Developing new temperature measurement techniques for harsh environments
Harsh environments are anywhere that a conventional sensor would be difficult, or impossible to use. Extreme environments are more common than we might imagine. For example, anywhere where there is combustion, temperatures can easily exceed 2000° C. In such an environment temperature measurements are very difficult, and yet the temperature can critically affect the efficiency of combustion, the amount of pollution and the corrosion of materials.
We are investigating a range of techniques, including thermographic phosphors, fibre-optics and laser diagnostics.
Temperature-sensitive phosphor coatings
A 'phosphor' is a material that absorbs light (typically UV/blue light) and emits a different colour of light (typically red light). The decay time (rate of dimming) of the emission and/or the emission strength varies with temperature and can be measured very precisely.
Once a surface is coated with a phosphor, its temperature can be measured without any physical contact by optical means.
NPL has demonstrated this technique for surface temperature probe calibrations and in-field monitoring of thermal aspects of welding. We have also developed camera based systems (2D imaging) that are independent of the surface emissivity and reflected background light. More recently, NPL have started to develop phosphor-tipped fibre-optic probes that may offer a non-electrical (optical) alternative to the traditional thermocouple.
In rapid combustion events, such as those which occur inside the cylinder of an internal combustion engine, the temperature reached during the power stroke critically affects the performance of the engine and the amount of pollutants that are formed. However, the exceptionally rapid processes make temperature measurements extremely difficult.
We are working on techniques that can rapidly, up to one million measurements per second, measure the temperature of the combustion fireball from the spectrum of light captured by an optical fibre embedded in the wall of the cylinder.
To support the traceable measurement of combustion flame temperatures, NPL has built and validated a standard reference flame. The temperature above the burner surface depends on the ratio of fuel to air, but is typically 2000 °C. The standard flame is reproducible to within just a few degrees Celsius, and has been characterised over a volume of 20 mm x 20 mm x 20 mm. It has been sent to laboratories around Europe to calibrate novel optical flame temperature measurement techniques.
Optical combustion thermometers: 2040 K to 2260 K
Improving welding quality for submarine assembly