You must know the temperature reference used by your company and for the application to accurately calibrate pressure instruments. Pressure Unit conversion charts do not always specify what temperature the conversion factor is referenced to, causing confusion.
There are three commonly used reference temperatures for inches of water column. They are 4° C (39° F), 60° F, and 20° C (68° F).
The difference between inches of water pressure units can cause about +/-0.1% to almost 0.2% error. For a 1% calibration, this might not matter but for higher accuracy, it’s significant.
You also need to know the reference temperature used by your inches of water pressure calibration standard. 20° C (68° F) is popular but some manufacturer’s use 4° C (39° F). Some calibrators have selectable units for inches of water.