The true accuracy of a pressure gauge under operating conditions can be difficult to determine, in part because manufacturers’ specifications are often very confusing. One of the major differences between brand names is whether they offer “% of reading” vs. % of full scale (F.S.)” accuracy. Specifications with % of F.S. are a legacy from mechanical gauges, whose resolution was how closely manufacturers could print the graduations on their dials. Today, digital pressure gauges that can display readings with sufficient resolution across their entire range use % of reading specifications. For example, when manufacturers define their accuracy as % of reading, they are describing the accuracy as a percentage of the reading currently displayed. I.e.. A gauge with 0.1% of reading accuracy that displays a reading of 100 psi would be accurate to ±0.1 psi at that pressure. At 50 psi, the same gauge would have an accuracy of ±0.05 psi (twice as accurate). By comparison, a gauge with an accuracy of 0.1% F.S. would be accurate to ±0.1 psi across its entire range. This give a clear advantage to gauges measuring their accuracy specification in % of Reading.
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