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 consistent in its indication, thermometer and sensors such as mercury-in-glass (MIG), thermocouples (T/C), thermistors, resistance thermometers (RTD) change their characteristics with time, 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 accurately, 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 labs 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 immersed.
Industrial calibration
Temperature indicators are often calibrated with electrical simulators which are connected to the instrument's sensor input. The device can then be quickly checked by programming the equivalent 'temperature' input into the indicator. The problem is that 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.
Heat sources
* There are a wide variety of heat sources for industrial calibration, the most popular type is the dry block calibrator. More recently multipurpose sources and liquid baths have become increasingly popular. 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 Modules are solid state heat pumps. Dry blocks using this type of device have the advantage of being able to cool as well as heat, and are often portable, typically capable of fast temperature changes. Peltier driven equipment has a typical operating range of -40 to 140°C.
* Electrically heated metal block baths have an electrical heater and commonly have a maximum temperature in the range 650°C to 700°C.
* 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 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 suitable for awkwardly shaped and short sensors. Accuracies are, in general, better than dry blocks due to lack of air gaps and the temperature uniformity of the stirred liquid.
The Isocal-6 range of products from Isotech is believed unique; it 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.
UKAS 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 the smallest 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.
e-learning
To help users involved in temperature calibration, Isotech is offering a comprehensive 'e-learning' program. The e-learning has been developed as part of the e-Learning Place. This £3-million project has enabled Isotech, in partnership with the University of Liverpool and Connect Internet Solutions, to create high-quality courses incorporating multimedia features and on-line interactive assessment. The benefits of the course include increased understanding of temperature calibration, the methods of calibration, the types of equipment available, how to select the appropriate equipment and how to avoid common pitfalls. The website is www.isotech.co.uk/elearning
For more information contact Jon Hughes, Lenton Laboratory & Scientific Equipment, 011 476 4662, [email protected], www.lenton.co.za
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