Accuracy and Resolution are Not the Same

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Introduction

Many people use the terms resolution and accuracy interchangeably, in relation to industrial scales. Yet there is no relationship between accuracy and resolution in industrial scales. Scales consist of a number of components: a single or multiple load cells; a junction box with summing card for multiple load cells, and a weighing instrument (such as a weight processor or weight controller).

Accuracy is established/defined by the load cell/s used. Resolution is established/defined by instrument used. Let take a more detailed look into each of these terms, i.e. scale specifications.

Accuracy is expressed as a PERCENTAGE OF FULL SCALE CAPACITY. The term full scale capacity means rated capacity and is referring to the “measuring device”. In the case of our industrial scale this would be the maximum capacity of a single load cell, or multiple load cells used by the scale. A load cell’s accuracy is defined (specified) as being less than or equal to a plus/minus percentage of its rated measurement capacity, expressed as a unit of weight e.g. lbs, kgs etc. As an example Hardy’s load cell’s accuracy specifications are typically <= ± 0.02% of rated capacity e.g. 1000 lbs, within in a defined temperature range of -10 to +40 degrees C (centigrade).

Resolution on the other hand, is expressed as the NUMBER OF, OR SIZE OF THE INTERVALS, INCREMENTS OR DIVISIONS (all these words mean the same thing in Industrial Scales) that can be resolved and displayed. Relating to the “granularity” of the measurement an observer sees on the scale. Resolution is usually expressed not as a percentage, but as 1 PART IN X, or by the size of the granularity that is the size of the increment e.g. 0.1lb.

The Ruler Example

Let’s take a look at an easier-to-understand example, by drawing a comparison between a ruler and a load cell. In the case of a 12 inch ruler (shown below) and using the same accuracy specification as a typical Hardy load cell, we would say the ruler is accurate to better than or equal to plus/minus 0.02% of 12 inches. Expressed in inches that would be accurate to <= ± 0.0024 inches. Why would the ruler vary in length? In this case, this ruler is made of wood. Actually wood fiber and it contains some moisture. So if we put this ruler in a controlled oven and vary the temperature between -10 and +40 degrees C, it would expand and contract ± 0.0024 inches.

If we use the ruler again as an example, the resolution on the first ruler shown above is 1 part in 12 (written as 1:12) or 1 inch. 12 increments of 1 inch. Looking at the second ruler shown above, the resolution the observer sees is 1:192 or 1/16th of an inch. Which is 0.0625% of an inch. So very clearly, the ACCURACY and RESOLUTION of the ruler above aren’t the same.

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