# 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 pressure instruments on technical data sheets can vary significantly across manufacturers and product types.

Contents

- 1 Specification Examples
- 2 Glossary
- 2.1 Accuracy
- 2.2 BSL – Best Straight Line
- 2.3 Drift
- 2.4 FRO – Full Range Output
- 2.5 FS – Full Scale / Full Span
- 2.6 FSO – Full Scale Output
- 2.7 Hysteresis
- 2.8 Linearity
- 2.9 Long Term Drift
- 2.10 Long Term Repeatability
- 2.11 Long Term Stability
- 2.12 NLH – Non-Linearity and Hysteresis
- 2.13 NLHR – Non-Linearity, Hysteresis and Repeatability
- 2.14 Non-Linearity
- 2.15 Non-Repeatability
- 2.16 Precision
- 2.17 Range
- 2.18 RDG – Reading
- 2.19 Referred Temperature Error
- 2.20 Repeatability
- 2.21 Resolution
- 2.22 Short Term Repeatability
- 2.23 Span
- 2.24 Span Offset
- 2.25 Span Stability
- 2.26 Span Drift
- 2.27 Stability
- 2.28 TEB (i) – Thermal / Temperature Error Band
- 2.29 TEB (ii) – Total Error Band
- 2.30 TSL – Terminal Straight Line
- 2.31 TSS – Thermal / Temperature Span Sensitivity
- 2.32 TZS – Thermal / Temperature Zero Shift
- 2.33 URL – Upper Range Limit
- 2.34 Zero Drift
- 2.35 Zero Offset
- 2.36 Zero Stability

- 3 Help

## Specification Examples

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

### Room Temperature Accuracy

- 0.25 % FS NLHR BSL
- 0.5 % FS TSL
- 0.25% FRO
- 0.2% URL
- 0.05% of rdg from 10 to 100% FS
- 0.15% Rdg + 0.15% FS + 1 digit resolution
- 0.02% full span hysteresis
- 0.01% full scale non-repeatability
- 0.025% full span non-linearity
- 0.35% FSO NLH BSL

### Zero and Span Setting

- 0.02mA zero offset
- 3% FS span offset
- 2 mbar zero and span offset

### 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

- 0.01 % FS drift / year
- Zero stability of +/-0.1% FRO per annum
- Span drift of +/-0.05% rdg per year

### Total Error Band

## 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

The difference in output signal between the minimum and maximum measurable pressure. Another way of describing the *full scale* for pressure sensors.

### FS – Full Scale / Full Span

The difference between the lowest and highest measured point.

### FSO – Full Scale Output

See *full range output*.

### Hysteresis

The shift in measurement when comparing between readings of the same pressure which were taken following an increase in pressure and a decrease in pressure.

### 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

An accuracy or precision specification that only considers one cycle of increasing and decreasing pressure and excludes any *short term repeatability* effects.

### NLHR – Non-Linearity, Hysteresis and Repeatability

An accuracy or precision specification that includes all room temperature uncertainties for a pressure sensing device. Occasionally may include *zero* and *span setting offsets*.

### Non-Linearity

See *linearity*.

### Non-Repeatability

See *repeatability*.

### Precision

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

### Range

Defines the limits of variation in measurement, i.e. 100% *span*

### 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, pressure 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.

## 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 pressure 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 pressure 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 pressure 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