dry calibration and wet calibration and how to do this?
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When calibrating a dP transmitter, there are two approaches to this. One is to isolate the transmitter from the process, drain and vent the transmitter to remove all of the process liquid, and then apply pressure to the high side of the transmitter using a dry gas such as air or Nitrogen (recommended for any transmitter containing oil or other volatile substance. This is a “dry” calibration.
A wet calibration involves the use of a “head chamber”, which consists of two cylindrical chambers mounted between two metal plates. The plates are grooved, with O-rings inserted in the grooves. The cylindrical chambers are sealed into the grooves in the plates using threaded rods which hold the assembly together. Holes are drilled in the plates and tapped to allow fittings to be installed in the holes. The chambers are filled approximately half way with water. The fittings on the bottom plate have valves which allow the water to stay in the chamber. There is also an equalizing valve between the fittings on the bottom to allow the water level in the two chambers to be equalized.
After tubing from the valves is conected to the transmitter, the water can be forced from one side of the head chamber, down through the transmitter valve block and out the transmitter vents. This process is then repeated on the other chamber so that all air is removed from the transmitter and it is solid with water. At that point the pressure is removed from the head chamber and the water levels are allowed to equalize.
Once that is done, the equalizing valves for both the transmitter and the head chamber are closed. Air pressure is then applied to the chamber connected to the high side of the transmitter. The water levels, being equal apply equal pressure to both sides of the transmitter, cancelling each other out. The only pressure then, which affects the output of the transmitter is the air pressure applied to the water in the chamber. (Pascal’s Law applies here).
When using a DP or P transmitter to measure tank level, you can calibrate while connected to the tank. (For wet leg you need to fill the LP line). you can set the zero when the level is bellow the point level does not change (depends on installation) ,process zero because bellow this point level is uncertain, and the top range when the tank is full (i.e. overflow point or other point), using a measurement system i.e. transparent pipe with ruler, manual measurement etc or simply set as 100%. This is not as accurate as test bench calibration, but as far as the level control is concerned for process operation it is sometimes sufficcient for the needs. It is fast to perform on site with no calibration equipment,takes into account the fluid density and also calibration chenges if someone hit the transmitter during transport or installation. This is also a wet calibration.
When you set the transmitter range by software (i.e. keypad, device integration tool) some people call it a dry calibration (I do not agree with this because it is really range setting).This can be done only if the transmitter has been previousely calibrated with a span larger then the new span, is accurate and has insignifficant drift.this is also fast to perform with no tools needed.