Industrial mechanical pressure gauges and digital pressure gauges typically specify errors as a percentage of F.S. (Full Scale). For digital pressure gauges, the convention in the specification sheet is to state “0.5% F.S.”. For mechanical pressure gauges, this is usually stated as their class. “cl 1.6” would be a class 1.6 gauge, meaning it is accurate to 1.6% of it’s F.S. value. 

As an example – one can expect that for a 400 bar, cl 1.0, mechanical gauge, readings at any point in its range would fall within 4 bar of the actual value. 

Another example – a 100 kPa, 0.2% F.S. digital gauge could be expected to read within 0.2 kPa of the actual value at any point in its range. 

More accurate devices may state their accuracy as a percentage of reading. For example – 0.1% of reading. On a 15000 psi gauge, this would mean that at 5000 psi the accuracy is 5 psi, and at 15000 psi the accuracy is 15 psi. 

One question that commonly arises at this point is – how does the calibration laboratory know what the actual value is? Generally, we use instruments which are 4x or more accurate than the accuracy specification of the instrument being calibrated. This ensures that we can say with confidence how well the instrument being calibrated is performing.