If you are monitoring low pressure and require a high level of accuracy, you will need to make sure that your pressure transducer breathes in dry conditions. How are low pressures and dry conditions related? It's all about the pressure type.
Pressure Type Matters
It's worth reviewing our pressure types quickly. We'll discuss the basic categories:
- A sealed pressure transducer is completely closed off. So the pressure inside the pressure transducer can is trapped, and becomes the reference point for all pressure measurements.
- A vented pressure transducer is not sealed shut. Instead a small opening releases the pressure inside the can, and the reference point for the sensor is the ambient pressure at the point of measurment.
This matters! A sealed pressure transmitter is ideal for wet and dirty applications because it best protects the inner electronics. However, as the pressure inside the transducer housing cannot adjust to atmosphere, the reading will appear to drift every time the weather changes. Think how your ear reacts to changes in elevation if its clogged. In other words, it messes with your pressure sensor's accuracy.
This small reaction to atmospheric pressure changes isn't noticeable in high pressure applications - above 500 PSI. However, in low pressure applications, a movement of a few PSI constitutes a large percentage of the total range, often well above the acceptable error band.
For example, if you have a sealed 0 – 15 PSI sensor, a 3 PSI change in barometric pressure will offset your measurement by 20%. On the other hand, if you have a sealed 0 – 1500 PSI sensor, a 3 PSI change would have a nominal effect - only 0.002%.
Bottom line? In order to achieve an accurate measurement, the pressure transducer can’t be sealed. It needs to be vented so that it will ignore barometric pressure changes. Yet, because it is vented, the sensor is at risk of getting damaged due to moisture.
Vented For Low Pressures
A vented pressure transducer will typically have some sort of protection from moisture. For example, a Gore® patch is used to protect the electronics in our transducers. However, this patch is only meant to protect the sensor in humid environments. It is not intended to protect the sensor if it will be submerged or subjected to direct spray.
In applications where the sensor is to be submerged, such as water tank level measurement, high accuracy is absolutely necessary. This is because a single PSI is equal to 2.3 feet of water. So if you are using a pressure sensor in a 20 ft. water tank, a barometric change in pressure of 3 PSI will offset your measurement by about 7 ft.! You will need to use a vented pressure transducer.
Submersible pressure transducers use a vent tube. The tube runs inside the shielded cable along with the conductors. Therefore, the sensor is vented to atmosphere inside the control box right where you interface with your controller.
To ensure that no moisture will get into the breathing tube, it is recommended that you install a desiccant cartridge on the end of the tube.
When It Doesn't Matter
In some applications you may not be worried about the accuracy of your pressure transducer. For example if you are using a 150 PSI sensor in a very wet environment, but a 3 to 4 percent barometric offset doesn’t bother you, then we recommend buying a sealed sensor. There is just no better way to ensure protection from moisture, or dust, or chemicals, etc.
If you have any questions on protecting your sensor from moisture damage or any other questions on pressure measurement, please let us know and we will be glad to help.
Image Credit: Tatiana Gerus
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