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27th May 2009 20:20

Alan Yates wrote...

Robert,

Very good point, there is a minimum activation energy.

Much the same as shock, friction and spark ignition, large transients are often tolerated without initiation if the total delivered energy is small. In a cap or igniter the bridgewire dissipates the delivered electrical energy and obviously has to heat the adjacent composition to its ignition point before it will fire.

The no-fire current represents the maximum safe current through the bridge wire ohmic losses which dissipates heat that can be sunk to the ambient continuously without the surrounding composition cooking off (plus a generous safety margin). The all-fire current is that which will deliver the largest activation energy observed in a batch of igniters within some given time. Both conditions are generally specified at some ambient temperature; clearly a igniter sitting in liquid nitrogen will fire much slower than one in an oven just below its self-ignition temperature. The absolute activation energy varies with ambient temperature because it basically represents how much more energy you need to put into the composition around the bridge wire to start the reaction.

Commercial igniters (of good quality at least) come with a chart showing the safe and all-fire regions for various currents and times, few if any seem to offer information for other than typical ambient temperatures.

I did consider talking about this when I first wrote the page, but I figured it was beyond the intended scope. Especially a full treatment considering the thermal capacity and conductivity of the igniter materials, ambient temperature, composition activation energy, etc. Compositions also frequently behave differently depending on how fast you heat them, for example slow-scanning of a temperature range such as in a Thermogravimetric Analyser will cause the entire bulk of the composition to be approximately the same temperature, allowing complete phase changes and violent bulk-reactions. That kind of thermal equilibrium is very uncommon in practical devices where a reaction proceeds from a starting point through the largely unaware bulk of the composition. Things get more complex in HEs of course with detonation shocks rather than simple melt/react style deflagrations.

One important consideration for all electrical initiators is that the no-fire current seen in the ambient specs may not be completely safe at elevated temperatures or in highly insulating conditions for extended periods of time. The lead-wires are reasonable heatsinks, but I don't know if anyone has ever tried cook-off tests with matches insulated in glass fibre or similar?

The converse it also true, in very cold ambient temperatures there might be significant delays to ignition or complete ignition failure if the system can't deliver enough energy.

When you buy a commercial device there have been lots of tests performed on multiple batches (well, there should have been anyway). Statistical analysis is performed to characterise the devices and individuals from each batch tested destructively to ensure they meet specs. Based on my experience with some Chinese sourced e-matches I suspect they often skip the quality control tests! But I hope at the least you can assume their no-fire and all-fire/X ms currents are accurate around room temperature.

In practice few people worry too much about the details, they just hook it up and duck... The actual ignition process is quite detailed and highly dependent on the match construction. This is one reason why I never ever do commercial work with homebrew devices. I could never make a large enough batch or make them identical enough to have sufficient statistical confidence in their properties. Saving a few bucks on e-matches just isn't worth it IMO, but I can understand the motivation when with cheap product the matches are a very significant fraction of the material cost of a display in an already squeezed budget.

Regards,

Alan

27th May 2009 13:36

robert wrote...

Hi

From time to time i beome involed with blasting jobs - I have a license. Any ways some time back I decied to build a very safe blaster. One term that is always left out is the relationship between current and time ie joules. The fuse link in detonators and electric match heads (the same thing) requires around 5mJ at 500ma min to fire. I used for the bit of gear I designed 10 mJ - just to be safe... Also the resistance I think was around 1 ohm per detonator. Hope this helps.