“Accuracy” as defined in the ANSI/ISA 51.1-1979 (R1993) standard Process Instrumentation Terminology is: “The degree of conformity of an indicated value to a recognized accepted standard value, or ideal value.”

Two common methods of rating or expressing accuracy are:
• As a percent of scale length (or percent of full scale). This rating method is most commonly employed with instruments equipped with an analog meter.
• As a percent of actual output reading. This rating method has become more popular for instruments provided with a digital meter.

The above referenced standard also notes that: As a performance specification, accuracy (or reference accuracy) shall be assumed to mean accuracy rating of the device, when used at reference operating conditions. Accuracy rating includes the combined effects of conformity, hysteresis, dead band, and repeatability errors.

What Accuracy Is Not

In the world of gas detection instrumentation, as in other places, confusion exists. Consensus standards are helpful, of course, but cannot be expected to address all practical issues.

Accuracy is Not Minimum Detectability

“Minimum detectability” is simply the lowest meter reading or other type of instrument output that can be unambiguously discriminated from noise. Some agencies set a standard that minimum detectability must be at least 2-2.5 times the noise level.

Note that any data garnered at the level of minimum detectability will not be very accurate. For example, in a typical case, the minimum detectability of a particular instrument, provided with an analog meter, is given as 1 percent of full scale, and accuracy is ± 2 percent of full scale. Thus, for a 0-100 ppm scale, the minimum detectable reading of 1 ppm would actually be 1 ppm ± 2 ppm—hardly a useful measurement.

Similarly, on a digital unit, the minimum detectability of a particular instrument is often given as the least significant digit. On a commonly used 31/2 digit meter, for a range of 0-199.9 ppm, this would be 0.1 ppm. In this case, accuracy is specified at ± 2 percent of reading ± 1 least significant digit. Here, the minimum detectable reading of 0.1 ppm would actually be 0.1 ppm ± 0.002 ppm ± 0.1 ppm. While technically better than the analog example, this is still of little value.

Even so, knowing the minimum detectability of an instrument can be helpful in situations when “go/no-go” readings are of interest. Given a properly calibrated instrument, the smallest observable response would be—by definition—the minimum detectable level, and would indicate at least the presence of the substance in question (any interferences notwithstanding).

Of course, such practices should only be done when instruments with more appropriate sensitivity are not available.

Accuracy is Not Precision

“Precision” as defined in ASTM Standard D 1356-05 Standard Terminology Relating to Sampling and Analysis of Atmospheres is:

The degree of agreement of repeated measurements of the same property, expressed in terms of dispersion of test results about the mean result obtained by repetitive testing of a homogeneous sample under specified conditions.

A classic illustration shows a target with many arrows closely clustered around the bull’s-eye. This scenario would be both accurate and precise. If many arrows were closely clustered far from the bull’s-eye, the situation would be precise, but not accurate. If arrows were all over the target, those closest to the bull’s-eye would be accurate, but the archery session was not precise.

However, while this understanding of precision holds true in the fields of science, engineering, and statistics, as the term is used in the world of computing, precision can be either:

The number of significant decimal digits or bits by which a particular value is expressed; for example, a calculation which rounds to three digits is said to have a working precision or rounding precision of 3.

Or…The units of the least significant digit of a measurement; for example, if a measurement is 25.371 meters then its precision is millimeters (one unit in the last place, or “ulp,” is 1 mm).

Accuracy is Not Resolution

“Resolution” as defined in the ANSI/ISA-51.1-1979 (R1993) standard Process Instrumentation Terminology is: The least interval between two adjacent discrete details which can be distinguished one from the other.

We can invoke another use of “resolution” to help visualize this concept. In the computer world, “resolution” is a measure of the sharpness of an image or of the fineness with which a device (such as a video display, printer, or scanner) can produce or record such an image. Resolution in this context is usually expressed as the total number or density of pixels in the image—typically as dots per inch or dots per millimeter.

For example, a 600-dpi (dots per inch) printer is one that is capable of printing 600 distinct dots in a line 1 inch long. Thus, it can print 360,000 dots per square inch.

Now, draw the analogy between all these “distinct dots” and the least significant digit on a digital meter. We referred earlier to a 31/2 digit meter, set up for a measuring range of 0-199.9 ppm. Our least significant digit here is 0.1 ppm. There is no way to read a digital meter beyond this least significant digit. Or, to put it another way, the resolution of the meter is 0.1 ppm.