In the upper part, the influence of static pressure on the performance of differential pressure transmitter is analyzed, and the causes of static pressure influence of metal capacitance sensor are analyzed. Next we will analyze: silicon sensor static pressure influence analysis, metal capacitance sensor static pressure influence data and result analysis.

4. Static pressure influence analysis of Lotensin silicon sensor
The silicon sensor developed by Rodinson is packaged as a whole. It is surrounded by sealed silicone oil, and its sensitive elements are stressed all around, and the static pressure has little influence.
Working principle introduction:

The sensitive element of silicon sensor is to diffuse p-type impurity onto n-type silicon wafer to form extremely thin conductive p-type layer. The lead wire is welded to form "silicon strain gauge". Its electrical performance is to make a full dynamic piezoresistive Wheatstone bridge. It is combined with an elastic element (i.e., its n-type silicon substrate). The medium pressure is transmitted to the positive cavity side of the silicon diaphragm through the sealing silicone oil, and forms a pressure difference with the silicone oil acting on the negative cavity side. The result of their joint action is that one side of the diaphragm is compressed and the other side is stretched. The pressure difference makes the bridge unbalanced and outputs a signal corresponding to the pressure change. After the output signal circuit of Wheatstone bridge is processed, 4-20 mm ADC standard signal output is generated which is linear with pressure change.

 It can be seen from the structural diagram of the silicon sensor in Fig. 5 that the silicon sensor is completely contained by silicone oil. From the stress distribution diagram of the silicon sensor under the working static pressure in Fig. 6, the stress in all directions inside and outside the silicon sensor is effectively offset. Only at the ring joint glue of the base silicon wafer and the oil guide tube, when there is working static pressure, there is a stress to compress the adhesive. This stress basically does not affect the measurement of silicon wafer. Therefore, the design structure of lodinson silicon sensor is less affected by the working static pressure, so it is suitable for high hydrostatic pressure measurement.

Analysis of static pressure influence data and results of 5 metal capacitance sensor
5.1 measurement method

In Figure 7, T1 is the reference differential pressure transmitter, T2 is the measured differential pressure transmitter, S1 is the negative cavity adjustable gas storage cylinder, and S2 is the positive chamber gas storage cylinder.
Working principle: high pressure gas source (7MPa) adds high-pressure nitrogen to T1 and T2 transmitters at the same time, then close the balance valve of three valve group, and adjust cylinder piston of S1 by micro motion to obtain the range output of transmitter. By comparing the data of the measured transmitter and the reference transmitter, the influence error of the range static pressure of the measured transmitter under this high pressure static pressure (7MPa) can be calculated.
5.2 measurement data
In this test, the reference differential pressure transmitter adopts silicon differential pressure transmitter developed by Rodinson (after static pressure compensation), and the measured differential pressure transmitter adopts metal capacitance differential pressure transmitter DP type capacitive differential pressure transmitter. The full range is 40kpa and the working static pressure is 7MPa. Before the test, all transmitters have been compensated linearly, and the basic error meets the requirement of ± 0.075%.
It can be seen from table 1 that:
a. Compared with the zero output under OmpA static pressure, part of the data is smaller and some data is larger.
b. When the static pressure difference is 7 MPa, the larger the differential pressure is, the smaller the differential pressure will be. The average static pressure deviation of full scale is - 0.020v, that is - 0.5%.
Table 1: record of voltage output value of Lotensin silicon differential pressure transmitter and metal capacitance differential pressure transmitter under the same range (40kpa) 7MPa working static pressure

      

6. Conclusion
Based on the analysis of the above test data and the simultaneous interpreting of different sensors' influence on hydrostatic pressure, it is known that the metal capacitance differential pressure transmitter has a larger offset in its output under hydrostatic pressure due to its own structural reasons. From the point of view of product design, in order to reduce or eliminate this kind of deviation, the following two schemes can be implemented:
a) The sensing element is designed with a pressure conducting medium package structure. As mentioned above, the silicon sensor can effectively reduce or eliminate the influence of static pressure;
b) The principle of software static pressure compensation is adopted
Firstly, an additional static pressure sensor is set inside the sensor to test the working static pressure;
Then, the static pressure error of each range of differential pressure sensor is tested repeatedly with special equipment. After accumulating certain data, the mathematical model of static pressure deviation is established by software method;
Finally, the mathematical model of static pressure deviation is downloaded to each differential pressure transmitter.
In this way, when the differential pressure transmitter after static pressure compensation is applied in the field, it will automatically correct the range output deviation according to different working static pressure, so as to achieve the purpose of reducing the influence of static pressure.
The rp1000 series high-precision differential pressure / pressure transmitter developed by Rodinson is based on the above two points, and has been well received in the market, which solves the problem of static pressure influence for customers. It is believed that the influence of static pressure on the accuracy of differential pressure sensor can be reduced to a lower level in the near future.