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Focus On Level Measurement
What is the difference between a pressure transmitter and a differential pressure transmitter
Differential pressure transmitters and pressure transmitters are often used in industrial production. Many newcomers are not clear about the general health conditions such as differential pressure and high pressure, and they have a lot of confusion in practice. In fact, this is a pressure instrument. The basic knowledge is the common sense that every person engaged in related instrument work must master. To clarify the differences between these instruments, we must first clarify the differences between these concepts.
1 Introduction to pressure/differential pressure transmitter
In addition to measuring the differential pressure value of the two measured pressures, the differential pressure transmitter can also be used with various throttling elements to measure flow. It can directly measure the liquid level of the pressure vessel and the liquid level of the atmospheric vessel, as well as the pressure and Negative pressure.
From the structure of pressure and differential pressure transmitters, there are ordinary and isolated types. The common type of measuring capsule is one, which directly senses the pressure and differential pressure of the measured medium; the isolated type of measuring capsule receives the pressure of a stable liquid (usually silicone oil), and this stable liquid is Sealed between the two diaphragms, the diaphragm receiving the measured pressure is the outer diaphragm. The diaphragm of the original ordinary diaphragm is the inner diaphragm. When the outer diaphragm receives the pressure signal, the pressure of the outer diaphragm is transferred to the ordinary diaphragm through the transmission of silicone oil intact, and the position of the outer diaphragm is measured. Feel the pressure.
The isolated transmitter is mainly used for the special measured medium. For example, the measured medium will crystallize after leaving the device, while the ordinary transmitter needs to take out the medium, which will block the pressure guiding tube and the bellows chamber. Can not work normally, so the isolated type must be selected. The isolation type is usually installed as a flange type, that is, an opening is added to the device under test to add a flange so that after the transmitter is installed, its sensing diaphragm is the bottom part of the device wall, so that it will not take out the measured medium, and generally will not cause crystallization Blocked.
When the measured medium requires a high crystallization temperature, a structure that protrudes the diaphragm can be used, so that the sensing diaphragm can be inserted into the device, so that the temperature of the sensed medium will not decrease, so the measurement is guaranteed , That is, the plug-in flange transmitter is used.
Isolated transmitters have remote transmission type and integrated type. The remote transmission type means that the outer capsule and the measuring capsule are connected by a reinforced capillary tube. Generally, the capillary is 3~5 meters. In this way, the outer capsule is installed on the device, and the inner capsule and transmitter can be installed on a support that is easy to maintain. ; Another form is that the outer diaphragm box and the transmitter are integrated directly on the equipment by flanges. For isolated pressure transmitters, it can also be made into a threaded connection type, that is, the outer diaphragm box or outer elastic element can be in front of the installation thread. As long as the internal thread boss is welded on the device under test, the transmitter can be directly connected. Screwed to the device, installation is very convenient.
The production of isolated pressure/differential pressure transmitter is complicated and the material requirements are high, so its price is usually 3 times that of the ordinary type.
1.2 Selection principle
The main basis for the selection of pressure/differential pressure transmitters: take the property index of the measured medium as the standard, and take the saving of capital, ease of installation and maintenance as a reference. If the measured medium is an occasion where the viscosity is easy to crystallize and strong corrosion, an isolated transmitter must be selected.
When selecting the type, consider the corrosion of its medium to the bellows metal, and the bellows material must be selected, otherwise the outer diaphragm will be corroded in a short time after use, and the flange will be corroded and cause equipment and personal accidents. , So the choice of material is very important. The diaphragm of the transmitter is made of ordinary stainless steel, 304 stainless steel, 316L stainless steel, tantalum diaphragm, etc.
The temperature of the measured medium should be considered when selecting the type. If the temperature is generally 200°C~400°C, the high temperature type should be selected, otherwise the silicone oil will vaporize and expand, making the measurement inaccurate.
The working pressure level of the equipment should be considered when selecting the model, and the pressure level of the transmitter must be consistent with the application. From an economic point of view, the material of the outer diaphragm box and the insertion part is relatively high and suitable, but the connecting flange can be selected from carbon steel and chrome plating, which will save a lot of resources.
The isolation type pressure transmitter is selected with a threaded connection, which saves resources and is convenient for installation.
For the selection of ordinary pressure transmitters and differential pressure transmitters, the corrosiveness of the measured medium should also be considered, but the temperature of the medium used can be ignored, because the ordinary pressure change is induced into the gauge, and the long-term working temperature It is normal temperature, but the maintenance of the ordinary type is larger than that of the isolated type. First is the problem of heat preservation. In the northern winter, the pressure guiding tube will freeze, and the transmitter will not work or even be damaged. This requires the addition of heat tracing and incubators.
From an economic point of view, when selecting a transmitter, as long as the medium is not easy to crystallize, ordinary transmitters can be used, and for low-pressure easy to crystallize media, a purge medium can also be added for indirect measurement (as long as the process allows the use of purge liquid or The use of ordinary transmitters requires maintenance personnel to conduct more regular inspections, including whether various pressure pipes are leaking, whether the purging medium is normal, whether the heat preservation is good, etc., as long as the maintenance is good, a large number of common transmitters are used. Sexual investment will save a lot.
In terms of the measurement range of the transmitter, generally the transmitter has a certain range adjustable range. Set the used range range to 1/4~3/4 of its range, so that the accuracy will be guaranteed. It is important for differential pressure transmitters. In practice, some applications (liquid level measurement) need to migrate the measuring range of the transmitter. The measuring range and migration amount are calculated according to the on-site installation position. The migration can be divided into positive migration and negative migration.