Vaporous liquids can be very difficult to monitor. And the difficulty increases with the volatility of the liquid. There are several side effects of measuring an off-gassing liquid that you need to know about.
Here are three common pitfalls that we want you to avoid:
When a liquid is corrosive, it stands to reason that a non-contact sensor could monitor that liquid without getting eaten alive. The problem with most corrosive liquids, however, is that they tend to be less stable (i.e. they readily react with other compounds) and are often vaporous.
This means the non-contact sensor still needs to have chemically compatible (non-reactive) materials if it is to survive.
It can get even more complex. Sometimes a sensor is compatible with the liquid, but not with the gas – meaning as soon as the liquid turns to vapor (and mixes with oxygen), it begins to corrode the sensor. To work in this environment, the sensor needs to be compatible with the material in both liquid and gas states.
Finally, pressure sensors are often vented to give a measurement using atmospheric pressure as its zero reference. If these vent holes and tubes are around vapor, you’ll find that the vapor makes its way into the electronics of the sensor – causing failure do to condensation or corrosion.
The basic tenant here is to make sure your sensor is compatible anywhere vapor can get. It doesn’t really matter if you’re using a contact sensor like a $500 submersible pressure transducer, or a fancy $3000 radar sensor.
Non-contact sensor technologies typically rely on an unobstructed path to the liquid it is measuring. Vapor can obstruct that path when it becomes thick enough. The two most vulnerable to thick vapor are ultrasonic and optic sensors.
Ultrasonic sensors struggle when the vapor thickens enough to either diffuse the sound waves or even cause a false echo. Optic sensors might be able to get their light waves through the vapor, but their lenses tend to fog up in these environments.
What’s the moral of the story? Non-corrosive vapor can still make measurement difficult when the sensor doesn’t match the application. It’s really best to stick to tried and true contact sensors like a float level transmitter or a submersible pressure transducer in vaporous environments.
Using pressure sensors for tank levels in vaporous environments is a good idea. It’s very simple and is not susceptible to many application challenges. However, if vapor is building in a vessel so is pressure, resulting in an inaccurate level reading.
You can still use a pressure transducer if vapor builds pressure, but you need to use a different kind. To know what type of sensor to buy, you really need to know if your tank is vented. And you need to know how it’s vented.
If the tank is open, then it’s obviously vented at all times. If the tank is covered and has a vent pipe, you’ll need to inspect the pipe. Some vents use a check valve, allowing pressure to build up to a point before opening. Other tanks are completely sealed.
You can use a standard pressure transducer, either externally mounted or submersible, in a fully vented tank. But if your tank is vented with a check valve or sealed, you’ll need a differential pressure reading to get an accurate level. A differential reading will subtract the added pressure from building vapor, leaving you with a good level measurement.
Related Post: How Does Level Measurement Work In A Pressurized Tank?
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Dealing with heavy vapor can be a challenge. Let us know if we can help. We carry a broad collection of level and pressure measurement technologies to give you an accurate sensor even with thick vapor.