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technical article
Notes on purchasing a transmitter
2011-3-9 9:51:41

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. Pressure transmitter, in addition to measuring pressure, it can also measure the liquid level in the device. In addition to measuring the differential pressure of two measured pressures, the differential pressure transmitter can also measure flow with various throttling elements. It can directly measure the liquid level of a pressure vessel and the pressure level of a normal pressure vessel, as well as the pressure and Negative pressure.

The selection of the pressure / differential pressure transmitter is mainly based on the properties of the measured medium as the reference, with reference to saving money and facilitating installation and maintenance.

When selecting the type, it is necessary to consider the corrosion of the capsule metal by its medium, and the material of the capsule must be selected, otherwise the outer diaphragm will be corroded in a short time after use, so the choice of material is very important. The material of the transmitter's capsule is ordinary stainless steel, 304 stainless steel, 316L stainless steel, tantalum capsule and so on.

When selecting the type, the temperature of the measured medium should be considered. If the temperature is generally 200 ° C ~ 400 ° C, a high temperature type should be selected, otherwise the silicone oil will generate vapor expansion and make the measurement inaccurate.

In the selection, the working pressure level of the equipment must be considered, and the pressure level of the transmitter must be consistent with the application. From an economic point of view, the material of the outer membrane box and the insertion part is more suitable

The best choice for isolated pressure transmitters is the threaded connection, which saves money and is easy to install.

For the selection of ordinary pressure and differential pressure transmitters, the corrosivity of the measured medium must also be considered, but the temperature of the medium used can be ignored, because ordinary pressure changes are induced into the table, and the long-term operating temperature is normal temperature. However, the maintenance amount of the ordinary type is larger than that of the isolated type. The first is the issue of thermal insulation. In sub-zero winters in the north, the pressure guiding tube will freeze, and the transmitter will not work or even be damaged. This requires additional heat tracing and insulation boxes.

From an economic point of view, when using a transmitter, ordinary transmitters can be used as long as it is not a crystalline medium, and purge medium can also be added for indirect measurement of low-pressure crystalline medium (as long as the process allows the use of a purge liquid or Gas), the application of ordinary transmitters requires maintenance personnel to perform more regular inspections, including whether various pressure vessels are leaking, whether the purge medium is normal, and whether the insulation is good. As long as the maintenance is good, the ordinary transmitters are used once. Sex investment will save a lot.

From the perspective of selecting the measurement range of the transmitter, the general transmitter has a certain range of adjustable range. It is best to set the range of use to 1/4 to 3/4 of its range, so that the accuracy will be guaranteed. It is even more important for differential pressure transmitters. In practice, some applications (liquid level measurement) require the measurement range of the transmitter to be migrated. The measurement range and the amount of migration are calculated according to the installation location on the site. The migration has positive and negative migration.