Differential pressure transmitter calibration method
Calibration of a conventional differential pressure transmitter:
First adjust the damping to the zero state, first adjust the zero point, then increase the pressure to full range, so that the output is 20mA. The adjustment in the field is fast. Here, introduce the zero and span quick adjustment method. Zero adjustment has almost no effect on fullness, but when fullness has an effect on the zero point, its effect is approximately 1/5 of the span adjustment without migration, ie 1mA of span is adjusted upwards and the zero point will move approximately 0.2mA upwards. ,vice versa. For example: input full-scale pressure is 100Kpa, the reading is 19.900mA, adjust the range potentiometer to make the output 19.900 + (20.000-19.900) × 1.25 = 20.025mA. When the range increases 0.125mA, the zero increases 1/5 × 0.125 =0.025. The zero point potentiometer makes the output 20.000mA. After the zero point and full scale are adjusted properly, check the middle scale to see if it is out of tolerance and fine tune if necessary. Then perform the adjustment of the migration, linearity, and damping.
The smart differential pressure transmitter calibration:
It is not possible to calibrate the smart transmitter using the above-mentioned conventional method because it is determined by the structural principle of the HART transmitter. Because the smart transmitter is between the input pressure source and the generated 4-20mA current signal, in addition to the mechanical and electrical circuits, there is also a microprocessor chip to calculate the input data. Therefore, the adjustment is different from the conventional method. Actually, the manufacturer's calibration of smart transmitters is also explained. For example, ABB's transmitters have calibration points such as "set range", "re-quantification range", and "fine adjustment". The "Set Range" operation is mainly done through the digital setting of LRV.URV. The "Re-ranging" operation requires that the transmitter is connected to a standard pressure source and guided by a series of instructions. The transmitter senses the actual pressure directly and sets the value. The initial and final setting of the range depends directly on the actual pressure input value. However, to see that although the analog output of the transmitter is correctly related to the input value used, the digital reading of the process value shows a slightly different value, which can be calibrated by a fine-tuning item. As each part needs to be adjusted separately and must be adjusted together, the following steps can be performed during actual calibration:
1. Do a 4-20mA trim once to correct the internal D/A converter of the transmitter. Since it does not involve sensing components, no external pressure signal source is required.
2. Do a full fine-tuning again so that the 4-20mA, digital readout matches the actual applied pressure signal, so a pressure signal source is needed.
3, finally do heavy ration, through the adjustment of the analog output 4-20mA and the additional pressure signal source, its role and the transmitter shell zero (Z), adjust the range (R) switch is exactly the same role .