Temperature Calibration in an ISO 17025 Calibration Laboratory

Calibration Baths, Comparators and Calibration Furnaces

The most used type of equipment in the secondary laboratory will be the stirred liquid bath. These calibration baths typically cover the temperature range of -80°C to 300°C. When selecting a calibration bath, it is important to choose one with sufficient depth and capacity to suit the thermometers being calibrated and having good temperature stability and uniformity. Isotech stirred liquid baths use sophisticated parallel tube or concentric tube designs to provide low uncertainty temperature calibration for a wide range of temperature sensor types, including liquid in glass thermometers, resistance thermometers and thermocouples.

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Higher Temperature Calibration, to 700°C

The maximum temperature of stirred liquid baths is dictated by the liquids used in them, most commonly the liquid being a silicone oil with an upper limit of around 300°C. Above this temperature, dry block calibrators and heated metal blocks can easily be used but the immersion depth and temperature uniformity will be a limiting factor for many laboratories. For those needing low uncertainty calibration there are alternatives of ‘Salt baths’ and ‘Fluidised Furnaces’.

Salt baths can offer good immersion depth and temperature uniformity, utilising a mixture of sodium and potassium nitrates up to a maximum temperature of around 550°C. These baths can be hazardous with a risk of eruption and explosion. The salt is solid at room temperature and again care is needed. A more convenient and safer option is a fluidised furnace with an inert powder being used rather than oil or salt. The furnace is connected to an air supply which helps the powder to flow around the bath. The Isotech fludisied furnace operates safely to 700°C and is sealed to prevent powder loss into the laboratory.

Find out more about our Fluidised Furnace Model 875

Standard Thermometers

In addition to the calibration baths and apparatus reference thermometers, working standards will be needed. These should be chosen according to temperature range, immersion length and required uncertainty.

Industrial Platinum Resistance Thermometers IPRTs ‘Semi Standard Thermometers’

These devices are relatively low cost, easy to handle and with different models can span the temperature range of -200°C to 660°C. They should be long enough to immerse into the calibration equipment to reach thermal equilibrium. Over narrow temperature ranges, say 0°C to 100°C they can be calibrated to an uncertainty around 0.01°C, much better than the tolerances of EN 60751. As the temperature is increased the uncertainty also increases, typically 0.05°C at around 660°C.

Standard Thermocouples

For higher temperatures and calibration in furnaces the standards are usually noble metal thermocouples, most commonly platinum rhodium types. There can be a benefit in switching to platinum palladium devices. The noble metal types are superior to base metal types and, comparing them to SPRTs, they are not fragile or difficult to handle. These devices can cover wide temperature ranges, to 1600°C or higher but the uncertainties are higher than for resistance thermometers. For example, over the temperature range 0°C to 1300°C the best uncertainties are around 0.3°C.

Find out more about our Standard Thermocouple Model 1600 NCJ

Thermometry Bridges

Thermometry Bridges are ideal for use with SPRTs and have an uncertainty of < 1ppm. The best Isotech bridges are better to 0.03ppm and uniquely support thermocouples as well as SPRTs. Bridges are more costly than precision thermometers. Bridges achieve superior performance as they make ratio measurements, and this technique can be employed to directly compare a test thermometer to a standard thermometer thereby achieving lower uncertainties than would be realised with precision thermometers.

Further Temperature Calibration Equipment

Measuring standards should be checked in between calibration periods. One way to check high accuracy thermometers and instruments is to use a water triple point cell. These devices can have an uncertainty of less than 0.001°C and are relatively inexpensive. Consideration should also be given to capacity – multiplexers, automation software and future expansion.