Hydrostatic Pressure As It Relates To Level Measurement

Liquid Level Measurement

One of the most common methods of liquid level measurement is via a pressure transmitter (preferably a D/P – Differential Pressure transmitter…more on this later).  When the time comes to calculate the instrumentation range for a tank level application, a few key pieces of information must be applied to ensure the proper setup is achieved.  The first thing that must be understood, is that hydrostatic level is only affected by the height of the liquid being measured.  What that means is that a 10” straw will exert the same pressure at the bottom of its column as a 10” deep lake will (assuming that both of them have the same liquid, specific gravity, and the same atmospheric pressure being applied to them).  The majority of tank level measurement is either in inches of water column or in feet (this depends on the size of the tank being measured).  With that being said, it is also imperative to know that 1” of water column with a specific gravity of 1 (1 is always assumed for water) exerts 0.0361 PSI.  This value is to be multiplied by the specific gravity of the liquid being measured if it is something besides 1 (this step must be taken or the instrumentation will not correctly read the level).  Note: the specific gravity of any chemical can be found on the associated MSDS sheets.  The next step is to determine the height of the tank from the pressure transmitter (D/P) to the desired top of the tank (this is different for each application).  For simplification of this discussion, let’s assume that we want to setup the transmitter for a 10” tank that will be used to store water.  This the formula to determine the PSI (transmitter range) for that application:

10’ of level X 12” inches per foot X 0.0361 PSI/per In. H2O = 4.332 PSI

Calibration

The next step is to calibrate the pressure transmitter to 4.332 PSI max range with the scale set to 0’ to 10’ (10’ of level will equal 100% of scale which is 20 ma or 5 vdc).  I mentioned a D/P transmitter earlier as the desired instrument for this application; I’d like to explain why I stated that.  If you use a simple single diaphragm pressure transmitter, it will only be accurate when the atmospheric pressure is the same as it was when you initially calibrated it.  The huge benefit of a D/P transmitter is the low side of the differential is open to atmosphere and therefore effected by it.  Assuming the tank is vented to atmosphere, its liquid level is as well.  As the atmospheric pressure changes, the pressure reading will be affected.  Without proper compensation, the level will seemingly change on the instrument without any physical change to the actual level.  The low side of the D/P in conjunction with the atmospheric pressure that is applied to both items effectively cancel out the effects any changes in the atmospheric pressure.  If your specific application has a non-vented tank which also happens to also be a pressurized vessel, you must connect the low side of the D/P to the top of the tank.  This configuration will perform the same task as the tank vent and the associated low side both being opened to atmospheric pressure.  In the pressurized vessel application, the level reading will not be affected by any changes in the pressure that is inside of the vessel which acts like the atmosphere pressing down on the liquid.  Without the low side of the D/P attached to the top of the vessel, the level reading will change as the pressure changes even if the level does not.

Regards,

-Ryan

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