Digital Gauge Dictionary: Zeroed or Nulled

Zeroed or Nulled - What’s the Difference?

Jim Bergman of Testo®:

“A null value is different than having a value of 0, since 0 is an actual value. When a variable has no value, it is considered being null. Because all standard pressure gauges are calibrated for sea level, [0 psig] if the sensor was zeroed rather than nulled, the current atmospheric conditions would not be considered. If the gauges were brought back to 0 psig outside of sea level, [a procedure often performed with mechanical gauges and some electronic] the accuracy of the gauge would be affected by the current elevation with respect to sea level. Because Testo® uses a nulling procedure rather than a  zeroing, the instrument sensor can be brought back into calibration relative to the current atmospheric conditions or ambient pressure, which thus serves as a reference pressure. Quite simply there is no refrigerant pressure on the sensor, the reference pressure and the sensed pressure are the same. Nulling the RSA sensor brings it back to a perfect zero psig regardless of the atmospheric conditions provided the pressure is removed from the sensors.”

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