Introduction to the principle of differential pressure sensor

Differential pressure sensors use a method of measuring flow by using different pressures before and after an orifice plate (other throttling devices), which is the Venturi principle. Within a certain flow range, the flow rate through the orifice plate has a linear relationship with the pressure difference before and after the orifice plate. Therefore, by detecting the pressure difference and calculating the flow totalizer, the flow rate of the fluid can be obtained. Nowadays, there are also bends and other pipes that need to look at the detection medium to determine the differential pressure device.

Working Principle Basic Principle When the fluid filling the pipe flows through the throttling member in the pipe, as shown in Figure 4.1, the flow rate will form a local contraction at the throttling member, so the flow rate increases and the static pressure decreases, thus creating a pressure difference before and after the throttling member. The greater the fluid flow, the greater the pressure difference generated, so the flow can be measured based on the pressure difference. This measurement method is based on the flow continuity equation (law of conservation of mass) and Bernoulli's equation (law of conservation of energy). The size of the pressure difference is not only related to the flow rate but also to many other factors. For example, when the form of the throttling device or the physical properties (density, viscosity) of the fluid in the pipeline are different, the pressure difference generated under the same flow rate is also different. http://www.chinaflow.com.cn/basic/images/4-1.gif Figure 4.1 Flow velocity and pressure distribution near the orifice plate 2.2 Flow equation 　 http://www.chinaflow.com.cn/basic/images/chayags-1.GIF In the formula, qm--mass flow rate, kg/s; qv--volume flow rate, m3/s; C--outflow coefficient ； 　　 ε--expandability coefficient ； 　　 β--diameter ratio, β=d/D; d--aperture of the throttling member under working conditions, m ； 　　 D--inner diameter of upstream pipeline under working conditions, m ； 　　 △P--differential pressure, Pa ； 　　 ρl--upstream fluid density, kg/m3. 　　It can be seen from the above formula that the flow rate is a function of six parameters: C, ε, d, ρ, △P, and β (D). These six parameters can be divided into actual measurements and statistics (C, ε). 　　(1) Actual measurement 1) d, D In formula (4.1), d has a square relationship with the flow rate, and its accuracy has a greater impact on the total accuracy of the flow rate. The error value should generally be controlled at around ±0.05%, and the impact of the operating temperature on the thermal expansion of the material should also be taken into account. The standard stipulates that the inner diameter D of the pipeline must be measured actually, and multiple measurements must be made on several sections of the upstream pipe section to find the average value. The error should not be greater than ±0.3%. In addition to the high requirements for numerical measurement accuracy, the serious impact of inner diameter deviation on the upstream channel of the throttle member causing abnormal throttling phenomena should also be considered. Therefore, when the throttling device is not supplied as a complete set, full attention should be paid to this issue during on-site piping. 　　2) ρ ρ is in the same position as △P in the flow equation. That is to say, when pursuing the high accuracy level of the differential pressure transmitter, never forget that the measurement accuracy of ρ should also match it. Otherwise, the increase in ΔP will be offset by the decrease in ρ. 　　3) △P The accurate measurement of differential pressure △P should not be limited to the use of a high-precision differential pressure transmitter. In fact, whether the differential pressure transmitter can accept the true differential pressure value also depends on a series of factors. Among them, the correct manufacturing, installation and use of the pressure tapping hole and pressure pipeline are the key to ensuring the true differential pressure value. Many of these influencing factors are difficult to determine quantitatively or qualitatively. Only by strengthening the standardization of manufacturing and installation can the goal be achieved. 　　(2) Statistical quantity 1) C Statistical quantity C is a quantity that cannot be measured actually (meaning that it is designed, manufactured and installed according to standards and used without calibration). When used on site, the most complicated situation occurs when the actual C value does not match the C value determined by the standard. Their deviation is caused by a series of factors in design, manufacturing, installation and use. It should be clear that all the above-mentioned links must strictly comply with the provisions of the standard, and only then will the actual value be consistent with the value determined by the standard. It is difficult to fully meet this requirement on site. 　　It should be pointed out that some deviations from standard conditions can be estimated quantitatively (can be corrected), while others can only be estimated qualitatively (magnitude and direction of uncertainty). But in reality, sometimes there is more than just one condition deviation, which leads to a very complicated situation, because general information only introduces the error caused by a certain condition deviation. If many conditions deviate simultaneously, there is a lack of relevant information to look up. 　　2) ε The expandability coefficient ε is a correction to the change in outflow coefficient caused by the change in density when the fluid passes through the throttling member. Its error consists of two parts.: One is the error of ε under common flow rates, that is, the error of the standard determined value. ； The second is the error caused by the fluctuation of ε value due to the change of flow rate. Generally, in the case of low static pressure and high differential pressure, the ε value has a non-negligible error. When △P/P≤0.04, the error of ε is negligible.