Why is it important to isolate the reference port when calibrating low gauge pressure?

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Why is it important to isolate the reference port when calibrating low gauge pressure?

A gauge pressure transducer is essentially a differential pressure transducer that measures the difference between the atmospheric pressure and the pressure applied at its pressure port.

Normally the reference port is left open to atmospheric pressure. When the reference and pressure ports are both open to atmospheric pressure, the transducer reads zero. Positive pressure applied at the pressure port will result in a positive pressure reading; a vacuum (subatmospheric pressure) applied at the pressure port will result in a negative reading. When the pressure port is connected to a closed system and the reference port is exposed to atmospheric pressure, changes in atmospheric pressure will cause a zero offset.

This effect as a percentage of the transducer range is more significant for low full scale pressure ranges, and becomes critical when performing a calibration. The transducer being tested and the transducer in the calibrator should have their reference pressures maintained at the same stable pressure, otherwise errors could be introduced.

For example, if a calibration is being performed on a transducer with a range of 0-10 in H2O using a calibrator of the same range. The accuracy of the calibrator is 0.01 % FS and the accuracy of the device under test (DUT) is 0.05% FS. The technician has set up a fan while performing the calibration, or has maybe positioned the calibrator close to an air vent.  The fan or vent happens to be pointed in the direction of the calibrator where the reference port is open to atmospheric pressure.  The impact pressure of the air, the Bernoulli effect or just a little turbulence can impart a force on the reference side of the transducer. Depending on the severity of the effect, an error – sometimes more than 10 times the accuracy of the calibrator – can occur.

If the reference ports of the calibrator and DUT are simply vented to atmosphere, whenever someone opens a door or the wind blows, atmospheric pressure will increase or decrease, causing intermittent error and noise. Another source of error easily overlooked is when the DUT and its reference port are an internal part of a larger instrument with an exhaust fan. There can be a significant difference between the pressure inside and outside the instrument case.  

All these pressure fluctuations can create relatively large differences between the reference of the DUT and the calibrator.

It is reasonable to think that transient fluctuations simultaneously experienced by the calibrator and the DUT would cancel each other, but this is not the case.  Different transducers have different response times so one transducer may lag behind the other when there is a rapidly changing pressure.

Ideally the two reference ports should be connected to the same stable atmospheric pressure, and if that pressure is changing significantly, a mechanism should be in place to ensure the change occurs slower than the slowest response time of either transducer. The setup below will ensure these conditions are met, using a volume as a buffer and a bleed valve as shown. The bleed valve should be slightly open so that the pressure in the volume changes very slowly with changes in atmospheric pressure.