Video tutorial: Measuring liquid depth using pressure

If you want to measure the level of liquid in a tank, one of the simplest approaches is to use a pressure gage. The fact that liquid has weight means that you can measure that weight in the form of pressure. By drilling through the side of a tank and inserting a gage, a reading can be translated into the depth of liquid above the gage.
January 1, 2010

If you want to measure the level of liquid in a tank, one of the simplest approaches is to use a pressure gage. The fact that liquid has weight means that you can measure that weight in the form of pressure.

By drilling through the side of a tank and inserting a gage, a reading can be translated into the depth of liquid above the gage.

If your gage is set up to read in inches of water column, the conversion is really easy. If it reads in PSI, one pound equals 2.307 feet of water, or 27.68 inches.

 

Control Engineering Senior Editor Peter Welander conducts liquid level measurement test in video at http://tinyurl.com/ye6e3yj.

Control Engineering Senior Editor Peter Welander conducts liquid level measurement test in video at http://tinyurl.com/ye6e3yj.

For all practical purposes, any unit of pressure can be converted to the required unit of depth. Most assume a liquid density equal to water, however there are conversion factors for liquids with other densities. If your tank is full of ethanol, one PSI equals about 3.00 feet of liquid since ethanol is less dense than water. If you want to be really precise, a temperature correction is required as well since density is affected by temperature.

Keep in mind that the gage is going to tell you what the depth is above itself. If it is mounted halfway up the side of a tank, that is its starting point. It can’t read anything lower. In the same way, if you put the gage on an impulse line (with no air in it), the position of the measuring device is still the relevant point. The position of the penetration into the tank doesn’t matter.

This type of application is very common given its simplicity, accuracy, and versatility:

  • You can use any type of pressure measuring device, electronic or mechanical;

  • It can be as precise as the process requires;

  • With the right type of device, you can interface with a control system, set alarm levels, etc., as needed; and

  • Once the device is permanently positioned, even if it isn’t at the bottom, you can compensate for most placement issues.

There are some practical limitations, including:

  • If there is sediment or debris in the liquid, it can clog impulse lines;

  • You have to add one or two more penetrations into the process; and

  • If a tank can hold multiple products, you have to use appropriate density corrections.