MEASUREMENT TERMINOLOGY

Accuracy

The accuracy of a measurement relates to the closeness of agreement between the measured value provided by the measurement system and the true value of the measurand (the dimension being measured).The true value of a measurement parameter (Volts,Amps,Kg,etc) is determined by National Standards laboratories working under international agreement and is disseminated by an unbroken chain of calibration.

Resolution

The smallest distinguishable increment provided by a measurement system whether digital or analogue systems are used.The resolution of a measurement system is by itself no indication of accuracy.

Precision

The precision of a measurement system relates to the closeness of agreement between measurements made of the same dimension.It is possible to have a measurement system, which is precise but not accurate.

Uncertainty

This factor comprises two elements, the first being a systematic error; the second being due to random variation. Uncertainty is an indication of the degree to which the variation in values obtained when measuring can be reasonably attributed to the measured itself.Uncertainty is normally expressed ratiometrically (%,dB,ppm,etc) or in the relevant engineering units (kN,mm,etc) with a minimum 95% confidence level.All errors affecting measurement uncertainty should be controlled by a documented measurement procedure.

Systematic Error

This element is one, which is constant throughout the time taken to perform a measurement. This can also be defined as the difference between the true value of the measurand and the mean value resulting from an infinite number of measurements carried out under repeatable conditions.The major contributing factor is likely to be the uncertainty of the measuring system as detailed in its calibration certificate.

Random Error

The second element in uncertainty of a random nature and concerns the inherently variable nature of repetitive, seemingly identical tasks such as successive measurements of a single item. This random variation is the variation in a series of measurements of the same item using the same measurement procedure.Contributing factors could be:

• Environmental: Temperature, Barometric Pressure

• Human: Staff training, Qualifications, Experience

• Physical: Working conditions, Jig & Fixture design 

Traceability

The concept of establishing a valid calibration of a measuring instrument or measurement standard, by step-by-step comparison with better standards up to an accepted or specified standard.In general, the concept of traceability implies eventual reference to an appropriate national or international standard. 

Non-Linearity Error That part of the instrumental error whereby a change of the indication, or response, departs from proportionality to the corresponding change of the value of the measured quantity over a defined range.Various methods can be employed for the numerical expression of non-linearity error, The MIRA Calibration Centre generally uses the departure from a "least squares" fit straight line applied to all the data.