Pressurized tanks make level measurement a little more difficult. It just comes with the territory. So what kinds of sensors work well in pressurized environments? How can you compensate for the static pressure of the vessel?
First, you need to recognize whether or not your tank or vessel is pressurized.
For vaporous liquids, tanks often have a pressure-relief valve that avoid building up too much pressure in the tank while keeping it isolated from outside world. This is a good way of keeping flammable liquids protected from ignition sources.
From time to time, we’ll have a customer call in with problems, only to realize that they have a pressure-relief valve that is holding pressure inside the tank. Either they didn’t realize it, or they don’t consider it a true pressurized vessel (semantics). But as far as the sensor is concerned, pressure is pressure.
A submersible pressure transducer by itself will have a hard time with a pressurized vessel as it uses pressure and specific gravity to infer a level measurement. The added static pressure is seen as additional feet of liquid.
An ultrasonic sensor has difficulty as well – simply because the housing material isn’t capable of withstanding high pressure. For our sensors, anything above 30 psi could cause the threads to fail, creating a projectile out of the sensor.
What Sensors Work?
There are a few sensors that work well for pressurized tanks. They are those that have a heavy duty housing to withstand the pressure, and are either unaffected by the static pressure, or can compensate for it effectively.
Our resistive chain and magnetostrictive level probes have a heavier duty build that can withstand up to 150 psi. While they are more expensive than several other options, they fall middle of the road in terms of list price – more than ultrasonic but less than radar. However, they are costly to ship and install due to their long stems and weight.
Despite the extra costs, they are well suited for many level measurements because they measure level directly with a float and are quite accurate. Unlike other technologies, the level reading will be reliable with very few exceptions.
Finally, what is most used for pressurized tanks is a differential pressure reading. Installing two pressure sensors - one at the bottom of the tank and one at the top – will allow you to subtract the influence of the static pressure and normalize the level reading.
How To Compensate For Static Pressure
It might take a little math, but compensating for static pressure is a relatively simple process. At its core, you’re simply subtracting the static pressure from the total pressure at the bottom (static + liquid column).
Of course, you have to calibrate the sensor for the right variables like span and specific gravity. You can find a great article on that topic here.
We offer the pressure transducers that would allow you to get a differential measurement. You would need to connect both sensors to a controller/PLC to do the math.
This method comes with its own set of challenges, including the potential for multiple specific gravities. However, it has proven to be relatively inexpensive and easy to do once you’re initiated with the process and all the variables.
We can help you evaluate your specific application. Contact us and we’ll point you in the right direction.
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