How should a pressure transmitter be calibrated before use?

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Hi, I’m the Industrial Control Assistant. The little piece of knowledge I learned today is about the calibration of pressure transmitters, which I’d like to share with everyone. Do you know what a pressure transmitter is? It is a commonly used measuring instrument that converts the physical parameters such as pressure of gases and liquids detected by the sensing element into standard signals for output. It boasts advantages such as accurate measurement, stable performance, high sensitivity, and easy operation. To ensure the proper operation of a pressure transmitter, users need to calibrate the device. So, how is a pressure transmitter calibrated? First, the true calibration of a pressure transmitter requires using a standard pressure source to feed into the transmitter. Adjusting the range (LRV, URV) without using a standard device is not calibration; adjusting the output (the transducer’s conversion circuit) by ignoring the input side (the pressure at the input transducer) is not proper calibration. Furthermore, the relationship between the pressure and differential pressure sensing elements, the A/D conversion circuit, and the current output is not proportional; the purpose of calibration is to determine the relationship between these three elements. The pressure source is connected to the homemade connector via a rubber hose; the balance valve is closed, and the airtightness of the gas circuit is checked. Then, the ammeter (voltage meter) and hand controller are connected to the transmitter’s output circuit, and calibration begins after powering on and preheating. 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 the 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 pressure, and use a custom-made connector to replace the vent and drain valves or plugs in the high-pressure chamber ； The negative pressure chamber, on the other hand, remains loose to allow it to be in contact with the atmosphere. First, set the damping to zero; adjust the zero point first, and then apply full-scale pressure to set the range so that the output is 20mA. On-site calibration is meant to be quick, and here is a method for quickly 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. It is not possible to calibrate the intelligent transmitter 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 processes the input data. Therefore, the tuning differs from conventional methods. The calibration of an actual pressure transmitter can be carried out following these steps: 1. First, perform a 4-20mA fine-tuning to adjust the D/A converter inside the transmitter; since this process does not involve any sensing components, no external pressure signal source is required. 2. Perform another full-scale fine adjustment to ensure that the 4-20mA signal and the digital readings correspond to the actual pressure signal applied; therefore, a pressure signal source is required. That’s all for today’s content. Learn together with the little assistant every day*. If you have any questions, the little assistant will do its best to find answers for you. New updates will be available regularly, so feel free to stay tuned