Head pressure question

[mjrx]

This is probably an easy question for some of you experts out there. I need to calculate depth of water in a pressurized vessel. I have a pressure transducer at the bottom of a tank. I know if this tank were open on top, I could just read the PSI and be done. Lets say theres 2.31 feet of water, so my transducer reads 1 PSI. Now if the tank gets pressurized with an air blanket in the top of the tank, lets say to 20 PSI, is my transducer just going to read the sum total of the two, .ie 21 PSI, or is there some other relationship between the two? Thanks.

[Ken Moore]

You will get the sum of the two. To measure heights of liquids in closed vessels a differential pressure transducer is required, it needs to be setup with one measurement at the bottom, and one measurement at the top. Then the pressure at the top is subtracted from the bottom reading and the difference is the weight of the liquid. The same arrangement can be used for pressure or vacuum. If you buy one of the industrial DP transmitters such as Rosemount, Foxboro, E&F, etc... the calculation is done withing the instrument and the output is the difference. Sometimes these are referred to as level transmitters, on open vessels the top reading is connecto to local atomospheric pressure as a reference. KEN

[STORMRIDER]

I WAS CALCULATING THE PROBLEM YOU HAVE ON WHETHER HOW MUCH IT WOULD SHOW AND CAME UP WITH 46.2 FT OF WATER IF THE PRESSURE IS AT 20 PSI. COULD YOU TELL ME THE SIZE OF YOUR TANK? MAYBE I COULD FIND OUT FOR SURE HOW MUCH OR WHAT TYPE OF HORSEPOWER YOU MIGHT NEED TO HAVE FOR A BETTER PUMP. JUST WONDERING!