When selecting a measuring instrument, recorder, or other such device, customers are basically making a selection of the input sensor type, measurement or sampling interval, accuracy, and other characteristics that will match an application in their industrial process. Therefore, this paper explains some of the terminology used in the µR Series General Specifications and other documents with respect to accuracy, one of the criteria used to select recorders, as well as to touch on the basics of how accuracy is expressed.

Accuracy is the precision of a measuring instrument under specified conditions as expressed by the limits of error (margin of error) occurring due to a variety of factors. Put differently, it is an expression of how close a value measured by a measuring instrument is to the true value, and it represents the limit value of the error relative to the input signal. In the case of measuring instruments, accuracy is most commonly expressed using both percentage error and absolute error, as in:
± (0.1% of measured value + 1 digit), or ± (0.2% of measured value + 1 mV).

*: According to the JIS, the true value is “the value when a given quantity is measured using a method with no inherent error,” but in actuality it is impossible to obtain a perfectly error-free measurement. Therefore a conventional true value that is as close to the true value as possible is used. The conventional true value is one that is traceable back to national standards, or a standard that has been mutually decided upon among users.

Also, since accuracy is influenced by temperature, humidity, and other phenomena, the general specifications list accuracies that are obtained under certain fixed range of conditions. For example, the μR series general specifications state the following.

Measurement/recording accuracy: Standard operating conditions: 23 ± 2°C, 55 ±10% RH, supply voltage of 90–132 VAC and 180–264 VAC, power supply frequency of 50/60 Hz ±1%, warm-up time of 30 minutes or more, without adverse conditions such as vibrations.

With µR series instruments, measurement error and recording error are described as specifications. As an example, the table below shows how accuracies are described in the µR series general specifications (for DC voltage).

Input	Range	Measurement (digital display)		Recording (analog)
		Measurement accuracy	Max. resolution	Recording accuracy	Resolution
DC V	20 mV	±(0.1% of rdg+2 digits)	10 µV	Measurement accuracy ± (0.3% of recording span)	Pen model dead band: 0.2% of recording span Dot printing model resolution: 0.1 mm
	60 mV		10 µV
	200 mV		100 µV
	2 V		1 mV
	6 V		1 mV
	20 V		10 mV
	50 V	±(0.1% of rdg+3 digits)	10 mV
	1-5 V	±(0.1% of rdg+2 digits)	1 mV

1. Measurement error
   Measurement error is expressed as measurement accuracy. For accuracy of 2 V range, this results in the following:

   Measurement accuracy: ±（0.1 % of rdg + 2 digits）
   	rdg:	reading (the read-in value)
   	digit(s):	A certain error independent of the input. 1 digit is the smallest value that can be displayed, which is the resolution of the µR series instrument, and in the 2 V range this 1 digit = 0.001 V (1 mV).
   Calculation Example
   	Determine the measurement accuracy when measuring 2 V in the 2 V range.

   
   
   
2. Recording error
   Recording error is expressed as recording accuracy. For accuracy of DC voltage, this results in the following:

   Recording accuracy: Measurement accuracy ± (0.3 % recording span)
   	Recording span: µR10000: 100 mm; µR2000: 180 mm.
   Calculation Example
   	On the µR10000, set the 2 V range, measuring span of 0–2 V, and recording width of 100 mm. Find the recording accuracy when measuring 2 V. (From the result of calculation in item a) above, the measurement accuracy at 2 V is ± 0.004 V). *1: Recording width *2: Recording span

   
   
   

3. Digital display resolution
   Because recorders convert analog input signals to digital, the display resolution is determined by the performance of the A/D converters. Resolution can be either display resolution or measuring resolution, and is expressed in units of µV or mV in the case of DC voltage, and °C in the case of temperature. The digital display resolution shown in the general specifications of the µR series is the display resolution (as opposed to the measuring resolution).

Further reading:
1. Practical Electronic Measuring Instrument Handbook, Tokyo Denki University Press
2. JIS Z 8103-1990 Compliant Measurement Terminology, JSA
3. Yokogawa Electronic Measuring Instruments, 1994 General Catalog, Yokogawa Electric Corp.
4. µR10000 General Specifications, Yokogawa Electric Corp.