Hydrostatic level measurement with over-pressure tank


Thread Starter


I need your guidelines or any recommends about how can we apply hydrostatic level measurement in over-pressure (~0.1 bar) liquid tank without air pressure on liquid effect.

Curt Wuollet

If your gage reference is the pressure over the liquid instead of atmosphere, you should be OK. Or am I missing something?


I'm not sure that I understand your question but if it is how to measure a level when you have pressure on top of the liquid you're measuring the level of, then you simply connect the top of the tank to the low side of the d/p cell. You have to concern yourself as to whether there are condensibles in the top the tank which will condense in the low leg, in that case, you can use a filled low side leg with a suitable liquid acceptable to the process that won't evaporate. Other possibilities depending on the application could be use sealed legs(capillary) and connect the low side to the top of the tank, use a pressure repeater to repeat the pressure to the low side of the D/P, or use two pressure transmitters and electronically calculate the level.

Bill Mostia
William(Bill) L. Mostia, Jr. P.E.
Worldwide Excellence in Dependable Automation
[email protected] (b) [email protected] (h)
www.exida.com 281-334-3169
These opinions are my own and are offered on the basis of Caveat Emptor.
I don't know much about this but could you not just put an additional sensor at the top of the tank to only measure the static pressure and subtract this from the bottom one which will be measuring both the hydrostatic and static pressures?

Hope I'm not oversimplifying a more complex problem.

Robert Scott

Just use a 2-port differential pressure transducer where one port is connected to the pressure at the bottom of the tank and the other port is connected to the internal air above the liquid. In order to read the pressure at the bottom, you will need a line filled with air, which is usually provided by a bubbler. Since you have a closed system, you might want to feed the input of the bubbler pump from the captive air above the liquid.

-Robert Scott

Jeff LeBlanc

Perhaps I'm not understanding the problem, but couldn't you use two pressure transmitters one at the bottom of the tank and one located in the top, subtract the top from the bottom and then you will have the pressure/weight/level?
most hydrostatic level transmitters have the abity to vent low pressure side of the measurement diapragm to atmosphere.

Michael R. Batchelor

Why on earth would you use two pressure transmitters? Just put the reference leg for the level transmitter to the top of the tank and the sense leg to the bottom. Then the over pressure is applied to both legs simultaneously, and the level transmitter simple measures the delta. You need to take steps to ensure that the reference leg stays dry if it's a dry leg setup or stays filled if it's a wet leg. But other than that it's straightforward.

Michael R. Batchelor - Industrial Informatics, Inc.
Contribute to society: http://www.distributed.net/ogr/
You should use a differential presure transmiter, it measure difference of pressure between the air at the top of the tank and at the mesure level. To do that you should connect a tube to the messure point and another to the top of the tank. At the startup you set the zero, fill the tank and set the end. Is a very reliable method.

You can also use (depending of the medium) capacitive transmiter, radar transmitter or ultrasonic transmitter.

Best Regards