NPL maintains a wide portfolio of internationally visible research programmes that advances measurement science, underpins the SI system and supports various cross-disciplinary technologies.
NPL's commercial services are built on 100 years of leadership in accuracy, innovation and scientific research. Experienced consultants and project managers draw upon a unique combination of industry know-how and world-leading scientific discovery to deliver real-world business solutions and enable innovation and secure competitive advantage.
We strive to actively promote public awareness and appreciation of science and technology, particularly the importance of measurement and the role of NPL. Here you can find out more about the interesting things we do, science outreach activities you can get involved in or take a look at some of our educational resources.
Instrumentation in the EU is subject to directives to ensure that it meets requirements for, among other things, EMC compatibility and health and safety.
A supplier has to guarantee that a product meets the particular specifications which are set out in documentary standards.
Specification standards which apply in Europe are either European Norms, designated EN, whereupon they also become British Standards (BS EN), or standards from international bodies such as the International Standardization Organisation (ISO) and the International Electrotechnical Commission (IEC).
As well as ensuring that manufacturers meet the specifications, and therefore that the sensors, thermocouple wires, etc, are interchangeable within the limits of their tolerances, the standards give the mathematical functions which are used to relate the sensor output (resistance or voltage) to temperature.
In the case of industrial Pt100 sensors, the standard uses the Callendar-Van Dusen (CVD) equation, which requires only three parameters to specify the relationship between the resistance R(t) in ohms and the temperature t in °C from –200 °C to 850 °C.
Rather more complicated equations are specified to relate thermocouple voltages to temperature, and these are used as ‘reference functions’ for the various types of thermocouple, against which manufactured thermocouple wires are checked. If necessary, the small deviations from the standard (V – Vref) can be represented by a simple equation which is then added to the reference function (Vref) to give V as a function of temperature.
Industrial thermocouples are often required to withstand harsh conditions of high temperature, vibration and rough use, in oxidising or reactive gases, such as in furnaces or combustion processes. Mineral-insulated metal-sheathed (MIMS) cables are normally used, manufactured according to IEC 61515 and drawn down to a final diameter between 0.5 and 8 mm. These provide good protection and some ‘bendability’, especially at the finer sizes, so they can follow a devious path into the process.
Once clear of the high-temperature zone, it is convenient to switch to flexible ‘extension’ cable, which can easily run to the instrument, where ‘cold junction compensation’ is applied. It is important that the extension cable is made from wires of nominally the same alloys as the main thermocouple, so that the circuit can be considered continuous all the way to the instrument, within a stated tolerance.
10 of 11
National Physical Laboratory | Hampton Road, Teddington, Middlesex, TW11 0LW | Tel: 020 8977 3222
Please note that the information will not be divulged to third parties, or used without your permission