A pressure transmitter is a kind of pressure measuring instrument widely used in many transmitters. It is widely used in petroleum, chemical, metallurgy, food, electric power, medicine, papermaking, textile and other industries. It is mainly used to detect the differential pressure, pressure, absolute pressure and liquid level of fluid.

1. Components of Pressure Transmitter

It is mainly composed of a load cell sensor, module circuit, display meter, case and process connector.

It can convert the received gas, liquid and other pressure difference signals into standard current and voltage signals to supply secondary instruments such as indicating alarm instrument, recorder and regulator for measurement, indication and process regulation.

2. Measurement Principle of Pressure Transmitter

The process pressure and the reference pressure act on both ends of the integrated silicon pressure-sensitive element respectively, and the differential pressure deforms the silicon wafer (displacement is very small, only μ m level) so that the full dynamic Wheatstone bridge made of semiconductor technology on the silicon wafer can output mv level voltage signal proportional to the pressure-driven by an external current source.

Because of the excellent strength of silicon material, the linearity and variation index of the output signal is very high. When working, the pressure transmitter converts the measured physical quantity into an mv level voltage signal and sends it to a differential amplifier with high magnification and mutual cancellation of temperature drift.

The amplified signal is transformed into the corresponding current signal by voltage and current conversion, and then through nonlinear correction, the standard current and voltage signal which is linear with the input pressure is generated.

3. Types of Pressure Transmitter

It mainly includes capacitive pressure transmitter, diffused silicon pressure transmitter, ceramic pressure transmitter, strain type pressure transmitter, etc.

3.1 Capacitive Type

When the pressure acts directly on the surface of the measuring diaphragm, the diaphragm will deform slightly.

The high-precision circuit on the measuring diaphragm transforms the tiny deformation into a voltage signal which is highly linear and proportional to the pressure and also proportional to the excitation voltage, and then a special chip is used to convert the voltage signal into an industrial standard 4–2oma current signal or 1–5V voltage signal.

Because the measuring diaphragm adopts the standardized integrated circuit, which contains the linear and temperature compensation circuit, it ensures the high accuracy and high stability of the pressure transmitter. The special two-wire system chip is used in the transmission circuit, which can ensure the output of the 4–2oma current signal of the two-wire system, which is convenient for field wiring.

3.2 Diffused Silicon Type

The pressure of the measured medium acts directly on the diaphragm of the sensor (stainless steel or ceramic), which makes the diaphragm produce a micro displacement proportional to the medium pressure and changes the resistance value of the sensor. And the electronic circuit detects the change and converts and outputs a standard measurement signal corresponding to the pressure.

3.3 Ceramic Type

The ceramic pressure acts directly on the front surface of the ceramic diaphragm, causing a slight deformation of the diaphragm. The thick film resistor is printed on the back of the ceramic diaphragm and connected to form a Wheatstone bridge (closed bridge). Because of the piezoresistive effect of the varistor, the bridge produces a voltage signal which is highly linear and proportional to the pressure and also proportional to the excitation voltage.

3.4 Strain Type

The resistance strain gauge is a kind of sensitive device which converts the strain change on the measured part into an electrical signal. It is one of the main components of the piezoresistive strain transducer.

The most widely used resistance strain gauges are metal resistance strain gauge and semiconductor strain gauge.

There are two kinds of metal resistance strain gauges: wire strain gauge and foil strain gauge. Usually, the strain gauge is tightly bonded to the matrix which produces mechanical strain by a special adhesive. When the stress of the matrix changes, the resistance strain gauge also deforms together, which changes the resistance value of the strain gauge, thus changing the voltage applied to the resistance.

4. How to Select Pressure Transmitter

The selection of pressure transmitter is based on its application. According to the following points to choose, respectively, is the accuracy, range, output signal, medium temperature, measurement medium, explosion-proof grade.

4.1 Accuracy Class

Every kind of electronic measuring meter will have an accuracy error. When using, you should choose the transmitter according to the actual accuracy requirements.

4.2 Range

Generally, it is best that the maximum measurement range of the sensor is 70% of the full scale of the sensor. For example, if we want to measure the pressure of 7MPa, we should choose 10MPa as the range of the pressure transmitter.

4.3 Output Signal

At present, due to the needs of various acquisition, there are many kinds of output signals of pressure transmitter on the market. There are mainly 4 ~ 20mA, 0 ~ 20mA, 0 ~ 10V, 1 ~ 5V and so on.

4 ~ 20mA is commonly used. Among the above output signals, only 4 ~ 20mA is a two-wire system. We say the output is several wire system, not including grounding or shielding wire. The other is the three-wire system.

4.4 Medium Temperature

As the signal of the pressure transmitter is partially converted by the electronic circuit, the temperature of the measuring medium of the pressure transmitter is generally — 30 to + 100 degrees. If the temperature is too high, we generally use the condensing bend to cool the medium. Relative to let the manufacturer specially produce a high-temperature pressure transmitter for you, the cost of doing so will be reduced a lot.

4.5 Measuring Medium

If the measurement is relatively clean fluid, we can directly use the standard pressure transmitter. If the medium you measure is easy to crystallize or viscous, we usually use the external diaphragm or use it together with the chemical seal, which will effectively prevent the medium from blocking the pressure measuring hole.

4.6 Other Info

After determining the above five parameters, we also need to confirm the process connection interface of your pressure transmitter and the power supply voltage of the pressure transmitter. If it is used on special occasions, explosion-proof and protection grade should also be considered.

5. Summary

Among all kinds of instruments, a pressure transmitter is widely used. Commonly used to measure pressure, differential pressure, etc. Different from the sensor, the transmitter can not only convert the non-electric quantity into the measurable electric quantity but also has a certain amplification effect.

In today’s industrial world, pressure transmitter plays an important role in the industrial control system. It not only displays the pressure of the system on the control screen but also participates in important logic calculation and parameter alarm functions.