One of the most versatile and inexpensive level sensors on the market is the submersible pressure transducer. It is easy to install, requires no set-up (other than to spec a full scale range when purchasing), draws very little current, and it’s immune to several application variables that so commonly upset other technologies.
However, it has an Achilles Heel of it’s own.
It relies on specific gravity, or density, to take an accurate measurement. Easy! You spec that out with the manufacturer as well. That’s usually good enough.
But not always.
How Density Changes
There are two factors that cause a liquid to change density: temperature and pressure. Controlling these two variables is the key to maintaining consistent density.
The density of many liquids is stable at room temperatures and at ambient pressure. However, other liquids require precise climate control in order to stabilize into a liquid with a consistent specific gravity.
Density will also change when two liquids of different density are stored in the same tank, or when naturally occurring liquids are inconsistent.
It May Be Best To Change Sensors
Some liquids change density easily. These are known as volatile liquids. Others have an unknown density, or their densities are estimated. Crude oil is a classic example of a naturally occurring liquid with a highly variable density.
When you need a liquid level measurement of one of these liquids, you should be using something other than a pressure sensor. If the air column above the liquid is clear, then an ultrasonic sensor might be the best option. If the liquid is highly vaporous or foamy, then a float based technology is probably the next best option – like a magnetostrictive level probe.
Radar sensors may be a better choice is you’re dealing with other variables like high temperatures or long distances. However, their cost is still very high and they can usually be avoided.
Unless High Accuracy Doesn’t Matter
If you only expect small changes in density, and the accuracy isn’t a top priority, then sticking with a submersible pressure transducer is likely a good idea. The accuracy will vary in direct relation to the density change. If this is minute, then the change in accuracy will likely be unnoticeable.
A Problem Of The Few
While a change in specific gravity is problematic for a pressure sensor, it’s not a problem for most applications. If it hasn’t been a problem for you in the past, it’s likely not something you need to worry about in the future.
At the end of the day, you need to be very familiar with your application, and how your liquid reacts to the environmental and chemical changes that it will experience in both routine and fault conditions. At that point, you can make some very good decisions about your level sensors and control.
We’re always happy to help you with those decisions. We’ll help you understand what strengths and weaknesses accompany our different level sensor technologies. Contact us to discuss your application.
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