Calibrating Temperature Measurement Devices

A White Paper from Omega Engineering discusses why and how frequently temperature sensors need calibrating.

In many industries controlling temperature is insufficient. It is important to also have
documentation of the temperature or thermal cycle to which the product was subjected. Such logs are required in food processing and pharmaceutical production and storage, for example. This can be accomplished by temperature controllers with recording capabilities. For safety-critical items such as wheel hubs, suspension linkages or brake components, documentation helps a manufacturer prove that a part was heat-treated correctly and was neither too brittle nor too soft.

Quality Management Systems universally require calibration of all measuring equipment that can affect final product quality. While calibration of gauging equipment is usually understood, the importance of calibrating temperature sensors is sometimes overlooked.

Why calibrate?
Every device used for process-critical measurements should be checked periodically. Where adjustment is possible, a device measuring outside of expected limits should be brought back to an acceptable performance level, but in the case of non-adjustable equipment the deviation or measurement performance should be recorded and a decision made on whether it remains fit for purpose.

In the case of temperature measurement equipment, the properties of bimetals and thermocouple wire change with use and time, especially when used at elevated temperatures, resulting in measurement drift. Additionally, a thermocouple probe may be damaged in service, possibly mechanically or by corrosion, resulting in rapid deterioration of the wire. RTD's and thermistors are also both fragile devices and easily damaged, so should be checked periodically. The same applies for IR thermometers and thermal imaging cameras.

Quality standards generally leave it to the user to decide how often a device should be calibrated. However, an auditor will expect a sound justification for whatever frequency is given. When establishing a calibration frequency, consideration should be given to the type of use the device sees, the risk of damage and the rate of drift, which can be determined from historical calibration records.

Procedures should be implemented defining the actions needed if calibration shows a device is operating outside of acceptable limits. For example, a product manufactured since the last calibration may need to be recalled (and the cost of doing so may influence calibration frequency). In safety critical situations, such as food or pharmaceutical production, calibration may need performing every day or even every shift.

In-house or calibration lab?
For most organisations the determining factors about whether to calibrate in house or to use a calibration lab will be the volume of calibration work to be performed and the availability of inhouse resources. If gauge calibration is already performed, adding temperature sensors to the list of equipment requires investment in a dry block probe or blackbody calibrator.

Just as ISO 9000 provides a Quality Management framework for manufacturing companies, ISO 17025 does the same for calibration laboratories. Defined procedures document the methods used for the calibration work undertaken, ensurinc methods are robust and provide an appropriate level of traceability. Significant emphasis is placed on communicating results to customers, and this includes information on measurement uncertainty.

A lab not meeting the requirements of ISO 17025 may perform satisfactory calibration work with appropriate levels of traceability. However, formal accreditation should give the confidence that appropriate procedures will be followed and negating the expense of having to verify this yourself.

Many manufacturing processes use heat to modify product characteristics. In some cases precise temperature control is essential to ensure fitness for purpose, and a paper trail - temperature logs plus evidence of calibration -verifies that the manufacturer took appropriate steps to maintain the quality of the items produced. Calibration of temperature sensors, whether performed in-house or contracted to a specialist lab, is an essential part of this activity.

More information >>

About Omega
Since its inception in 1962, OMEGA has grown from manufacturing a single product line of thermocouples to an established global leader in the technical marketplace, offering more than 100,000 state-of-the-art products for measurement and control of temperature, humidity, pressure, strain, force, flow, level, pH, conductivity and automation. OMEGA also provides customers with a complete line of data acquisition, electric heating and custom engineered products.

For more information, please contact :-

Omega Engineering Ltd.
One Omega Drive, River Bend Technology Centre, Northbank, Irlam,
Manchester, M44 5BD, UK
0800 488 488 ( UK only)
Tel: +44(0)161 777 2211
Fax: +44(0)161 777 6622

March 2019

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