Purpose of a Calibration

Three main reasons to calibrate instruments:

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

Measurements provide the most value when comparable to others taken at different times, locations, by different people or instruments. These measurements keep manufacturing processes controlled across days and between factories. Manufacturers and exporters require such measurements to know that they will satisfy their clients’ specifications.

Most countries have a system of accreditation for calibration laboratories. Accreditation is the recognition by an official accreditation body of a laboratory’s competence to calibrate, test, or measure an instrument or product. International standards define the criteria used to make the assessment.  Accreditation ensures that the links back to the national standard are based on sound procedures.

Uncertainty: how accurate are your measurements?

All measurements guide decisions, and poor measurement quality leads to poor decisions. Measurement uncertainty estimates the range of values reasonably attributed to a quantity.  It is a measure of the quality of a measurement and provides the means to assess and minimize the risk and possible consequences of poor decisions.

For example we may want to determine whether the diameter of a lawn mower shaft is too big, too small or just right. Our aim is to balance the cost of rejecting good shafts and of customer complaints if we were to accept faulty shafts, against the cost of an accurate but over engineered measurement system. When making these decisions the uncertainty in the measurement is as important as the measurement itself. The uncertainty reported on your certificate is information necessary for you to calculate the uncertainty in your measurements.

Reliability: can I trust the instrument?

Many measuring instruments read directly in terms of the SI units, and have a specified accuracy greater than needed for most tasks. With such an instrument, where corrections and uncertainties are negligible, the user simply wants to know that the instrument is reliable. Unfortunately a large number of instruments are not. MSL finds that approximately one in six instruments sent for calibration are unreliable or unfit for use.

This failure rate is typical of that experienced by most calibration laboratories and is not related to the cost or complexity of the instrument. Reliability is determined mainly by the absence of any signs indicating the instrument is or might be faulty.

Achieving Traceability in your measurements

Many quantities of practical interest such as color, loudness and comfort are difficult to define because they relate to human attributes. Properties such as viscosity, flammability, and thermal conductivity depend on measurement conditions. These properties cannot always be traced directly to SI units.

For this reason, the international measurement community establishes documentary standards and procedures. These standards define how to measure such properties consistently and accurately. They enable reliable product quality comparisons and help meet safety and health requirements.

Three elements are required for traceability:

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

Companies that outsource measurement services should choose laboratories independently assessed for technical competence.

Adjustment: what a calibration is not

Calibration does not usually involve the adjustment of an instrument so that it reads ‘true’. Indeed adjustments made as a part of a calibration often detract from the reliability of an instrument because they may destroy or weaken the instrument’s history of stability. The adjustment may also prevent the calibration from being used retrospectively. When MSL adjusts an instrument it normally issues a calibration report with both the ‘as received’ and ‘after adjustment’ values.

What a calibration certificate contains

Your calibration certificate must contain certain information if it is to fulfil its purpose of supporting traceable
measurements. This information can be divided into several categories:

• it establishes the identity and credibility of the calibrating laboratory;
• it uniquely identifies the instrument and its owner;
• it identifies the measurements made; and
• it is an unambiguous statement of the results, including an uncertainty statement.

In some cases the information contained in your certificate might seem obvious but ISO Guide 25 grew out of the
experience that stating the obvious is the only reliable policy.

Calibration overview provided by the Measurement Standards Laboratory.