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Sealed Pressure – How, When, and Why

Plastic bottle affected by changing atmospheric pressureLike your ear, a sensor is sensitive to atmospheric pressure changes unless it is properly vented. I took a trip to New Orleans last fall for a tradeshow. While I’m not afraid of flying, the trip was very difficult for me.

I had an ear infection. As fun as it sounds, an ear infection is caused when the vent tube from your ear to your sinuses becomes clogged. This seals fluid inside your inner ear cavity, which makes changing elevations quite painful. Hence, the flight to New Orleans was unpleasant. A sealed pressure transducer acts in the same way as my poor ear.

How

A sealed sensor is made when the area where the gages (sensing elements) are placed has been sealed air-tight, creating a “sealed” chamber. This chamber has the same air pressure that was present when it was sealed, becoming the permanent internal reference pressure of the sensor. This also means that the sensor will be sensitive to changes in atmospheric pressure, just like your ear.

Why

Sealing a pressure transducer is typically done to protect the sensing elements from damage. Moisture and dust can penetrate a transducer that has not been sealed, and can cause failure. Therefore, sealed pressure transducers are common for applications with high humidity, dust, or frequent cleaning.

When

Because of its sensitivity to atmospheric changes, it is most commonly used for pressure ranges of 1000 psi and above where this effect is so small it is not noticeable. For applications less than 1000 psi, care should be taken to make sure that variations caused by changes in atmospheric pressure would be within an acceptable level. For example, a 1 psi shift on a 15 psi transducer would typically be unacceptable performance, well outside the common error band of 0.25%.

There are some who want a sealed pressure transducer even in low pressure applications. One of our customers, a paint sprayer manufacturer, uses sealed pressure to protect the sensor from paint particles, and from frequent cleaning. However, because the pressure range is so low, the sensor has to be calibrated often – ideally every time the atmospheric pressure changes. If they don’t calibrate, then the readings would be constantly changing and unreliable.

The sealed pressure type is a tool to protect your pressure transducer from harm. However, it has its drawbacks, and has to be used appropriately. Let us know if you have any questions about sealed pressure, or if it may be an answer to your application. Drop us a line in the comments below, or contact our application support at 888-525-7300.

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