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Level Measurement with Pressure Transducers: Sealed Tanks

Pressure transducers have always been popular for level measurement in tanks. They are typically low-cost compared to alternatives, and this is a powerful driver even when competing with more purpose-built technology. In addition, installation is usually quite easy. However, some tanks are more difficult than others.

Sealed tanks tend to be a bit more difficult than most. If you want to use a pressure transmitter, you’ll need to install two. Let’s get into the details:

A sealed tank behaves differently as it is filled. Because the air in the tank is trapped, it is compressed as liquid fills the cavity. In fact, both the air and the liquid become compressed as they are forced to share space. This has an effect on the pressure sensor, reading a higher pressure than normal.

In order to normalize the pressure reading, both the air pressure and the liquid pressure have to be measured. Then, the air pressure has to be subtracted. This will normalize the pressure reading, allowing operators to see a true measurement of liquid level.

This is done by installing a pressure transducer at the top, for the air pressure, and on the bottom, for the liquid column. The bottom transducer can be mounted outside of the tank, or dropped into the liquid inside the tank.

Pressurized tanks, including negative or vacuum, need pressure sensors of the appropriate type (gauge, compound, vacuum, etc).

Obviously, this method requires a PLC, or some kind of controller, to do the math and normalize the data. When done properly, this simple method can accurately measure tank levels when other technologies may be impractical.

Of course, there are a few ways to go about this. The above is simply our recommendation. Has anyone else found themselves needing pressure sensors for liquid level measurement in a sealed tank? How did you go about it?

If you have any questions about this application, or others like it, reach out to our application engineers.

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