2020-07-27

The following is the discussion on the static pressure influence of differential pressure transmitter by lotingson technology R & D center. You are welcome to read it.

1. Introduction

When the differential pressure transmitter is in linear calibration, it is usually carried out under the condition that the negative pressure chamber is open to the atmosphere. In other words, the static pressure is 1 atmospheric pressure. However, once installed in the field for actual use, a certain working pressure will be added to the positive and negative pressure chambers. At this time, it will be found that the zero position is offset and the full position output is also offset (the full position offset is generally read out by comparing with the standard instrument). When the working static pressure is added, the zero position and full position output of the transmitter will deviate from the zero position and full position during atmospheric calibration, which is called static pressure influence error.

2. The influence of static pressure on transmitter performance and field examples

The static pressure error of differential pressure transmitter directly affects its comprehensive accuracy. The comprehensive accuracy (%) of differential pressure transmitter generally consists of three factors, which are accuracy (%), influence of ambient temperature change (% / 30o) and static pressure change (% / 7MPa). Their calculation formula is as follows:

Thus, static pressure error is a very important factor for the comprehensive accuracy of differential pressure transmitter.

This point has also been confirmed in various practical application conditions.

For example, when the differential pressure transmitter is applied to the field application of orifice flow detection, orifice plate or nozzle and other throttling parts are installed in the pipeline. Because the orifice diameter of the throttling piece is smaller than the inner diameter of the pipe, when the fluid flows through the throttling part, the beam cross-section suddenly shrinks and the flow rate accelerates. After throttling, the static pressure of the fluid at the back end decreases, so there is a static pressure difference before and after the throttling piece. There is a definite numerical relationship between the static pressure difference and the fluid flow, which conforms to q = K. The differential pressure transmitter is used to measure the differential pressure before and after the throttling parts to realize the flow measurement.

See Figure 1：

When it is used to measure the flow rate of high pressure steam in power plant, if the static pressure effect is not corrected or compensated, it will bring large error to the flow measurement, especially when the relative flow rate is small, the influence is more significant. For example, a metal capacitive differential pressure transmitter and throttling device constitute a differential pressure flowmeter. Under the condition of 32Mpa working static pressure, the static pressure error of full scale is ≤± 2% FS. Although the zero position error can be eliminated by zero adjustment, the full output error is always unavoidable. Therefore, this error directly affects the flow test and has a great influence. In this application condition, the static pressure performance of differential pressure transmitter is particularly important. If the static pressure error is compensated or its static pressure error is very small, its measurement accuracy will be greatly improved.

Causes of static pressure influence of metal capacitance sensor

Metal capacitive sensor is a kind of structural sensor, its static pressure effect is particularly prominent. This is related to its own structural characteristics.

Working principle introduction:

The medium pressure is transmitted to the measuring diaphragm located in the center of "δ" chamber through isolation diaphragm and silicone oil, and the measuring diaphragm deforms with the differential pressure on both sides. The displacement of the diaphragm is directly proportional to the differential pressure, and the maximum displacement is 0.1 mm. The differential capacitance between the measuring diaphragm and the capacitor plate is converted into a two-wire 4-20madc output signal through an electronic conversion circuit.

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It can be seen from Fig. 2 and Fig. 3 that both sides of the metal capacitive type are pressed, and the pressure is transmitted to the inner central diaphragm through the isolation diaphragm.

From the simplified stress distribution diagram and deflection change diagram of the metal capacitive sensor in Fig. 4, it can be seen that the internal pressure of the sensor is distributed from the center to the surrounding direction, and the stress in the X direction is completely offset, but the stress Q in the Y direction is all added to the shell of the sensor. Because of the size of the structure, the closer to the center, the thinner the structure, the worse the compressive capacity of the sensor, especially at the central diaphragm, the structural strength is the weakest. Under high hydrostatic pressure, there is a maximum deflection f at the center point. The result is that under high static pressure, the tension force of the central isolation diaphragm increases, and the tension degree of the diaphragm is strengthened when the working static pressure is zero. And the greater the static pressure, the greater the degree of tension.

When the tension is increased, the displacement of the central diaphragm with the differential pressure will become smaller, and then the differential capacitance between the measuring diaphragm and the capacitor plate will be converted into two-wire system through the electronic conversion circuit, and the output signal of 4-20madc will also become smaller. Finally, it leads to measurement error and static pressure influence error, and the absolute error of static pressure influence has a certain linear relationship with the added working static pressure. The larger the working static pressure is, the greater the static pressure error of its range is.

As for the static pressure error of the zero position, the direction is uncertain, which is mainly related to the welding stress and the personality of the sensor.

It can be seen from Figure 5 that when static pressure is applied to both sides of the sensor, the curved surfaces of the metal capacitive type are pressed simultaneously. However, the curved surface is composed of metal and glass, which will produce small deformation under the action of external force. Therefore, the thickness L1 and L2 of the curved surface seat on both sides decrease linearly with the increase of static pressure P. As a result, the electrode distances H1 and H2 of capacitor plates on both sides increase.

According to the definition of capacitance, the capacitance of parallel plate capacitor is C = ε o × s / h

Therefore, when the electrode distance h of the capacitance increases, the two-stage capacitance C1 and C2 of the sensor will become smaller. According to the working principle of capacitance sensor, the differential capacitance between measuring diaphragm and capacitor plate is converted into two-wire system by electronic conversion circuit, and the output signal of 4-20madc becomes smaller. Finally, it leads to measurement error and static pressure influence error, and the absolute error of static pressure influence has a certain linear relationship with the added working static pressure. The larger the working static pressure is, the greater the static pressure error of its range is.

From the analysis of the above two reasons, the metal capacitance sensor will inevitably produce measurement drift error under the influence of working static pressure. There is a certain linear relationship between the full position drift and the working static pressure, and the direction uncertainty for the zero position drift.