problem of insulation, in the north of the winter below zero, the pressure tube will freeze, the transmitter can not work or even damage, which needs to increase the heating and insulation box.