# Explanations for Measurement Accuracy Specifications

**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.

## 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.