Wet and Dry Calibration and the Isolation Valve

As a best practice, level transmitters must be calibrated on a regular basis to ensure accurate measurements and optimal functionality. Proper calibration helps the instrument produce accurate measurements of liquids, slurries and solids.

Inaccurate measurements can be costly, and in some instances, dangerous. It’s important to have a comprehensive understanding of both wet and dry calibration, and the role that an expertly designed isolation valve can play in each process.

Dry Calibration Offers Ease-of-Use

Dry calibration of differential pressure transmitters is performed in a closed, pressurized tank. This is considered by some to be the easier method of calibration. It involves making the inlet pressures – at the high and low pressure points – the same as the atmospheric pressure by opening the two separate valves below those high and low pressure points. This then completes the minimum span calibration.

The operator must close the two separate valves and open the isolation valve. This ensures the high pressure inlet is subject to maximum span, thereby calibrating the transmitter to 20mA. Once the operator opens the dry reference leg valve, the low pressure point will be subject to minimum span. After the transmitter is re-ranged, the dry calibration method is completed and the operator has an accurate understanding of the equipment’s performance.

Wet Calibration is Designed for Liquids Much Colder or Warmer than Ambient Temperature

While dry calibration is ideal for applications in which the liquid in the tank is room temperature, when the liquid is much hotter or much colder than the ambient environment, wet calibration is required. That’s because the liquid condensate will fill the dry leg of the equipment, making any attempts at dry calibration impossible.

For wet calibration, the low pressure leg of the differential pressure transmitter is filled with a buffer solution, which is usually diesel, glycol or the same type of liquid as is in the tank. The operator then follows the same process as with dry calibration and re-ranges the differential pressure transmitter.

Considerations for Both Wet and Dry Calibration :

  • Liquid density variations impact results. During wet and dry calibration work with an isolation valve, operators may discover data errors if the density of the process fluid changes. There are a number of reasons for fluid density variations, including temperature and process changes. This means it’s important to maintain consistent process fluid density to achieve verifiable calibration results.
  • Piping must be designed for easy draining and flushing. Companies must have access to optimal piping and valves on both the tank and the leg to ensure draining and flushing can be completed easily.

By using an isolation valve when wet or dry calibrating a level transmitter, the instrument technicians can reduce their long-term maintenance expenses and ensure the accuracy of their equipment. To learn more about wet and dry calibration applications, contact the trusted team members at Indu-Tech.

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