Application of Intelligent Digital Pressure Calibrators in the Calibration of Pressure Controllers

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Authors: Cui Yaoyao, Sun Bo (Tianjin Institute of Metrological Supervision, Inspection and Research; Tianjin Bicycle Research Institute). When reproducing this article, please acknowledge the copyright of the authors. Abstract: In the calibration of pressure controllers, it is difficult to achieve accurate data recording using traditional calibration methods. By using an intelligent digital pressure calibrator, it is possible to automatically record the pressure values at the moment when the switch is turned on or off; therefore, it can be utilized in the calibration of pressure controllers. In terms of application performance, the data obtained using an intelligent calibrator are generally low, with a switching difference ranging from 0.06 to 0.08 MPa; the fluctuations are relatively small. It takes only 2–3 minutes to complete the calibration of the controller. The results of the measurement uncertainty assessment are p=0.695 MPa, U=0.002 MPa, and k=2, which meets the accuracy requirements for calibrating pressure controllers. In aspects such as medium pressure signal detection, display, and control, pressure controllers play a vital role in ensuring the proper operation of the system. Using an intelligent digital pressure calibrator for the calibration of pressure controllers ensures that the error in the pressure readings of the equipment is corrected, thereby enabling the equipment to function effectively. 1 Calibration requirements for pressure controllers: A pressure controller is a device designed specifically for controlling pressure; it initiates control actions or issues alarms once the input pressure reaches a set value, and is thus referred to as a pressure switch. At the current stage, there are two types of pressure controllers: one mechanical and the other electronic. Among them, mechanical pressure controllers use Bourdon tubes, diaphragms, and similar components as pressure-sensing elements; when the internal pressure of the system exceeds a certain value, the free end of these elements moves, thereby exerting a pushing force on the control switch through the connecting ducts. As required, the pressure controller must be calibrated, during which its appearance, insulation strength, repeatability error, pressure control range, setpoint deviation, and other aspects need to be verified. In this process, to determine the pressure control range, the set point must be adjusted to its maximum position; then, the pressure is increased from zero until the contacts activate, and the pressure value on the standard device is read – this value represents the upper switching value. Setting the setpoint to the minimum and then slowly reducing it until the contact acts will yield the lower switching value. Based on the obtained values, it can be determined whether the controller is within the pressure control range. To address the setpoint deviation, the setpoint should be placed near the lower limit of the scale; then pressure is increased until the controller indicates that the pressure is close to the setpoint. After that, the pressure is increased gradually until it approaches the detection point that triggers the action, and the value read at this point constitutes the upper switching value. Following the reverse process, the next switch value can be read. By repeating the cycle three times, an average value can be obtained; then, the setpoint is placed near the upper limit of the controller’s range, and the aforementioned verification steps are repeated. In terms of calibration methods, for pressure controllers, personnel need to use traditional calibration techniques, that is, by employing a gas-piston pressure gauge as a standard source to ensure that the pressure value meets the quality standards. Due to its large size and inconvenience in carrying, it is difficult to use this type of pressure gauge for field testing. During the actual calibration process, personnel not only need to operate the pressure gauge but also must constantly monitor changes in the signals, and quickly record any changes in pressure values; hence, this task presents certain difficulties. 2 Application of Intelligent Digital Pressure Calibrators in the Calibration of Pressure Controllers 2.1 Application Method In the calibration of pressure controllers, when using intelligent digital pressure calibrators in practice, they must also be used as standards. A pressure calibrator is used to calibrate the J120 pressure controller. Using this calibrator, the maximum allowable indication error is ±0.05% of the reading; air must be used as the medium for calibrating pressure controllers with a range of 0.7 to 3.9 MPa and an accuracy of 0.5%. Before testing, it is necessary to connect the tester and the pressure controller together. Select an appropriate pressure module based on the range, and connect the module’s pressure port to the generator to prevent leakage issues. The pressure generator selected has a pressure range of –0.098 MPa to 10 MPa; it is a CST1002B type desktop pneumatic pressure pump that uses air as the medium for transmitting pressure, and can meet the requirements for pressurization and depressurization during the calibration of pressure controllers. The range of the tester is between 0 and 4 MPa; the SW and COM electrical connectors can be selected. A connection cable is used to link the N0 terminal of the controller to the COM terminal, after which the dual-trigger mode can be chosen. The tester will output a standard pressure value; when testing the pressure during the forward stroke, it is necessary to start from zero and use a pressure pump to increase the pressure until the contacts activate. 2.2 Application effect: Based on the testing results, during the gradual increase in pressure, the terminals NO and COM will be disconnected. And when the pressure reaches 1.435 MPa, NO and CMO will become connected. When the pressure is reduced to 1.428 MPa, NO and CMO will disconnect, resulting in a switching difference of 0.007 MPa. To determine the testing effectiveness, a multimeter is used to monitor the on/off state of the switch, while conventional testing is carried out through manual counting. As shown in Table 1, it presents a comparison of the results from the two testing methods. As can be seen from the data in the table, the values obtained using the intelligent calibrator for the P丑_0罅_history measurements are generally low; the switching difference ranges from 0.06 to 0.08 MPa, with relatively little fluctuation. With the manual calibration method, since pressure adjustment and data recording need to be carried out simultaneously during the differential test, the pressure keeps increasing or decreasing throughout the calibration process, preventing personnel from recording the data at the exact moment the signal is generated. Affected by this factor, the data obtained through manual testing are generally on the higher side and exhibit significant fluctuations, with the switching difference ranging between 0.15 and 0.18 MPa. Furthermore, in terms of time, manual inspection takes 8 to 9 minutes per inspection, whereas an intelligent calibrator only requires 2 to 3 minutes. Therefore, overall, using an intelligent digital pressure calibrator for the calibration of pressure controllers not only yields more accurate measurement results but also reduces the calibration time. http://www.sanchangyb.com/d/file/zs/js/2018-11-23/bc98ea77fa0e60fba0263a54eab270dc.jpg Table 1 Comparison of the results obtained from the two calibration methods for pressure controllers 2.3 Measurement uncertainty In practice, when using a calibrator to test pressure controllers, it is necessary not only to analyze the calibration process for pressure reading errors but also to assess the measurement uncertainty of these readings in order to determine their accuracy. Specifically, during the measurement process, the mathematical model Ps = PZ + Px is to be used, where Ps, PZ, and Px represent the reading of the controller, the reading of the calibrator, and the correction value for the controller, all in MPa. The standard uncertainty u1 introduced for the indication repeatability also needs to be assessed. During actual evaluation, the Class A method must be used to perform repeated measurements on the pressure controller within the scope of the regulations. Specifically, by performing 3 repeated measurements at the set point of 0.7 MPa, three values of 0.694 MPa, 0.696 MPa, and 0.695 MPa can be obtained. According to the standards for a single experiment, it can be determined that s = (Pmax – Pmin) / Cn = 0.0011 MPa. Here, Pmax and Pmin represent the highest and lowest readings of the calibrator across the three measurements, respectively, while Cn is the coefficient of range, which is 1.69. It can be seen that u1 = s/ = 0000 6 MPa. By using an intelligent digital pressure calibrator, its accuracy will also contribute to the uncertainty u2. During the evaluation process, since the maximum allowable measurement error of the instrument is ±1MPa×0.05% = ±0.0005MPa, it is determined that a = 0.0005MPa. Moreover, as the values are uniformly distributed, it follows that u2 = a/ = 0.00029MPa. 3 Conclusions Through research, it can be found that with the development of science and technology, intelligent digital pressure calibrators have been developed. These calibrators are used for testing pressure controllers, and they not only help improve the accuracy of the calibration results but also meet the needs for automated on-site calibration. When actually using a calibration instrument to test pressure controllers, it is also necessary to follow relevant regulations in order to improve the efficiency and quality of the calibration process and ensure its smooth progress.