Thread Content
The true calibration of a pressure transmitter requires using a standard pressure source to feed it. Adjusting the range (LRV, URV) without using a standard is not calibration; adjusting the output (the transducer’s conversion circuit) while ignoring the input (the pressure at the input transducer) is not proper calibration. Furthermore, the relationship between the pressure and differential pressure sensing components, the A/D conversion circuit, and the current output is not symmetrical; the purpose of calibration is to determine the relationship of changes among these three elements. It should be emphasized that only by tuning both the inputs and outputs (the pressure at the input transmitter, the A/D conversion circuit, and the loop current output circuit) together can it be considered a true calibration. I. Preparation work: The pressure source is connected to the homemade connector via rubber hoses; the balance valve is closed, and the airtightness of the gas circuit is checked. Then, the ammeter (voltage meter) and the manual controller are connected to the transmitter’s output circuit. After powering on and allowing it to warm up, calibration begins. We know that for differential pressure transmitters of any model, both the positive and negative pressure chambers are equipped with vent and drain valves or plugs ； This facilitates on-site calibration of differential pressure transmitters, meaning that it is possible to calibrate them without having to remove the pressure guiding tubes. When calibrating a differential pressure transmitter, first close the positive and negative valves of the three-valve assembly, open the balance valve, then loosen the vent and drain valves or plugs to release air and liquid, and use a custom-made connector to replace the vent and drain valves or plugs in the high-pressure chamber ； The negative pressure chamber remains loose to allow it to be in contact with the atmosphere. II. Calibration of conventional differential pressure transmitters First, set the damping to zero; adjust the zero point first, and then apply the full-scale pressure to set the range so that the output is 20mA. On-site calibration is meant to be carried out quickly, and here are methods for rapidly adjusting the zero point and the range. Zero adjustment has almost no effect on full scale, but adjusting the full scale does affect the zero point. In the absence of drift, this effect is approximately 1/5 of the range adjustment amount; that is, if the range is increased by 1 mA, the zero point will shift upward by about 0.2 mA, and vice versa. III. Calibration of Intelligent Differential Pressure Transmitters It is not possible to calibrate intelligent transmitters using the conventional methods mentioned above, as this is determined by the structural principles of HART transmitters. Because in an intelligent transmitter, between the input pressure source and the generated 4-20mA current signal, in addition to mechanical and electrical components, there is also a microprocessor chip that performs calculations on the input data. Therefore, the tuning differs from conventional methods. In fact, manufacturers also provide instructions for calibrating smart transmitters; for example, ABB’s transmitters offer options such as “setting the range,” “resetting the range,” and “fine tuning.” The “set range” operation is primarily carried out by configuring the values via LRV and URV, whereas the “reset range” operation requires connecting the transmitter to a standard pressure source; through a series of instructions, the transmitter directly senses the actual pressure and sets the corresponding value. The initial and final settings of the range depend directly on the actual pressure input value. However, it should be noted that although the analog output of the transmitter corresponds correctly to the input value used, the digital reading of the process value will show a slight difference, which can be calibrated using the fine-tuning option. Since each component needs to be adjusted individually as well as in combination with the others, the actual adjustment can be carried out following these steps: 1. First, perform a 4-20mA fine-tuning to calibrate the D/A converter inside the transmitter; since this process does not involve the sensing elements, no external pressure signal source is required. 2. Perform another full-scale fine adjustment to ensure that the 4-20mA signal and the digital reading correspond to the actual pressure signal applied; therefore, a pressure signal source is required. 3. Finally, the full measurement range is set; by making adjustments, the simulated output of 4-20mA is made to match the external pressure signal source. This function is exactly the same as that of the zero adjustment (Z) and range adjustment (R) switches on the transmitter housing. For more information, please visit the company’s official website at http://www.yb1518.com/. Please retain this link when reproducing the content! http://www.yb1518.com/UploadFiles/2010728141131670.jpg