You have just had your measuring instrument calibrated, and are now the proud owner of a calibration certificate. What should you do with it? This guide explains. It is arranged in three main sections:
What is a calibration? A brief introduction to the concept of traceability and the national measurement system, and how to ensure that your measurements are traceable.
How to use the information on your certificate. How to use the certificate to calculate corrections and uncertainties for your measurements.
How to ensure that it is still valid to use the information on the certificate sometime after calibration. How to look after your instrument and how to determine when the instrument should be re-calibrated.
Using your calibration certificate
In this section we give a brief introduction to the methods used to characterize the accuracy of measuring instruments.
This will help you to make corrections to your instrument readings and calculate the uncertainty in your measurements. Throughout the discussion we refer to ‘the instrument’. The term includes all indicating instruments such as thermometers, voltmeters and mass balances. It also includes ‘material measures’ such as standard masses, standards of electrical quantities, and photometric reflectance.
Characterising the accuracy of an instrument
From experience we all know that measuring instruments are not perfect. One person may get nearly the same result every time a measurement is made, but another person may define the measurement differently and get a different result because the instrument is sensitive to the measurement setup. Even if a measurement is repeated by the same person with the same instrument a slightly different reading is expected. For example, when a wooden rule is used to measure a length some of the variation between measurements is due to an inability to position the rule against the object in exactly the same place every time. Some of the variation is also due to an inability to read the scale accurately, changes in the length of the rule with temperature and humidity, and similar changes in the length of the object. These variations are typical of every measurement.
Some measuring instruments are better than others. For example a steel rule may have better quality scale markings, and be less sensitive to environmental influences than a wooden rule. Repeated measurements made with the steel rule usually result in smaller variations between readings. One of the tasks of the calibration laboratory is to measure just how accurate each measuring instrument is.
To measure the variations in readings associated with a particular instrument, a number of comparisons are made with a more accurate instrument. The differences between the readings of the two instruments provide us with information for calculating the uncertainty.
We can describe the variations in terms of the centre and the width of the distribution of readings.
· The centre of the distribution of readings is characterized by the mean, which will usually be biased away from the reference value.
· The width of the distribution is characterized by the standard deviation or a range of values.
The calibration certificate provides you with:
· The means to apply corrections to the instrument’s readings, which removes the bias. For material measures the certificate will give the average value, e.g. the certificate for a standard mass will report the average mass.
· A number which measures the width of the distributions of the measured values for the corrected instrument readings or values. This is the uncertainty.
The results supplied on the certificate are sufficient to relate the instrument readings to the SI over the entire range of interest. Note that the range of interest can be less than the full range of the instrument as specified by the manufacturer. Where non-SI units are used e.g. pounds, gallons etc., MSL will provide a conversion factor to the appropriate SI units.