Most pressure sensors are calibrated and set up at the factory. The zero and full-scale outputs are set to precise values by the manufacturer, using techniques such as computer-controlled laser trimming to set these values within limits.
Like anything else in this world, everything has its tolerance. Manufacturing errors are permanent and the values will not change. However, if necessary most control units can normally compensate for such errors.
The null offset is the electrical output present when the pressure on both sides of the diaphragm is equal, for example 0mV +/-5mV (tolerance)
Full-scale output (FSO)
The full-scale output is the algebraic difference between the output end points. Normally the end points are null and full scale, for example full-scale output 100mV +/- 4mV (tolerance).
From these examples we have a sensor which gives -5mV worst-case output at zero pressure and +109mV at full-scale pressure ((100mV + 4) -5mV = 96mV). If we have prepared our associated electronics to adjust for these tolerances, especially zero, we are half way to eliminating the fixed zero and span on each sensor.
Therefore, in this example, 0mV at zero pressure will give an output at full scale of 101mV (worst case).
This principle is applicable to any sensor, whether amplified or nonamplified, current output, voltage output etc. Every sensor has a set-up tolerance and it all depends on how accurate the manufacturer wants it to be.
Therefore it is important to ensure that your electronics can work with the range of the sensor in order to give the best possible reading. And in some cases the electronics need to be able to adjust a negative offset.
Also some sensor manufacturers are unable to go all the way to zero, and so their sensors will always have a small positive output.
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