What Is Calibration Tolerance and How Do I Calculate It?

Calibration tolerance is defined by the International Society of Automation (ISA) as “permissible deviation from a specified value; may be expressed in measurement units, percent of span, or percent of reading.“ When it comes to scale calibration, tolerance is the amount the weight reading on your scale can differ from the nominal value of the mass standard that has optimal accuracy. Of course, ideally, everything would match up perfectly. Since that’s not the case, tolerance guides ensure that your scale is measuring weights within a range that will not negatively affect your business.

 

While the ISA specifically states that tolerance can be in measurement units, percent of span or percent of reading, it’s ideal to calculate the measurement units. Eliminating the need for any percentage calculations is ideal, as those additional calculations only leave more room for error.

The manufacturer will specify accuracy and tolerance for your particular scale, but you should not use this as your only source to determine the calibration tolerance you will use. Rather, in addition to the manufacturer’s specified tolerance, you should consider:

Regulatory accuracy and maintenance requirements

Your process requirements

Consistency with similar instruments at your facility

Let’s say, for example, your process requires ±5 grams, test equipment is capable of ±0.25 grams, and the manufacturer states accuracy for your scale is ±0.25 grams. Your specified calibration tolerance would need to be between the process requirement of ±5 grams and the manufacturer’s tolerance of ±0.25 grams. To narrow it down even further, the calibration tolerance should be consistent with other, similar instruments at your facility. You should also use an accuracy ratio of 4:1 to decrease the chance compromising the calibrations. So, in this example, the accuracy of the scale should be ±1.25 grams or finer(5 grams divided by 4 from the 4:1 ratio). Furthermore, to properly calibrate the scale in this example, the calibration technician should be using a mass standard with an accuracy tolerance of at least ±0.3125 grams or finer(1.25 grams divided by 4 from the 4:1 ratio).