Basics of Calibrating Pressure Transmitters

Pressure transmitters are used to measure pressure and gases—a sensitive role in process manufacturing. Periodic maintenance and calibration is required to ensure maximum performance from your pressure transmitters.

thumb-cerabar-t-pmc131-2Knowing how often to perform maintenance on your pressure transmitters can save both time and money. If you overcompensate and perform maintenance too often, you’re contributing to needless downtime and wasted employee hours. Too little, however, and you risk imprecise measurements, which could then lead to product loss and negatively impact the bottom line.

Frequency of Calibration

As a general rule, the frequency of calibration depends on the conditions under which the pressure transmitters operate.

Direct-mounted transmitters installed outside with stable process conditions must be calibrated every one-to-four years. For transmitters installed inside a controlled environment with stable process conditions, calibration every four-to-six years should be enough.

For pressure transmitters with a remote diaphragm seal, the calibration interval should be halved since this condition subjects the transmitter to extra mechanical stress.

There might be a need for a custom calibration interval if the following conditions apply:

  • Government-mandated safety and/or environmental regulations
  • Process conditions such as the presence of a homogenous process fluid with a stable pressure or temperature; significant fluctuations in process conditions; risk of buildup, corrosion, or abrasion; and heavy vibration
  • Ambient conditions such as exposure to varying weather conditions, high humidity, or harsh environments

Calibration Accuracy Requirements

Pressure transmitters must meet a predetermined maximum permissible error (MPE) to pass the calibration test. In many cases, adopting the manufacturer’s recommended MPE will not suffice, and might lead to the pressure transmitters not meeting the standards required for your specific operational situation. This is because this MPE is based on tightly controlled conditions that are not often present in the real world.

A more viable MPE takes into account the fact that all instruments will experience slight accuracy degradation as they age, and that normal wear and tear will affect their mechanical components. To ensure accuracy of test results, make sure that your test equipment meets the standards set by the National Institute of Standards and Technology. The reference equipment should be at least three times more accurate than the pressure transmitters undergoing the calibration.

Performing the Calibration

Pressure transmitters may be calibrated once the calibration interval and MPE are set. The general process is as follows:

  1. Mount the transmitter in a stable fixture. Make sure that it is not moving or vibrating.
  2. Exercise the sensor/membrane by applying pressure and raising the level to approximately 90% of its maximum range.
  3. Perform a position zero adjustment (zero the transmitter) to correct for any differences between the fixture where the transmitter is mounted during testing and the location of the transmitter in the actual process.
  4. Begin the calibration procedure by starting from 0%, then moving on to 50%, and then 100%. From 100%, go back down to 0%. Make sure to allow each test point to stabilize before proceeding to the next one. You may add more test points if you’re not confident in the result.
  5. Compare the results of the pressure transmitter to the reference device.
  6. Document the results.

Who Should Perform the Calibration?

Trained technicians should perform the calibration. In some cases, accredited labs can simplify the calibration audit process, as they will have access to the latest technology and training in the most recently accepted testing methods. Since these labs apply a repeatable process and methodology during calibration, most auditors have a higher level of confidence in their results. Moreover, accredited labs are subject to annual audits to ensure that they meet current standards for their registered scope of work.

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When calibrating pressure transmitters, there is a need to follow correct calibration procedures, establish correct and realistic MPEs, have the calibration performed by trained personnel, and properly document calibration results.

Established in 1970 and now one of the country’s largest instrument manufacturers,  Endress+Hauser maintains an ISO 17025-certified laboratory and field calibration services. To learn more about calibrating your pressure transmitters, download our PDF.

 

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Topics: pressure transmitter, improving calibration


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