(1) A grade AA test gage is defined as that type gage wherein the error in
pressure indication at any point of scale is not to exceed 1/2 percent of the
maximum pressure for which the scale is graduated (that is, 1/2 percent of 500 psig
in the example given above, or 2.5 psig).
Grade AA gages are often said to be
"guaranteed accurate within 1/2 percent of scale range." Grade AA gages are
normally used for the following purposes:
Research laboratories where accurate measurements are required.
Standards to check less accurate gages in the field or in a maintenance
(2) Even though this classification by A.S.A. is called a "test gage,"
gages with this accuracy are often used for other than strictly "test"
Also, deal indicating gages can be obtained with accuracies even
better then €1/2 percent (such as 1/4 percent) for more exacting measurements.
(3) Classification of a gage by the A.S.A. standard accuracy may affect
other gage specifications. For example, Grade AA gages are not generally offered
in dial sizes below 4 1/2 inch diameter, because it is difficult to read this
degree of accuracy on a deal any smaller than 4 1/2 inches.
manufacture of gages to this high degree of accuracy is much more difficult as the
gage size decreases.
(4) In a grade A high grade commercial gage, as shown in Figure 7b, the
error in pressure indication is not to exceed 1 1/2 percent of the scale range at
any point within the middle of the scale (above 25 percent