London, October 2 : Engineers in the UK have developed a new kind of thermometer that is tough enough to measure the temperature inside big explosions, while managing to survive the powerful impacts as well.
According to a report in New Scientist, the thermometer, developed by the UK's National Physical Laboratory, comprises of an optical fibre protected inside a steel tube with one end exposed picks up thermal radiation that reveals the temperature inside an explosive fireball. Conventional temperature sensors such as thermocouples, which measure the voltage produced when a metal is subject to a temperature gradient, are too slow to react and are easily damaged.
"An explosion generates a shock wave, significant amounts of heat, and particulate matter such as soot, all of which could degrade or damage a thermometer," said Gavin Sutton, leading the project at the UK's National Physical Laboratory.
Bouncing laser light off objects can reveal temperature, too, but it is an expensive approach that is hard to use.
So, the team set out to find a new design to make a bomb-proof thermometer that could be used time and again.
The strength and finesse of different designs were tested.
One candidate was a pyrometer, which remotely measures the thermal radiation from a hot object. But, it was unable to detect temperature gradients inside an explosion.
"We needed something right inside the fireball," said Sutton. The answer was an optical fibre 400 microns (0.4 mm) across, protected from the blast by a sand-packed steel tube with one open end.
When the charge is detonated, thermal electromagnetic radiation enters the exposed end of the fibre and passes along it to a safe area beyond the reach of the explosion.
There the fibre splits into four, with each branch piping its own specific set of wavelengths to devices that convert the radiation they receive into a voltage.
After performing calibration tests with bodies of known temperature, it is possible to transform those voltages into temperature readings.
The thermometer can take 50,000 measurements per second, producing a detailed profile of temperature changes during a split-second detonation.
"The lab tests of our portable thermometer regularly measured fireball temperatures of 3000 kelvin and above without any damage," Sutton told New Scientist.
The researchers have also carried out simple field trials, and now plan to look inside much larger explosions.
The findings should help refine computer models that predict how large explosions will be, and what effect they will have on their surroundings.