Hydrostatic level measurement is one of the most popular ways to measure liquid levels. This is due to its simplicity, ease of installation, and reliability. But that reliability hinges on how you protect one important part: the vent tube. Moisture in a vent tube can spell disaster for a submersible pressure sensor. So how do we keep it out?
A submersible pressure transducer rests at the bottom of a tank or well, and measures level by detecting the pressure exerted by the volume of liquid above it. This is converted to a level based on the dimensions of the tank or well, and the specific gravity of the liquid being measured.
In order to keep the level measurement accurate, the sensor must be vented to atmospheric pressure. If a sealed unit were to be used, a 1 PSI change in barometric pressure would appear to be over a 2 ft. change in water level! Few operators can deal with that uncertainty.
Since these units are submerged in liquid they can’t be vented through their housing like other pressure instruments. So a vent tube is run from inside the housing through the entire length of cable and is terminated along with the electrical wires at the end of the cable run. This is how the sensor references atmospheric pressure while still protecting the electrical components.
But take note that this vent tube is a direct pathway to the components. If moisture were to get inside the tube and travel down to the components the sensor would fail at worst and no longer be accurate at best.
This leads us to today’s discussion on different ways to protect the vent tube from moisture. There are a variety of methods but we will focus on the 3 that we see the most often.
One moisture prevention method is to connect the vent tube to bellows. This actually seals the vent tube so that the zero pressure reference is inside the bellows. The bellows, in turn, compress and decompress due to barometric pressure changes. This way the vent tube is completely protected but is still compensated for changes in atmospheric pressure.
The primary drawback with this method is that the bellows are not barometrically perfect: The pressure in the bellows may not represent true atmosphere and so the possibility of error is introduced. It is by far better than using a completely sealed sensor but it is not perfect.
Desiccant cartridges are plastic tubes filled with desiccant material. They are connected to the end of the vent tube and typically stored in a panel alongside other control components.
This method is great since the desiccant material “pulls” moisture out of the sensors vent tube into the cartridge.
A common complaint we hear with this method is that it requires more maintenance than other methods. This is because the cartridge has to be replaced every few months or so depending on climate conditions.
Finally there are hydrophobic vent caps. These are typically small plastic tubes with an entry hole on one end for the sensor’s vent tube and a hydrophobic membrane on the other end. The advantage with these is that they don’t require replacement and they will reference real atmospheric conditions.
One disadvantage is that they do not actively “pull” moisture out. So if moisture was already in the tube prior to the cap being placed then that moisture will remain there.
The other disadvantage is that moisture collecting on the outside of the membrane can seal off the tube. This can easily be prevented though by installing the tube so that the cap is both faced down and elevated.
There is no perfect solution to protecting your submersible sensor's vent tube. Some methods may make more sense than others depending on how you operate. If you are not sure which option makes the most sense for your application, please reach out to us, and we will be happy to help.
