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Thursday, December 13, 2012

Question from a Customer

Q. What is the difference between percentage of full-scale and percentage of reading?

A. The accuracy of a device describes how close a measurement is to the actual value. It is usually presented in one of two forms: percentage of full-scale or percentage of reading.

Percentage of full-scale, usually shown as %FS, is a fixed error, and therefore, has a greater influence at lower measurement values. For example, if a load cell has a 200 lbf capacity and an accuracy of 0.3%FS, the error is 0.6 lbf throughout the measurement range. Therefore, at a measurement of 20 lbf, the 0.6 lbf error is 3% of the reading. This is outside of the ASTM E4 requirement that measurements should be within 1% of reading.

Percentage of reading is usually shown as %RO. For example, if a load cell has a 200 lbf capacity and an accuracy of 1% RO, then at a measurement of 20 lbf the error is 0.2 lbf or 1% of the reading, and within ASTM E4 requirements. Devices that specify accuracy as a percentage of reading typically have a wider range of use, since this is a more difficult specification to meet.

2 comments:

Norberto said...

Accuracy spec should also define the range within which the accuracy is defined

Unknown said...

There are two ways to specify this, although in essence they mean the same thing:

(a) Specify an accuracy as a percent of reading and then a range over which this accuracy is defined (as Norberto said).

(b) Specify an accuracy as a percent of reading and another accuracy as an absolute value. These are also referred to as "relative error" and "fixed error" respectively. This is usually followed by the phrase "whichever is greater".

For example, if a load cell has 100 N capacity and its accuracy is specified as "±1% of reading or ±0.01 N whichever is greater" this means from 1 N to 100 N, the relative error is predominant, but at forces less than 1 N, the fixed error is predominant because it will be greater.