Zero Adjustment

In any measurement environment, sensors and instruments must provide an output that is an accurate, predictable and repeatable function of their input. A 0 – 10 Bar pressure transmitter, for example, may have a 0 – 10 V output corresponding to its 0 – 10 Bar measurement range in a linear relationship, starting at 0 V output for a 0 Bar pressure measurement.

However, the transmitter must be calibrated to ensure that, for a 0 Bar input, its output is actually 0 V. There must also be a facility to adjust, or ‘zero’, the output when this isn’t so. An electronic device will use a bias voltage (adding or subtracting a certain amount of potential) to make the adjustment; Mechanical instruments can do this using a bias force (spring or mass force applied to a mechanism) or mechanical offset (adding or subtracting a certain amount of motion).

Some instruments use a ‘live zero’; it’s very common, for example, for transmitters to have a 4 – 20 mA output range corresponding to 0 – 10 Bar or whatever their input span may be. The ‘zero’ adjustment would then serve to ensure that the transmitter’s output is 4 mA for 0 Bar measured input.

It is also possible to change the instrument’s span, by adjusting the slope of its response. In an electronic device, this can be done by changing an amplifier gain setting. In analogue instruments, gain and zero settings can interact with one another, making the calibration task more challenging.