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The differential pressure scale of a differential pressure transmitter is usually calibrated with the negative pressure chamber exposed to atmospheric pressure. When it is installed on-site and zeroed using the actual static pressure under operating conditions, it is often found that the zero output differs from that obtained during calibration with the negative pressure chamber exposed to atmospheric pressure. The zero-point output obtained by applying the same static pressure to these positive and negative pressure chambers is referred to as the static pressure error, which is the zero point when calibration is performed using atmospheric pressure. If the static pressure error of the differential pressure transmitter is not corrected, it will introduce errors in flow measurement; this effect is particularly significant when the relative flow rate is low. For example, a DDZ-Ⅲ type differential pressure transmitter, together with a throttling device, forms a differential pressure flow meter. Under normal pressure conditions, its static pressure error is 0.5% FS; if it is put into operation without adjusting for this static pressure error, the flow reading of the instrument can reach 7.1% FS when the actual flow rate is zero. Although the low-signal rejection function masks this issue, its impact still exists objectively, and a differential pressure deviation of ±0.5% FS is always present across the entire range of flow rates. The static pressure error of a differential pressure transmitter is caused by the unequal effective areas of the diaphragm chambers for the positive and negative pressures. In the DDZ-Ⅲ differential pressure transmitter, the static pressure error can be as high as ±0.5% FS. In intelligent differential pressure transmitters, a static pressure sensor is installed, and the deviation of the zero-point output when the static pressure changes within a specified range is measured experimentally; thereafter, the static pressure error is corrected using a microcontroller built into the device. For differential pressure transmitters with online correction of static pressure error, the residual static pressure error can generally be reduced to below ±0.1%, thereby significantly improving their performance. Some differential pressure transmitters come equipped with a square-root function and a low-level signal rejection feature; when checking the static pressure error, this low-level signal rejection feature should be temporarily disabled in order to observe the true zero point. Before leaving the manufacturing plant, the zero point of the differential pressure transmitter is checked as an important parameter; however, the residual static pressure error still needs to be verified once again at the installation site by applying actual static pressure. The method involves applying the same static pressure to both the positive and negative pressure chambers. In the system diagram shown in Figure 3.14 of Chapter 3 of this book, one of the high-pressure and low-pressure valves in the three-valve assembly is opened while the other is closed; the balance valve is then opened. If it is suspected that the chambers have not yet been filled with the medium under test, any air (or liquid) accumulated there can be removed by using the exhaust (or drainage) valves on those chambers, after which the output of the transmitter can be checked. The output of the differential pressure transmitter can also be read on flow display instruments or DCS systems; to obtain the true zero-output value, it is also necessary to temporarily disable the small-signal rejection function. Combine the differential pressure transmitter with the flow display instrument to check the zero output; if there is a deviation at zero, the possible reasons are as follows: 1. The incorrect installation location of the differential pressure transmitter causes a shift in the zero point. 2. Zero offset of the flow display meter. 3. Differential pressure transmitter static pressure error. After the differential pressure flow meter has been calibrated by addressing the static pressure error and zero-point issues as mentioned above, the balance valve can be closed, the high and low pressure valves can be opened fully, and the meter can be put into operation. The algebraic sum of such deviations is not large, and it is ultimately eliminated through the zero-point calibration of the differential pressure transmitter. Therefore, this inspection and calibration step before putting the instrument into operation is an important part of the process of commissioning it.