How To Calibrate The Pressure Transmitter

- Dec 24, 2018-

How To Calibrate The Pressure Transmitter

Performing the calibration

Once you have established the calibration interval and MPE, you are ready to perform the actual calibration procedure on your pressure transmitter. The best-practice recommendation is:

  1. Mount the transmitter in a stable fixture free from vibration or movement.

  2. Exercise the sensor or membrane before performing the calibration. This means applying pressure and raising the level to approximately 90 percent of the maximum range. For a 150 psi cell that means pressurizing it to 130–135 psig. Hold this pressure for 30 seconds, and then vent. Your overall results will be much better than if you calibrate “cold.”

  3. Perform a position zero adjustment (zero the transmitter). This is important because the orientation of the fixture used for calibration may be different than the way the transmitter is mounted in the process. Failing to correct for this by skipping this step can result in nonconformance.

  4. Begin the calibration procedure. Typically this means three points up (0 percent/50 percent/100 percent) and then three points down. The 4–20 mA output should be 4 mA, 12 mA, and 20 mA at the three points (or the correct digital values for a smart transmitter). Each test point should be held and allowed to stabilize before proceeding to the next. Normally that should take no more than 30 seconds. You can use more points if you require a higher confidence in the performance of the instrument.

  5. Compare the results of your pressure transmitter to your reference device.

  6. Document the results for your records.

The calibration should be performed in as stable an environment as possible, because temperature and humidity can influence the pressure transmitter being tested as well as the pressure reference. If the results of your calibration are within the MPE, do not attempt to improve the performance of the transmitter.

One mistake many end users make is to regularly perform a sensor trim adjustment of their pressure transmitter—even on new units. A sensor trim corrects the digital reading from the sensor after the analog/digital conversion. Performing a sensor trim on a new transmitter is essentially a single-point calibration under the current plant environment conditions, as opposed to sticking with the original factory calibration.

Factory calibrations of pressure transmitters are performed in a tightly controlled environment and incorporate as many as 100 test points. Performing a sensor trim on a new pressure transmitter under field conditions will cause the unit to operate at less than optimal capacity. A sensor trim should only be performed by a qualified technician under the manufacturer’s guidance.

Who should perform the calibrations?

Even with the sophisticated calibration and reference equipment currently available, there is no substitute for a properly trained technician when it comes to calibrating pressure transmitters. Not only does the technician need to be trained on the mechanics of the calibration process, he or she also must be equally qualified in completing and maintaining the documentation. Repeatability is the key. In the world of calibration, if it is not properly documented, it did not happen.

How To Calibrate The Pressure Transmitter? Do You Know These Methods?

Pressure transmitters have always played a key role in controlling industrial processes and pressure changes. In fact, calibration of pressure transmitters requires the use of a standard pressure source input transducer.

Since the range (LRV, URV) is not calibrated without the use of a standard, ignoring the input section (pressure input to the transmitter) for output regulation (transmitter conversion circuit) is not the calibrate calibration. Furthermore, the relationship between the pressure and differential pressure detecting components, the A/D conversion circuit, and the current output is not equal. The purpose of the calibration is to find the relationship between the three. One point to emphasize: Only the input and output (input transmitter pressure, A / D conversion circuit, loop current output circuit) can be called a true calibration.

How to calibrate the pressure transmitter

Are You Ready?

The pressure source is connected to the self-made joint through the rubber tube, the balance valve is closed, and the air circuit sealing condition is checked, and then the ammeter (voltmeter) and the hand-held device are connected to the output circuit of the transmitter, and the calibration is started after the power is preheated. We know that no matter what type of differential pressure transmitter, the positive and negative pressure chambers have exhaust, drain valve or cock; this is convenient for us to calibrate the differential pressure transmitter on site, that is to say, it is not necessary to remove the guide. The differential pressure transmitter can be calibrated with a pressure tube. When calibrating the differential pressure transmitter, first close the positive and negative valves of the three-valve group, open the balance valve, and then loosen the exhaust, drain valve or cock to empty, and then replace the positive pressure chamber with a self-made joint. The exhaust, drain valve or cock; while the negative pressure chamber remains loose, allowing it to pass through the atmosphere.

Calibration Of Intelligent Differential Pressure Transmitter

It is not acceptable to calibrate the smart transmitter using the conventional method described above, as this is determined by the structural principle of the HART transmitter. Because the smart transmitter is between the input pressure source and the generated 4-20 mA current signal, in addition to the mechanical and electrical circuits, there is also the operation of the microprocessor chip on the input data. Therefore, the adjustment is different from the conventional method.

In fact, the manufacturer also has a description of the calibration of the smart transmitter. For example, ABB's transmitter has the following functions: “set range”, “heavy quantitation” and “fine adjustment”. The “set range” operation is mainly through the digital setting of LRV.URV to complete the configuration work, while the “re-quantity” operation requires the transmitter to be connected to the standard pressure source, guided by a series of instructions, The transmitter directly senses the actual pressure and sets the value. The initial and final settings of the range are directly dependent on the actual pressure input value. However, it should be noted that although the analog output of the transmitter is calibrately related to the input value used, the digital reading of the process value will display a slightly different value, which can be calibrated by fine-tuning the item. Since the parts must be adjusted separately and must be adjusted, the actual steps can be followed by the following steps:

1. A 4-20 mA trim is done first to calibrate the D/A converter inside the transmitter. Since it does not involve sensing components, no external pressure source is required.

2. Perform a full fine-tuning to match the 4-20 mA, digital reading to the actual applied pressure signal, thus requiring a source of pressure.

3. Finally, the heavy-quantity process is performed, and the analog output 4-20 mA is matched with the applied pressure signal source by the adjustment, and the action is exactly the same as the zero-setting (Z) and the range-adjusting (R) switch on the transmitter casing.