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Setra Blog

May 12, 2016 8:48:46 AM

What is the difference between opening and closing gap sensors?

Measuring pressure is complicated enough now that we understand that there are three pressure sensor types; capacitance, strain gauge and MEMS based. Nonetheless, it does not stop there. There are two types of capacitance based sensors; closing and opening gap for Unidirectional and Bidirectional applications.

Closing Gap

In a closing gap style sensor there is a stretched membrane that moves towards a fixed electrode, changing the capacitance value. This style sensor can be ordered with either a unidirectional or bidirectional range. A closing gap capacitance based sensor is typically used for measuring low pressure applications with a range of <15 PSI.


Ordinarily with a unidirectional range the stretched membrane diaphragm moves towards the fixed electrode plate, reducing the gap between the two, thus changing capacitance. As the diaphragm moves towards the electrode and the gap decreases, capacitance increases. In a unidirectional sensor the analog output (VDC or mA) is only linearized from the point at which the diaphragm is flat through the full scale pressure range.



With a bidirectional range, depending on the bidirectional.gifdirection of applied pressure, the diaphragm can deflect either towards or away from the fixed electrode respectively. If the diaphragm deflects from flat to negative full scale output (away from the electrode), the gap between the diaphragm and electrode plate will increase, thus decreasing capacitance levels. If the diaphragm deflects from flat to positive full scale the gap between the membrane diaphragm decreases, increasing capacitance. In a bidirectional sensor, the analog output is linearized in both the positive and negative direction.

Opening Gap

An opening gap capacitance based sensor is Finale_Opening_Gap.gifused for measuring higher pressure ranges than the closing gap sensor, from 15 PSI to 10,000 PSI. The opening gap sensor can measure gauge, compound, and vacuum pressures and is best used for high pressure OEM applications. With the opening gap sensor, applied pressure causes the diaphragm to bow, causing the distance between the diaphragm and the electrode to increases as the pressure increases, thus decreasing capacitance. This change is then linearized to a selected analog output (VDC or mA) proportional to the capacitance change.


CLICK HERE to learn more about Setra's pressure sensors.

Topics: General Industrial, HVAC/R

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