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Measurement Accuracy

There are many contributing error factors which go into a total uncertainty calculation and the proportion contributed by each one will be different from one measurement technology to another.

The way accuracy is defined for measurement instruments on technical data sheets can vary significantly across manufacturers and product types.

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Specification Examples

Please click on an example term below to be guided to the associated glossary explanation:

Room Temperature Accuracy

Zero and Span Setting

Thermal Errors

  • 1% FS TEB, compensated temperature range -20 to +80 degC
  • 0.5% FRO RTE @ 23degC, from -10 to +50 degC
  • 0.005 % FS / deg C TZS, over -20 to +80 degC
  • 0.005 % span / deg C TSS, comp temp range -20 to +80 degC

Stability

Total Error Band

  • +/-1.5% FS TEB, comp temp range -20 to +80 degC, plus 90 days drift

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Glossary

Accuracy

How close the measured reading is to a reference point or value.

BSL – Best Straight Line

A virtual line derived from a set of non-linear points which is used to demonstrate the best accuracy that can be achieved from the product.

Drift

See stability.

FRO – Full Range Output

This can have two meanings. The first meaning is it is the difference in output signal between the lowest and maximum measurement. The second meaning is that it is the actual reading a full range, which often is the same value because the lowest reading is zero, but this is not always the case, for example 4-20mA output has a reading of 20mA at full range, and also a difference of 16mA between the lowest and highest value.

FS – Full Scale / Full Span

The difference between the lowest and highest measured point. Often used to define errors as a percentage instead of measurement units.

FSO – Full Scale Output / Full Span Output

See full range output.

Hysteresis

The shift in measurement when comparing between readings at the same point which were taken following an increasing and a decreasing change in measurement reading.

Linearity

The straightness of a set of measured points compared to a perfectly straight line.

Long Term Drift

See stability.

Long Term Repeatability

The amount of change in measured points following many measurement cycles from low to high, then to low again over a long period of time.

Long Term Stability

See stability.

NLH – Non-Linearity and Hysteresis

A way of expressing the accuracy or precision specification of a device by combining the effects of errors when increasing the measured parameter over the full range at one temperature, and the errors attributed to hysteresis when the measured parameter is decreased.

NLHR – Non-Linearity, Hysteresis and Repeatability

A way of expressing the accuracy or precision specification of a device by combining the effects of errors when increasing the measured parameter over the full range at one temperature, the errors attributed to hysteresis when the measured parameter is decreased and the errors attributed to repeating the increasing and decreasing of the measured parameter for a defined number of cycles.

Non-Linearity

See linearity.

Non-Repeatability

See repeatability.

Precision

A measure of the proximity of all measured points to a virtual reference line such as bsl or tsl.

Range

Defines the limits of variation in measurement, or the difference between the lowest and highest measurement.

RDG – Reading

Used to distinguish a percentage accuracy which varies proportionally to the measured span (% of reading) from one which is a fixed percentage of the maximum measurement reading (i.e. % of full scale).

Referred Temperature Error

A fixed temperature reference is defined (usually room temperature) which is representative of the average operating temperature. The temperature error is then defined as a +/- value of the largest error.

Repeatability

The amount of change in measured points following a number of measurement cycles from low to high, then to low again over a period of time.

Resolution

The ability of a device to distinguish a measurement via a reading or an signal output. In most cases the resolution should be much better than the overall accuracy, but in some cases the resolution can become a significant part of the total measurement uncertainty.

Short Term Repeatability

The amount of change in measured points following a few measurement cycles from low to high, then to low again over a short period of time.

Span

The difference between any measured point and the lowest value.

Span Offset

The variation in measured span compared to the perfect span reading, which is either represented as a percentage, measurement unit or output value error.

Span Stability

The amount of long term measurement variation which is only attributed to the span.

Span Drift

See span stability.

Stability

The amount of measurement change expected over a long period of time.

TEB (i) – Thermal / Temperature Error Band

The difference between the most negative and positive error across the whole temperature range. The difference is then halved and expressed as a +/- error.

TEB (ii) – Total Error Band

A combined error that includes linearity, hysteresis, repeatability, zero setting, span setting and thermal errors. It may also include stability error if a time factor is included with the total error band.

TSL – Terminal Straight Line

The line created by joining the lowest and highest measured points together. The error of all other measured points is referred to this line

TSS – Thermal / Temperature Span Sensitivity

How a measured value at any point in the range is affected by changes in temperature, normally expressed as a % span or % span / degC.

TZS – Thermal / Temperature Zero Shift

How much the lowest measured reading will vary with temperature, typically shown as % full span or % full span / degC.

URL – Upper Range Limit

Used to define the accuracy as a factor of the maximum range of a rangeable device rather than an adjusted (turndown) range.

Zero Drift

See zero stability.

Zero Offset

The amount of variation of the lowest measured reading compared to a perfect reading, which can be expressed as a percentage of full scale (%FS) or measurement units.

Zero Stability

The amount of long term measurement variation which only affects the zero offset.

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Help

Resolution vs Accuracy

Is Measurement Resolution the same as Accuracy?

You will find mentions of resolution and accuracy on many product information sheets for measuring equipment, however when discussing the performance of equipment the two terms often get confused as meaning the same.

Resolution defines the ability to distinguish one reading from another. For a digital gauge the resolution is normally referred to as the number of readable digits, e.g. 2 bar range with a 5 digit display would have a 0.1 mbar resolution.

A strain gauge sensor without any amplification is described as having a signal output with infinite resolution, since there is no signal conditioning to limit it.

Accuracy refers to the worse case error in measuring a particular reading compared to the actual value. If the resolution is of a similar value to the accuracy it should be included in the accuracy statement, since the true uncertainty of reading should also encompass readability.

The following are examples of how resolution and accuracy are described in specifications of pressure measuring equipment:

Digital Pressure Gauge
Pressure Range: 200 bar
Accuracy: 0.05% Full Scale = 100 mbar
Display Resolution: 5 digits = 10 mbar

Amplified Pressure Transducer (0-10Vdc out)
Pressure Range: 200 bar
Accuracy: 0.25% Full Scale = 500 mbar/25mV
Digital to Analog Amplifier Resolution: 0.002% Full Span = 4 mbar/0.2mV

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