Using Pressure to Measure Level

Yes, liquid level within a tank, or other body/vessel of liquid, can be measured by using a pressure transmitter. The weight of water creates a known amount of pressure on a submerged item, depending on its depth below the surface. The further something is under the water, the higher the pressure on it will be because of the “weight” on it.

The height of liquid above a submerged item is known as the “water column” or hydrostatic head pressure. The longer the distance between the surface and the submerged item, the higher the pressure. This is a known constant and can be used for determining liquid level.

The full equation for determining liquid level via a pressure transmitter is as follows.

Height of liquid being measured = pressure reading/media specific gravity.

A variable that must be taken into account for using a pressure transmitter to measure level within a tank is whether the tank is sealed or open/vented. Depending on what gas occupies the space above the water level within the tank, extra pressure may be exerted by whatever gas occupies that space (which is common in any sealed tank).

Any extra pressure created within that head space exerts additional pressure on the liquid below and also increases the total pressure on the pressure sensor. If that pressure is subject to change, based on environmental conditions, then using another level reading technology type is advisable.

If a pressure sensor method is really desired, a bypass/capillary tube can be installed on the outside of the tank. The high side of the bypass tube must be high enough that it is always exposed to the “empty” head space and not any liquid. The lower end should be installed at the lowest point of measurement possible.

The pressure sensor in this instance must be a differential pressure sensor, like our PAD Differential Pressure Transmitter, that will measure the pressure difference between the head space area and the pressure at the bottom of the tank where liquid is exerting pressure. To use this type of system, environmental conditions should not vary much in temperature because that could throw off the readings. You must also make sure that the differential pressure transmitter can withstand any environmental challenges like exposure to elements if outside the tank.

Vented or open tanks generally are not subject to any additional pressure besides the liquid and using a pressure transmitter is a good choice for many applications where the tank allows for exposure to normal atmospheric pressure.

If the media is something that coats or crystallizes, using a pressure sensor is not a good idea as the buildup would also create additional pressure on the instrument and potentially cause a false reading.

You must also ensure that the sensor materials are chemically compatible with the media within the tank.

Installation considerations must also be taken into account. Can the tank be tapped in the bottom to insert/install the transmitter? If not, is there access from the top to use a fully submersible model that will rest on the bottom of the tank?

If the application has sanitary/hygienic requirements due to food and beverage regulations, you must also ensure that the transmitter can meet those requirements.

Pressure sensors are also not prone to wear as they have no moving parts, making them a popular choice for level applications with water in open/vented tanks. They also are great for foaming media as the measurement is unaffected by the foam. They are also usually a cost-effective option in comparison to some other liquid level technologies.