Water U tube monamometer. Or calibrate against the actual water level. Inches of H2O means exactly that. Most sensors specify accuracy, but if repeatable, can be calibrated to better than the production limits.
Accuracy will be dependant on several variables, turndown, amb. temp swing as well as xmtr specifics such as stability, reference accuracy, ambient Temperature effect and line pressure effect.
Be cautious... not all xmtrs are created equal... basically what I mean is that all xmtrs can be accurate on a test bench @ ambiant conditions.
Hi:
Each trend must give you, your the accuracy in the specs sheet, but if you want to test, needs a amperimeter and to make histerisis method, that consists in take reads a diferents values, up the pressure and to make a graphic, when you are reached the maximum level of your transducer, now you must to take the same reads down. Finally you should obtain a graphic seemed a "S", and betwen both curves, you can view the accuracy in the medium point, because rarely at diferents values the psi vs mAmps would give you the same value up and down. This method is recomended when an transducer is used long time and you wants to see the mistake on the reads.
By the way, there a some trends of transducer of level of water that working perfect, with accuracy of milimeters.
