How to Calibrate a Pressure Transmitter

  • Are we calibrating our transmitters too often, resulting in excessive downtime and unnecessary maintenance expenses?
  • Are we calibrating our transmitters too infrequently, resulting in quality issues and possible loss of product?
  • Are we calibrating our transmitters correctly?

How often?

  • Are there any local, national, safety, or environmental regulations that must be observed?
  • What is your reason for requiring calibration: quality, safety, or standard maintenance?

Process conditions

  • Is there a homogeneous process fluid with a stable pressure/temperature?
  • Will the process conditions fluctuate significantly?
  • Is there risk of buildup, corrosion, or abrasion to the pressure transmitter?
  • Will heavy vibration be present?

Ambient conditions

  • Will the pressure transmitter be installed in a well-controlled environment with low humidity, normal or stable temperatures, and few contaminants, such as dust or dirt?
  • Is an outdoor transmitter exposed to widely varying weather conditions or high humidity?
  • Direct-mounted pressure transmitters installed inside in a controlled environment on a process with stable conditions should be calibrated every four to six years.
  • Direct-mounted pressure transmitters installed outside on a process with stable conditions should be calibrated every one to four years, depending upon ambient conditions.

How accurate?

Performing the calibration

  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.

Who should perform the calibrations?

  • Accredited labs can simplify the calibration audit process.
  • The process and methodology used by an accredited lab is extremely repeatable, thus there is a high level of confidence in the results from an auditor’s perspective.
  • Annual audits of the accredited lab ensure it is consistently performing at a high level for its registered scope of work.
  • establishing correct and realistic MPEs
  • following correct calibration procedures
  • properly training the person performing the calibration
  • properly documenting calibration results

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The International Society of Automation (isa.org) is a non-profit professional association founded in 1945 to create a better world through automation.

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The International Society of Automation (isa.org) is a non-profit professional association founded in 1945 to create a better world through automation.

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