There are three main reasons for having instruments calibrated:

1. To ensure readings from an instrument are consistent with other measurements.
2. To determine the accuracy of the instrument readings.
3. To establish the reliability of the instrument i.e. that it can be trusted.

Traceability: Relating your measurements to others
For measurements to be compared they must share a common measurement system.

Uncertainty: How accurate are your measurements?
A measurement with a stated uncertainty can be compared to others
Measurements are traceable when they can be related to a recognised measurement system, through an unbroken chain of comparisons, each with stated uncertainties

Reliability: Can I trust the instrument?
A calibration certificate will be issued only if an instrument is found to be reliable, and will satisfy its intended purpose

Achieving Traceability in your measurements
To make a traceable measurement three elements are required:

1. An appropriate and recognised definition of how the quantity should be measured,
2. A calibrated measuring instrument, and
3. Competent staff who are able to interpret the standard or procedure, and use the instrument.

Traceability is ensured only if these three factors are present in the measurement process