Pressure Gauge Accuracy Grades
One of the most important considerations you will make when purchasing a pressure gauge is that of accuracy. Though accuracy is not a complex concept—it
is simply the degree to which a measurement conforms to the correct value—the way in which gauges are graded for accuracy can be confusing.
To ensure a pressure gauge has the accuracy required by your application, it’s necessary to understand how accuracy is graded by gauge manufacturers as
these grades are often used more prominently than the actual accuracy.
There are two sets of standards which define the accuracy grades of pressure gauges: ASME B40.1 defines the accuracy grades of dial type gauges while
ASME B40.7 defines the accuracy grades of digital gauges.
A few important considerations when discussing the accuracy of pressure gauges:
- The accuracy of pressure gauges is determined under very specific environmental conditions: a temperature of 73.4°F (23°C) and barometric
pressure of 29.92 in Hg (1013 mbar). Conditions other than these can effect gauge accuracy.
- Accuracy can be expressed as percent of span or percent of indicated reading. Percent of span is the most common method with percent of
indicated reading being generally limited to precision test gauges or high resolution digital gauges. Accuracy as a percent of span means that
a 100 psi gauge with a 2% accuracy is accurate to within 2 psi whether the gauge is reading 1 psi or 100 psi. Accuracy as a percent of indicated
reading means that a gauge with 0.1 % accuracy displaying 100 psi is accurate to 0.1 psi while the same gauge displaying 50 psi is accurate to
0.05 psi—twice as accurate.
- Some accuracy grades divide the gauge’s range into quartiles for the purpose of determining accuracy. This is because some gauges are more
accurate over the middle half of their range than over the first or last quartile. For example, a 100 psi gauge with Grade A accuracy is accurate
to 2% of span from 0 to 25 psi and 76 to 100 psi while being accurate to 1% of span from 26 to 75 psi.
ASME B40.1: Accuracy for Pressure Indicating Dial Type, Elastic Element Gauges
ASME B40.7: Accuracy for Pressure Indicating Digital Gauges