Meter deviation

As gauges deviation deviations of the display (generally the output signal) of a measuring device from the true value, which is caused solely by the meter, respectively.

Demarcation

According to the language regulation of DIN 1319, there are measuring devices

• with display,
• without display, these are transmitters and converters that

• a pneumatic signal (impressed pressure) or
• an electrical analog signal (e.g. standard signal ) or
• an electrical digital signal (e.g. for coupling to Profibus )

output.

The interaction of a measuring device with a measurement object can result in a measurement deviation or a measurement error. For example, a voltmeter typically consumes a small amount of current that it draws through its input terminals. Depending on the internal resistance of the voltage source and depending on the line resistance, the power consumption generates a not insignificant voltage loss. One measures less than the open circuit voltage of the voltage source (which can be measured with an ideal voltmeter). There is a feedback deviation (circuit influence error), a systematic deviation that is always negative. Their size is not only indicated by a measuring device label, e.g. B. by the internal resistance of the measuring device, but also by the characteristics of the measuring object. Because of this coupling, deviations due to internal consumption are not dealt with here .

Observer influences are not dealt with here either .

In the following it should therefore be about deviations of a measuring device with display, which are exclusively properties of the device itself.

A distinction must be made between these devices

• analog measuring devices with dial display ,
• digital measuring devices with numeric display .

There is also

• analog measuring devices with numeric display,

z. B. Energy meters (so-called kilowatt hour meters) with numeric rollers; these have a continuously scrolling number roll at the lowest point, provided with a small graduated scale; with regard to their deviations, they are to be treated like devices with a dial display,

• digital measuring devices with dial display,

z. B. Station clocks that mostly do not contain a second hand; In terms of their deviations, these are to be treated like devices with a numeric display (due to the coupling of these clocks to the time reference, reduced to the quantization deviation; see also digital measurement technology ).

Device deviations from measuring devices

Common

If the output signal (read value) is plotted as a function of the input signal (measured variable) in a right-angled, linearly divided coordinate system, the characteristic curve is obtained , which is aimed for as linear (for digital devices, linear, if only the left (or right) ) Corners of the stepped characteristic connects). Three deviations are possible for the characteristic:

Deviations from the proportional relationship (dashed line)
a) additive, b) multiplicative, c) non-linear

1. Shift of the approximate straight line: starting point deviation (often zero point deviation),
2. Twisting of the approximation straight line: slope deviation or sensitivity deviation ,
3. Deviation from the approximate straight line: deviation from linearity.

The measurement errors are made as small as possible by adjustment during production ; however, imperfections in design, manufacture and adjustment do not make them zero.

In practical use, a measuring device is subject to various environmental influences that cause further measurement errors, e.g. B. if it is operated at a different temperature than during the adjustment.

Measuring devices with dial display

For these devices, an adjuster for the zero point is usually freely accessible so that the zero point deviation can be avoided. A summary statement is made for the measurement error for the other reasons by specifying a class symbol . This describes

1. the amount of the maximum intrinsic deviation , i.e. the measurement deviation when operating under the same conditions as during adjustment, the so-called reference conditions ,
2. the amount of the maximum influencing effects, i.e. the additionally occurring measurement deviations, if the device is not operated under reference conditions, but at least still in a permissible proximity to the respective reference condition, in the nominal range of use .

Reference is made to examples under the keyword accuracy class .

Measuring devices with numeric display

The zero point cannot be adjusted within the width of a step of the characteristic curve ( zero point deviation ). When reading a measured value, there is an additional measurement deviation, the quantization deviation - also up to the width of a step; both together result in the error limit of ± 1 digit step (on the lowest digit) or ± 1  digit . For some measurement tasks, e.g. B. in AC measurements, this error limit can be greater. It applies to the entire measuring range and is often expressed as a percentage of the final value (vE).

The next deviation comes from the slope of the approximated characteristic. The limit value of this sensitivity deviation is given in percent of the measured value (v. M.) or the display (vA). The third deviation mentioned above, due to the non-linearity of the analog-to-digital converter (ADC), is often so far below 1 digit that it does not need to be considered. The total error limit is thus made up of two parts, which should correctly be given both as a sum.

There are no class symbols here. Information on the error limit only applies to conditions that correspond to the reference conditions. However, each manufacturer determines this at its own discretion. Some manufacturers are very cautious when it comes to specifying this and specifying the extended error limit, which includes influencing effects.

Example of handling the error limits:

Measuring range (MR) 200 V, resolved in 20,000 steps (digits), so that 1 digit 0.01 V. ${\displaystyle {\hat {=}}}$

For the DC voltage range, the device is specified as = 0.02% of the value. M. + 0.005% FS ${\displaystyle G_{g}}$

For the AC voltage MB, the device is specified as = 0.2% of the value. M. + 0.015% FS ${\displaystyle G_{w}}$

In the specific case of an applied voltage of 100 V this results

${\displaystyle G_{g}}$= 0.02% ⋅100 V + 0.005% ⋅200 V = 0.02 V + 0.01 V = 0.03 V 3 digit ${\displaystyle {\hat {=}}}$

${\displaystyle G_{w}}$= 0.2% ⋅100 V + 0.015% ⋅200 V = 0.2 V + 0.03 V = 0.23 V 23 digit ${\displaystyle {\hat {=}}}$

Note: This second result is perhaps surprising, but quite realistic even for a high-quality, fairly high-resolution voltmeter: The penultimate digit can already differ by two.

literature

• Thomas Mühl: Introduction to electrical measurement technology . 4th edition, Springer Fachmedien Wiesbaden, Wiesbaden 2014, ISBN 978-3-8348-0899-8 .
• Rainer Parthier: Measurement technology . Basics for all technical fields and industrial engineers, 2nd improved edition, Springer Fachmedien Wiesbaden GmbH, Wiesbaden 2004, ISBN 978-3-528-13941-4 .

See also

• Measurement deviation
• Measurement uncertainty budget
• Margin of error
• Digital multimeter
• Basic accuracy

Web links

• Handling of measurement errors (accessed October 12, 2015)
• Measurement errors (accessed on October 12, 2015)