This article is about how to measure level in Closed Tank Level Measurement and on which formula it is based on.
You will see below figure for which tank should be closed, and a gas phase exist on top of the liquid, the gas pressure must be compensated for.
• A change in the gas pressure will cause a change in transmitter output.
• The pressure exerted by the gas phase may be so high that the hydrostatic pressure of the liquid column becomes insignificant.
• Compensation can be achieved by applying the gas pressure to both the high and low pressure sides of the level transmitter.
We have: Phigh = Pgas + S · H
Plow= Pgas
AP = Phigh – Plow= S · H
Figure shows Typical Closed Tank Level Measurement System.
• The effect of the gas pressure is canceled and only the pressure due to the hydrostatic head of the liquid is sensed.
• When the low pressure impulse line is connected directly to the gas phase above the liquid level, it is called a dry leg.