Accuracy class

The accuracy class of a measuring device defines the maximum expected deviation of a measured value from the true value of the physical quantity to be measured , provided the deviation is caused by the measuring device itself. On the one hand, a measuring device cannot be set exactly; on the other hand, its properties can change due to external influences. With the classification in an accuracy class, a quality feature is provided, to what extent these causes may lead to a measurement deviation.

Standards use the term e.g. B. for current transformers , weighing systems or direct-acting measuring devices with a scale display . Such classes are not known for the widespread current and voltage measuring devices with numeric displays ; see digital multimeter , measuring device deviation , resolution (digital technology) .

Scale of a moving coil measuring device of class 2.5 for vertical operating position (symbols on the right).
Under reference conditions, the limit value of the deviation for this measuring device is 2.5% of the end value of the measuring range 10 A, i.e. 0.25 A.

Terms

Accuracy class

In DIN 1319 , which is fundamental for metrology , the term accuracy class is defined as a class of measuring devices that meet specified metrological requirements so that the measurement deviations of these measuring devices remain within defined limits .

accuracy

In EN 60051, the accuracy of a measuring device is defined as the degree of correspondence between the displayed and correct value. The accuracy ... is determined by the limits of the inherent deviation and the limits of the influencing effects . The terms are explained below.

Notation

Measuring devices that meet specific accuracy requirements can be assigned to an accuracy class. This class is identified by a class symbol in the form of a number. In the picture above this is 2.5. An addition, e.g. B. a circle that encloses the number can be added.

Error limits for direct-acting measuring devices with a scale display

DIN EN 60051
title	Direct-acting indicating electrical measuring devices and their accessories; Measuring instruments with a scale display
Area	Measuring device
Regulates	Part 1: Definitions and general requirements for all parts of this standard Part 2: Special requirements for current and voltage measuring devices Part 3: ... for active and reactive power measuring devices Part 4: ... for frequency measuring devices Part 5: ... for phase shift angle Measuring devices, power factor measuring devices and synchronoscopes Part 6: ... for resistance and conductivity measuring devices Part 7: ... for multiple measuring devices Part 8: ... for accessories Part 9: Recommended test methods
Publishing year	German version DIN EN 60051-1: 1999; -2… -9: 1991… 96
Remarks	replaces: DIN 43780; VDE  0410 basis: IEC  60051

The EN 60051 issued for this is extremely diverse, so that only the basics are explained here. Older measuring devices were manufactured according to the similar predecessor regulations DIN 43780 or VDE 0410.

In addition, this list is limited to current and voltage measuring devices in the preferred versions according to EN 60051-2.

A manufacturer who qualifies his measuring device by specifying a class symbol guarantees compliance

• the limits of intrinsic deviation (previously the basic error ),
• the limits of the effects of influence .

Intrinsic deviation

If a measuring device is operated under reference conditions (the same conditions as during adjustment ) and within the measuring range , a measurement deviation that then occurs is called intrinsic deviation .

limit

The intrinsic deviation must not exceed the values ​​given as an example for the class symbol 2.5 (in the sense of an error limit according to the amount)

• 2.5% of the full scale value if the zero point is at one end of the measuring range,
• 2.5% of the measuring range end value if the mechanical or electrical zero point is outside the measuring range,
• 2.5% of the sum (regardless of the sign) of the measuring range end values ​​if the zero point is within the scale .

In the case of an addition to the class symbol, e.g. B. circle, a different reference value applies.

Example: Ammeter with measuring range 0 to 100 mA, linearly divided, class symbol 1

The limit of the intrinsic deviation is = 1% · 100 mA = 1 mA. This limit is a constant over the entire measuring range. ${\displaystyle G}$

Note: The relative error limit of a measured value only has the value = 1% at 100 mA ; it is greater for every other measured value. At 25 mA it is already 4%, since the reference value for the relative error limit of the measured value is the respective measured value. ${\displaystyle g}$ ${\displaystyle g}$

${\displaystyle g}$= = 0.01 = 1% ${\displaystyle {\tfrac {1\ \mathrm {mA} }{100\ \mathrm {mA} }}}$

${\displaystyle g}$= = 0.04 = 4% ${\displaystyle {\tfrac {1\ \mathrm {mA} }{25\ \mathrm {mA} }}}$

Reference conditions

The definition of the reference conditions (reference value or range) is part of the definition of the intrinsic deviation . Essentially, it is defined:

Influencing factor	Reference condition	permissible limits of the reference condition
Ambient temperature	23 ° C (previously 20 ° C)	2 K for class symbols 0.5 or greater, otherwise 1 K
location	according to labeling	1 °
External magnetic field	complete absence	Earth field allowed
External electric field	complete absence
Frequency of an alternating quantity	45 ... 65 Hz
Curve shape of an alternating quantity	sinusoidal
Ripple of a uniform size	zero

Measuring range

Display range 0… 12 A
Measuring range 0.6… 6 A
In class 2.5 Limit value of intrinsic deviation 2.5% · 6 A = 0.15 A

Since the information on the above limit value only applies within the measuring range , the measuring range must be recognizable if it does not match the length of the scale. There are three ways of marking the measuring range on the scale :

• No fine division outside the measuring range,
• Measuring range limit marked by point,
• reinforced (wider drawn) scale arc in the measuring range.

Influence Effects

If the measuring device is not operated under reference conditions, further deviations can arise in addition to the intrinsic deviation.

Single influencing effect

In the case of a single influencing variable that is not complied with, the influencing effect caused by it must also not be greater than the limit value specified above by means of the class symbol, but still provided with a correction factor. However, this only applies in a certain nominal range of use :

Influencing factor	Limits of the nominal range of use	Correction factor
Ambient temperature	Reference temperature ± 10 ° C	100%
location	from the reference position 5 ° in each direction	50%
frequency	Reference range ± 10% of the respective limit	100%

Multiple influence effects

If two or more influencing variables deviate from their reference conditions up to a value within the nominal range of use, the resulting influencing effect must not be greater than the sum of the permissible individual effects.

Example: The measuring device described above is operated at 28 ° C and inclined by 4 °.

Then the limit value of the measurement deviation = (1 + 1 + 0.5) mA = 2.5 mA ${\displaystyle G}$

(Intrinsic deviation + deviation due to temperature influence + deviation due to position influence).

Example: The measuring device described above is operated at 28 ° C and inclined by 10 °.

No guarantee that the measurement deviation will be adhered to, since the nominal range of use is not adhered to.

Deviating reference conditions and nominal areas of use

It is permissible to deviate from the above specifications of the standard if the deviation is indicated by labeling. For example:

labeling	Reference value (range)	Nominal range of use
27 ° C	27 ° C	17 ... 37 ° C
35… 50 … 60 Hz	50 Hz	35 ... 60 Hz
23… 23 … 37 ° C	23 ° C	23 ... 37 ° C
35… 45… 55 … 60 Hz	45 ... 55 Hz	35 ... 60 Hz

Requirements associated with class assignment

For the classes, not only requirements for accuracy, but also various other specifications such as

• Conditions that must be observed when it comes to compliance with the limits,
• Electrical and mechanical requirements, e.g. B. Overload capacity , damping ,
• Inscriptions,
• Test procedure to determine compliance with standardized behavior.

history

According to the VDE 0410 rules for electrical measuring devices, which was valid until August 1976 , these devices were divided into the following groups:

• Precision measuring devices with the classes 0.1 - 0.2 - 0.5
• Industrial measuring devices with classes 1 - 1.5 - 2.5 - 5

Accuracy classes for other measuring devices

• Pressure gauges : display ranges for pressure gauges, division spacing and numbering of the scale according to EN 837 (accessed on October 12, 2015)
• Current transformers : Simplified explanation: Accuracy classes (accessed on October 12, 2015)
• Legal-for-trade measuring devices such as gas meters , energy meters , heat meters , measuring standards :
  EU Directive 2014/32 / EU on the provision of measuring devices on the market , appendices from p. 205, accessed on July 26, 2020

literature

• Thomas Mühl: Introduction to electrical measurement technology. 4th edition, Springer Fachmedien Wiesbaden, Wiesbaden 2014, ISBN 978-3-8348-0899-8 .
• Reinhard Lerch: Electrical measurement technology. Analogue, digital and computer-aided processes, 6th edition, Springer Verlag Berlin, Berlin 2012, ISBN 978-3-642-22608-3 .