2282Accuracy

In the realm of instrument calibration and accuracy specifications, there are two important terms: "percent of reading" and "percent of span." It is not enough to say "+/- 5%" when specifying a calibration tolerance or instrument accuracy rating. The tolerance should be stated as either +/- xx degF, +/- xx psi, etc., +/- xx% of reading, or +/- xx% of span. Usually the instruments that are rated in terms of +/-% of reading are the higher quality instruments.

The following example illustrates the important difference between "% of reading" and "% of span" concepts. Consider a 0-100 psig (span) transmitter that is indicating 10 psig (reading).

Accuracy Spec "+/- 5% of reading," Acceptable Range 9.5-10.5 psig

Accuracy Spec "+/- 5% of span," Acceptable Range 5-15 psig

Measurement

Instrument Tolerance

Note:

Industrial instrument tolerances are much tighter.

Temperature (space or room)

+/- tOdegF

Temperature (duct)

+/- 0.5 degF

Pressure (duct static)

+1- 0 5 in w.c.

Pressure (other)

+/- 5.0 pet of reading

Flow rates for air

+/- 5 0 pet of reading

Flow rates for water

+/- 5.0 pet of reading

Relative Humidity

+/- 5.0 pet of reading

Oxygen or C02 monitor

+/-1.0 pet of span

Figure 22.9. Suggested Tolerances For Commercial Instruments.

Figure 22.9. Suggested Tolerances For Commercial Instruments.

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