Video tutorial: Measuring liquid depth using pressure

One of the simplest and most reliable ways of measuring liquid depth is via pressure. This article, and its accompanying video demonstration, offers some practical suggestions. See outtake and other photos.

12/07/2009

 Photo inset, upper right, shows an outtake from the pressure gage tutorial demonstration, flipping the demo's lid, says Peter Welander.

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.

If you drill through the side of a tank and insert a gage, the 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. ( Watch the video demonstration .)

 A 500,000 gallon fire water tank The sensor measures static pressure from the liquid weight

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.

The accompanying photos illustrate a 500,000 gallon fire water tank at Arkema Chemical . The depth of water in the tank is monitored by a pressure sensor mounted near the bottom. It is configured to monitor the level and provide an alarm if it drops below a specific point. (Source: ISA100 Wireless Compliance Institute )

This discussion so far is based on the assumption that the tank is open to atmosphere and that you are taking a normal gage-pressure reading. If the tank is sealed, there is the possibility that the process might cause the pressure to go above or below atmospheric which will throw off your reading. In these situations, you have two choices: Either adjust your level reading to compensate for the internal pressure, or use a differential pressure device.

A manual correction is simple enough, but it means you need a second gage mounted on top to read the interior tank pressure. If it's reading 2 psi, you deduct that from the depth reading on the bottom.

It's simpler to use a differential pressure gage. The high end should be connected to the bottom of the tank, or at least below the liquid level, just as you would if the tank were open. The low end of the pressure gage should be connected to the top of the tank, where the line will be above the liquid level. This line should be filled with air or whatever gas is in the headspace of the tank. With both sides of the device connected thusly, the gage will self correct. You don't need to pay attention to changing pressure levels within the process.

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.

Pressure measuring devices are available from a wide variety of suppliers, including:

-Peter Welander, process industries editor, PWelander@cfemedia.com
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