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Pressure transmitter selection basis
Jan 09, 2019

Pressure transmitter selection basis

Among the various types of instruments, transmitters are the most widely used and widely used, 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. There are two-wire transmitters. There are intelligent and non-intelligent points, and there are more intelligent transmitters. There are pneumatic and electric parts, and most of the electric transmitters. In addition, according to the application, there are intrinsic safety type (intrinsic safety type) and explosion-proof type; according to the application conditions, the main types of transmitters are as follows:

Low (micro) pressure / low (micro) differential pressure transmitter; medium / medium differential pressure transmitter; high pressure / high differential pressure transmitter; absolute / vacuum / negative pressure differential pressure transmitter; high temperature / Pressure, differential pressure transmitter; corrosion resistant / pressure, differential pressure transmitter; easy to crystal / pressure, differential pressure transmitter.

Pressure transmitter selection basis

Transmitter selection is usually based on installation conditions, environmental conditions, instrument performance, economy, and media usage. In practical applications, it can be divided into direct measurement and indirect measurement; its uses include process measurement, process control and device interlocking. Common transmitters include ordinary pressure transmitters, differential pressure transmitters, single-lamp transmitters, dual-lamp transmitters, and plug-in flange transmitters.

Pressure transmitter selection basis

The selection of pressure/differential pressure transmitter is mainly based on the measured medium, temperature environment, accuracy grade, installation and maintenance cost, etc. After determining the manufacturer and model of the transmitter used, the transmission is determined according to the requirements of use. The range of the device. The range selection should be based on the principle that the range is adjustable up and down, the highest precision, and the price is suitable.

1, Measuring medium

If you are measuring a relatively clean fluid, use a standard pressure transmitter directly. If the measured medium is high viscosity, easy to crystallize and corrode, then an isolated transmitter must be used. When selecting the type, consider the corrosion of the medium to the bellows metal. Be sure to choose the material of the bellows. Otherwise, the outer membrane will be corroded if it is used for a short time, and the flange will be corroded and damaged. Personal accidents. The diaphragm of the transmitter is made of ordinary stainless steel, 304 stainless steel, 316L stainless steel, and diaphragm case material.

2, Range

From the selection range of the selected transmitter, the general transmitter has a certain range adjustable range. It is better to set the range of the range to 1/4~3/4 of its range, so that the accuracy will be guaranteed. It is even more important for differential pressure transmitters. In practice, some applications need to migrate the measurement range of the transmitter. In this case, the measurement range and migration amount should be calculated according to the site installation position, and then the positive or negative migration should be performed.

3, Accuracy level

Each meter will have an accuracy error, but the accuracy level is different for each country. For example, the accuracy of the Chinese and American standards is the best part of the linearity of the sensor, which is what we usually call the measurement range. Accuracy between % and 90%; and European standard accuracy is the worst part of linearity, which is what we usually call 0% to 10% of the measurement range, and between 90% and 100%. If the accuracy of the European standard is 1%, the accuracy of the standard in China is 0.5%.

Pressure transmitter selection basis

4, Wetted material

The contact medium part of the general pressure transmitter is made of 316 stainless steel. If the measuring medium is not corrosive to 316 stainless steel, then basically all pressure transmitters are suitable for measuring the medium pressure; if measuring medium pair 316 Stainless steel is corrosive, so it is necessary to use a chemical seal, which not only can measure the pressure of the medium, but also effectively prevent the contact between the medium and the pressure transmitter, thus protecting the pressure transmitter and prolonging the service life. The role.

5, The output signal

At present, due to various collection needs, there are many kinds of output signals of pressure transmitters on the market, mainly 4~20mA, 0~20mA, 0~10V, 0~5V, etc., 4~20mA and 0 are commonly used. ~10V two, and only 4~20mA signal is two-wire system (excluding grounding and shielding line), the others are three-wire system.

6, Medium temperature

Since the signal of the pressure transmitter is partially converted by the electronic circuit, in general, the temperature of the pressure transmitter is -30 to +100 degrees. If the temperature is too high, a condensation bend is generally used to cool the medium.

7, Other

After determining these parameters, the process connection interface of the pressure transmitter and the supply voltage of the pressure transmitter should also be confirmed; if it is used in special occasions, the explosion protection and the degree of protection should be considered.