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A method of calibration without removing the pressure guide tube from the differential pressure transmitter

Author:QINSUN Released in:2023-06 Click:134

Differential pressure transmitters are widely used in factories. In order to ensure their normal operation and accuracy, regular inspection and calibration are necessary. Now introduce a field calibration method without removing the pressure guide tube.

1. Preparatory work:

We know that the differential pressure transmitter is connected to the pressure guide tube in the application. The connector is disassembled and then connected to the pressure source for calibration. This is very troublesome, and the labor and labor intensity is high, and the worry is that the pressure guide tube will be broken or leakage will occur when the joint is disassembled. It is known that whatever the type of differential pressure transmitter, the positive and negative pressure chambers include exhaust valves, drain valves or taps; this allows us to calibrate the transmitter prdifferential pressure on site, i.e. without removing the guide The pressure tube can calibrate the differential pressure transmitter. When calibrating the differential pressure transmitter, first close the positive and negative valves of the three-valve group, open the balancing valve, then unscrew the exhaust, drain valve or tap to empty, then replace the positive pressure chamber with a self-made gasket Exhaust, liquid relief valve or tap; while the vacuum chamber remains unscrewed to make it open to the atmosphere. Connect the pressure source to the joint made by yourself through the rubber tube, close the balancing valve and check the tightness of the air circuit, then connect the ammeter (voltmeter) and the manual device to the circuit transmitter output, and start calibration after power-on and warm-up.

Second, the calibration of the transmitterconventional differential pressure:

First adjust the zero state damping, first adjust the zero point, then fill the full scale pressure and adjust the full range, so that the output is 20mA, on-site adjustment is fast, here is the zero point and span fast adjustment method. When adjusting the zero point it has almost no effect on the full scale, but when adjusting the full scale it has an impact on the zero point. When there is no migration, its impact is about 1/5 of the range adjustment amount, i.e. if the range is adjusted upwards by 1 mA, the zero point will move up approximately 0.2 mA, vice versa. For example: full scale input pressure is 100Kpa, reading is 19900mA, adjust range pot so output is 19900+(20000-19900)×1.25= 20.025mA. Increase the span by 0.125 mA, then increase the zero point de 1/5 × 0.125 = 0.025. Adjust the zero point potentiometer so that the output is 20,000 mA. Once the zero point and full scale adjustments are normal, check the intermediate scales to see if they are out of tolerance? Adjust if necessary. Then adjust migration, linearity and damping.

Thirdly, the calibration of the intelligent nt differential pressure transmitter

It is not possible to calibrate the intelligent transmitter with the conventional method mentioned above, because it is based on the structural principle of the HART transmitter It\'s decided. Because the intelligent transmitter is between the input pressure source and the generated 4-20mA current signal, in addition to the machinery and the circuit, there is also a micro-processing chip for the calculation of the data of entrance. Therefore, the adjustment is different from the conventional method. ;V???_7@) m7v#F In fact, the manufacturer alsoalso instructions on calibrating smart transmitters. For example, ABB transmitters have: \"set range\", \"re-range\", \"fine-tuning\". Among them, the \"set amount\" operation The \"range\" operation mainly completes the configuration work by digitally setting LRV.URV, while the \"re-range\" operation requires the transmitter to be connected to a standard pressure source, guided by a series of setpoints, und detected directly by the transmitter The actual pressure and set the value. The initial and final setting of the range is directly dependent on the actual pressure input value. However, it is necessary to see that although the analog output of the transmitter has a correct relationship to the input value used, the digital readout of the process value indicates the value will be slightly different, which can be calibrated to the help of fine-tuning elements. Since each part must be adjusted individuallyand must be adjusted together, the actual calibration can be done according to the following steps:

1. First do 4-20mA fine-tuning to calibrate the D/A converter inside the \'transmitter. Since it does not involve component sensing, no external pressure signal source is required.

2. Perform another large-scale fine-tuning, match the 4-20mA readings and digital signals with the actual applied pressure signal, so a pressure signal source is needed.

3. Make a heavy range and make the 4-20mA analog output and external pressure signal source by adjusting It is consistent with the functions of the zero adjust (Z) and span adjust (R) switches on the transmitter shell.

Four, some suggestions

Adjustment After the work is completed, the exhaust and liquid discharge valves or taps should be returned to their original position, and the rRibbon of raw material should be wrapped around them, and they should be tightened to ensure no leakage, but before tightening, the positive and negative pressure chambers should be exhausted, drain work. At this point, the process pressure can also be used to check the static pressure error of the transmitter.