All thermometers need to be characterised or calibrated. In industry the need to calibrate is often very real and practical.
No thermometer will remain entirely constant. Thermometers and sensors such as mercury-in-glass thermometers (MIG), thermocouples (T/C), thermistors and resistance temperature detectors (RTD) change their characteristics with time and temperature cycling, and they can become contaminated.
There is a need to calibrate to meet quality standards, which for medical and safety applications may be vital to safeguard life. Calibration can improve efficiency - often if a thermometer's characteristics are known, more accurate savings in cost can be made. The requirement to calibrate temperature sensors continues to grow.
Isothermal Technology has focused on providing calibration solutions for the calibration of all types of thermometers.
Three levels of temperature calibration
Primary laboratories
Primary laboratories are often national laboratories, now known as National Metrology Institutes (NMIs).
A role of the NMI is to directly realise the International Temperature Scale of 1990 and to maintain standards for the nation. NMIs also carry out inter-comparisons between themselves and other NMIs. Most NMIs also look to support and educate industry.
Secondary laboratories
The secondary laboratory is so called because it does not maintain 'Primary Standards'. These labs generally calibrate thermometers against standards in comparison baths. The comparison equipment creates a volume of constant temperature into which a standard thermometer, calibrated by a primary laboratory, together with the unknown thermometers is situated.
Industrial calibration
Temperature indicators are often calibrated with electrical simulators which are connected to the instruments sensor input. The device can then be rapidly checked by programming the equivalent 'temperature' input into the indicator.
However, the temperature sensor will have the potential for larger drift so it is very important to test and calibrate the temperature sensor with a heat source.
Types of equipment
There are a wide variety of heat sources for industrial calibration with the most popular type being the dry block calibrator. More recently multipurpose sources and liquid baths have become increasingly popular.
Dry block calibrators
Dry blocks have a metal block into which the temperature sensors to be calibrated are placed. The block is heated or cooled to the desired temperature. Dry blocks fall into three temperature ranges, dictated by material availability, heater and cooling technology.
* Peltier driven equipment: Peltier modules are solid state heat pumps. Dry blocks utilising this type of device have advantages of being able to heat and cool, are often portable and typically have fast heating and cooling rates. Peltier driven equipment has a typical operating range of -40 to 140°C.
* Electrically heated metal block baths: with this type of equipment an electrical heater replaces the heat pump. These are commonly made with maximum temperature ranges of 650 to 700°C.
* Small furnaces: at high temperatures conventional metal sheathed electric heating elements are replaced with ceramic furnace designs. These devices are commonly used to calibrate to 1200°C.
Portable liquid baths
Portable stirred liquid baths have a well which is filled with a liquid. The liquid is stirred and heated or cooled to the desired temperature. They are ideal for awkward shaped and short sensors. Accuracies are in general better than dry blocks due to the lack of air gaps and the temperature uniformity of the stirred liquid.
Isotech Isocal-6
The Isocal-6 range of products from Isotech can be used in six different modes. As standard, the units can be used as dry block calibrators with a removable metal block drilled to accept temperature sensors. The block can be removed and replaced with a tank of liquid which is stirred as it is heated or cooled. Other accessories allow it to be used to calibrate surface sensors and infrared thermometers. The Isocal-6 can be considered as a portable calibration laboratory.
In addition to manufacturing temperature calibration products, Isotech maintains a full scale UKAS accredited laboratory providing a valuable service to both industrial users and laboratories that need very small uncertainties.
Isotech is known for its innovation and has introduced a tympanic thermometer calibration system for medical thermometers. The use of infrared tympanic thermometers is replacing more traditional thermometry such as mercury-in-glass. Besides obvious health concerns over the use of mercury, the increase in the use of tympanic thermometers is related to their speed and relatively non-invasive method of operation. The design is licensed from the UK's National Physical Laboratory and meets a very real, easy to use and cost-effective calibration system.
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