Eye in the firestorm

UK engineers have developed a thermometer that will carefully monitor the extremes of temperature in previously uncharted territory: the fiery heart of a bomb-explosion.

The re-usable device, pioneered by engineers at the National Physical Laboratory (NPL) for the government's Defence Science and Technology Laboratory (DSTL) is expected to provide enormous improvements to the understanding of explosions.

Capable of withstanding the shock, intense heat and soot of a fireball while taking split-second measurements of the temperature at its core, the fibreoptic radiation probe could be used in everything from the development of new bomb-proof materials, to the forensic study of explosions.

To understand bomb-blasts at present, scientists use computational fluid dynamics (CFD) computer simulation techniques. Data relating to the size of a simulated explosion is entered into a computer and the simulation is run. The problem is that typically this relies on a little bit of guesswork as there is no reliable technique for measuring one of the key parameters — the temperature at the heart of the blast.

Gavin Sutton, the NPL scientist behind the development, explained that DSTL wanted a way of acquiring this data so it could improve the realism of its computer simulations.

'There's very little data regarding the conditions at the centre of an explosion,' he said. 'DSTL wanted to make sure that temperature of an explosion was getting to a particular level and they wanted it to augment CFD simulations.'

While this is not the first effort to take a peek at the temperature inside an explosion it is certainly the most promising. In the past scientists have attempted to use a variety of techniques. These include the use of thermocouples, which do not respond quickly enough, and optical thermometers, known as pyrometers, which have been used to probe explosions from a distance. 'But no-one,' said Sutton, 'has ever stuck something directly inside a fireball — it's uncharted territory.'

Calibrating the thermometer

Developed in a small pyrotechnics lab set up specially for the project the probe, made of optical fibre measuring 400 microns across, is contained within a rigid sand-packed steel tube with one open end.

During operation, the armoured section of the probe, which is about 2m long, is placed at the heart of the explosion site and triggered to begin measuring by a voltage switch that is tripped by a detonation button. Though the current system has just one probe, Sutton said its capabilities could be further enhanced by the addition of another three or four probes, all hooked up to the same system.

Over the split-second duration of the explosion, the thermometer will continually sample visible and infrared optical radiation, taking up to 50,000 measurements/sec. This data is sent to the main instrumentation part of the device that is safely situated at the end of a 100m optical cable. Here the single fibre is split into four, and the radiation is passed through optical filters designed to only pass optical radiation/light at very particular wavelengths. This allows the device to separate the light into four different wavelengths, collecting far more information about the thermal physics of the explosion than could be obtained from one wavelength.

From the filters, the light is passed to high-speed detectors, which convert optical radiation into a voltage that can be related directly to temperature. Sutton would not be drawn on the full cost of the system, but said it is relatively inexpensive, requiring a high-speed data acquisition system that typically costs in the region of £2,000, and off-the-shelf fibreoptic cables.

So far, the system has only been tested on quite small charges of 5-10g. However, Sutton is confident that once tested outside NPL's pyro-lab the clever device will survive far bigger explosions time and again. 'The thing about explosions is that although they are very violent, the actual amount of energy released isn't very big. It's just that it happens very quickly that does the damage,' said Sutton.

'If the thermometer was put in a Bunsen flame for a few minutes it would melt, but because explosions happen very quickly it survives — we just need to clean the end off each time,' he added.

The system, designed to cope with temperatures of at least 4,000ºC, could be used to monitor blasts equivalent to those created by a car blowing up, said Sutton.

He said the DSTL is using the equipment to generate reliable temperature data for its explosion simulations, and although the government agency declined to comment, it is likely the technology will feed into two of its key areas: the development of more advanced armour, and the forensic analysis of bomb blasts. For example, while the skills of the agency's forensic explosives lab in Kent are called upon whenever explosive material is recovered from a terror plot, a separate team is researching the tough lightweight armour that will be a key component of the British Army's next generation of vehicles, the Future Rapid Effect System (FRES).