Dry calibration and wet calibration are two methods used to calibrate instruments, particularly in the context of pressure measurement. Here's an explanation of each method and how they are performed:

1. Dry Calibration:

   • Dry calibration involves calibrating the instrument using a reference standard without any direct contact with the process fluid.
   • In dry calibration, the instrument is calibrated using a controlled environment, typically a calibration bench or chamber, where the pressure is applied using a pressure source such as a pneumatic pump or pressure generator.
   • Dry calibration is suitable for instruments that are sensitive to contamination or damage from contact with process fluids, such as sensitive electronic pressure transmitters.
   • Dry calibration is often performed in a laboratory or workshop environment where precise control over pressure conditions can be maintained.

   Steps for Dry Calibration:

   1. Set up the calibration bench or chamber with the necessary pressure source and reference standards.
   2. Connect the instrument to be calibrated to the pressure source using appropriate fittings, tubing, and adapters.
   3. Apply a known pressure to the instrument using the pressure source and record the instrument's response.
   4. Compare the measured pressure readings from the instrument with the reference standards and adjust the instrument's settings if necessary.
   5. Repeat the process at multiple pressure points across the instrument's range to ensure accurate calibration.
   6. Document the calibration results, including the applied pressures, measured readings, adjustments made, and any observed errors or deviations.

2. Wet Calibration:

   • Wet calibration involves calibrating the instrument by exposing it to the actual process fluid under controlled conditions.
   • In wet calibration, the instrument is installed in the process line or vessel where it will be used, and the process fluid is used to apply pressure to the instrument.
   • Wet calibration provides a more realistic calibration environment since the instrument is calibrated under actual process conditions.
   • Wet calibration is suitable for instruments that are designed to withstand contact with process fluids and for applications where accuracy under real process conditions is critical.

   Steps for Wet Calibration:

   1. Install the instrument to be calibrated in the process line or vessel according to the manufacturer's instructions.
   2. Isolate the instrument from the process and ensure that it is safely accessible for calibration.
   3. Apply pressure to the instrument by introducing the process fluid into the calibration setup and measuring the instrument's response.
   4. Compare the measured pressure readings from the instrument with the reference standards and adjust the instrument's settings if necessary.
   5. Repeat the process at multiple pressure points across the instrument's range to ensure accurate calibration.
   6. Document the calibration results, including the applied pressures, measured readings, adjustments made, and any observed errors or deviations.

In summary, dry calibration involves calibrating the instrument using a reference standard in a controlled environment, while wet calibration involves calibrating the instrument under actual process conditions using the process fluid. The choice between dry and wet calibration depends on factors such as the instrument's design, sensitivity to process fluids, and the required accuracy under real process conditions.