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Ignition by laser irradiation

Two international projects with PTB participation have, for the first time, determined limiting values for laser radiation in potentially explosive atmospheres.

Ignition of an ethene/gas mixture by a laser pulse impinging on the absorber (absorber marked in yellow)

Drawing-up recommendations for the intrinsically safe use of optical radiation in a potentially explosive atmosphere has become an important task in the field of explosion protection because of the manifold applications of lasers in research and industry, for example, in material machining or data transmission. In particular environments optical radiation can be an ignition source if the irradiance exceeds a critical value. PTB has therefore participated in two projects of the European Community to determine limiting values for optical radiation.

For this purpose, the lower power limits to ignite various gas/air mixtures were determined for near infrared radiation. As a result different radiation limits must be laid down for pulsed and continuous radiation, respectively, as the ignition mechanism is different in both cases. To investigate the case of pulsed radiation, nanosecond pulses and modulated microsecond pulses from lasers were compared in experiments with and without solid absorbers. The pulse energies required for ignition were found to be clearly reduced when solid absor-bers were used. Comparison of different absorber materials shows that the lowest ignition-capable energies are obtained with flammable materials. The energies lie in the range of the minimum electric ignition energies of the gases investigated, they are, however, never lower than the latter. Non-flammable materials are found to induce ignition at distinctively higher energies. Contrarily however, these materials were identified as optimum absorber materials for ignition by continuous irradiation.

It was not possible to induce ignition below a limiting value of 50 mW continuos radiant power in any of the fuels and gas mixtures investigated. Taking an adequate safety distance into account a recommended limiting value of 35 mW radiant power should not be exceeded. This value applies to the wavelength range extending from the near infrared to visible light and under the condition that non-flammable or averagely-flammable absor-bers are used. For laser beam of greater diameter (above 7 mm2), this value can be increased to a less restrictive value, i.e. an irradiance of 5 mW/mm2.

The results obtained prove that, in isolated cases, it can be possible to determine a less restrictive limiting value for a specific beam/absorber combination. In other cases, it will be useful to encapsulate the radiation by suitable means instead of basing the design on an assessment of the intrinsic safety. The results of the work performed will have an impact on international standardization, i. e. they will be incorporated in a standard concerning the use of instruments in mines.