The accuracy and tolerance described in the product specifications for many Hioki measuring instruments are given in terms of rdg., f.s., and dgt. Measurement uncertainty is often defined, and the uncertainty limit is obtained from the extent of these values, but we have found that many customers are not familiar with the terms.  Let's explore more about Understanding Accuracy Specifications.

(1) f.s. (full scale: maximum displayed value, maximum scale, scale length)

Generally, f.s. represents the range value currently used, and for analog displays, the maximum deflection of the needle.  For example, when using the 300.0V range to measure, the f.s. will be 300.0.

(2) rdg. (reading: read value, displayed value, indicated value)

Refers to the value actually measured, i.e., the value that is currently indicated on the instrument.  For example, a conductor being measured gives a value of 300.0 V on the clamp meter's display.  300.0 V is therefore the reading, or rdg.

(3) dgt. (digit: resolution)

Refers to the least significant digit of the digital display, i.e., the lowest digit in the displayed value, located at the far right of a string.  In the sample on the right, that refers to the 10th decimal place.

Understanding accuracy specifications goes further.  Now let's take a look at how those specifications should be interpreted.

In the example below, the clamp meter is set to the 300.0V range to measure a conductor, and shows a reading of 100.0V.  The accuracy specifications of the clamp meter defines how far the true value is above and below the reading of 100.0V.

To calculate, first, take ±1.0% of the reading of 100.0, or ±1, which gives us a first range of values: 99.0 to 101.0V.  Then, additional margin in terms of the .dgt value must also be taken into consideration, in this instance ±3dgt. or ±0.3V because the 10th decimal place is the lowest digit displayed.

Example: Measuring voltage using the 3285 Clamp On AC/DC HiTester

Accuracy specifications: ±1.0%rdg.±3dgt.  (300.0 V range)
Measurement range: 300.0 V
Measured value:100.0 V
(A) Reading uncertainty: ±1.0% of 100.0V → 99.0 to 101.0V
(B) Digit uncertainty: ±3dgt. → ±0.3V
Total uncertainty limits (range of true value) (A+B)98.7V to 101.3V


Adding and subtracting 0.3V to both ends of the first set of values gives us the final uncertainty limit of 98.7V to 101.3V.  That is, the true value of the signal is between this range.

More recently, Hioki clamp meter accuracy is being defined in terms of % of rdg. ± a value in terms of the actual signal, such as  ±1.5% rdg. ±0.08 A on the AC A range of Model CM4141.  This is essentially equivalent to using the .dgt indication, but helps to avoid confusion and makes it easier to interpret the uncertainty of the measured value.  Both formats are valid.