Earlier tests indicated I was in for trouble with my homemade e-matches. My e-matches have always been black powder green meal and NCL (nitrocellulose lacquer) based, they worked fine, until... I ran a series of tests deployment charge tests in a vacuum chamber, much to my surprise the e-matches failed almost 100% of the time at pressures equaling 30,000' and higher. At 25,000' to 30,000' the matches were hit and miss, under 25,000' they worked fine. As I understand the theory, the failure at low pressure is a result in the lack of air molecules to transfer the heat from the nichrome bridge wire to the pyrogen coating on the match head.
Short of sealing the deployment charge in an air tight container, there are a couple of things a person can do to improve e-match performance. The best approach is to use a pyrogen primer coating that is more sensitive, the second is to make sure you have a high density coating on the bridge wire. From what research I've done, it seems a potassium chlorate and antimony trisulfide mixture makes for a very sensitive primer coat. A higher density primer coating can be achieved by using a fairly high concentration of NC in the NCL, in real terms that means keep the NCL syrupy, not watery. If the NCL is too thin, you'll end with the acetone evaporating out of the mixture leaving a lot of little holes in the primer coating.
Now for the disclaimer: What follows are my tests with a very unstable mixture of PC (potassium chlorate) and ATS (antimony trisulfide). Unless you're flying over 25,000', you don't need this to make e-matches. Even if you are flying over 25,000' I would suggest buying your e-matches or a kit to make them. PC is shock and friction sensitive, ATS only makes it much more so. ATS is also poisonous. So, don't do this at home kids!
I'd like to make this process as easy as possible, since I make e-matches on site to stay AFT compliant. So I'd like to make it a single dip, dry and fly type of thing. Of course you can make the uncoated match heads and lead wires in advance since they would not yet have a pyrogen coating. But I don't want to compromise quality or reliability, I'll perform tests and go with whatever ends up being the best e-match.
I've always made e-matches by wrapping the nichrome wire over the ends of the lead wire, while that has worked fine, I'd like to take it one step farther and make the match performance as repeatable as possible. With that in mind I'm going to switch to nichrome wire soldered to a PCB board chip. That should make each and every match head fire with almost exactly the same current. I'm using 40 gauge nichrome wire. While I could get better low current firing using 50 gauge wire, I've got a large amount of the 40 gauge and it's stronger, and well proven to fire from all my altimeters in the past. The PCB material I'm using is a thin, double sided standard copper clad board. I'll post some details of the chip making later.
This batch consisted of 7 parts PC, 1 part ATS and 1 part lamp black.
All chemicals were used "as obtained" right out of the container. I did feel the PC was not as finely powdered as it should have been, but went ahead and mixed up a batch as it was. This was a 9 gram batch, mixing the PC into some NCL, then adding the ATS and lamp black. To give me a better "real world" idea of the current required to fire these, I found a couple of several year old 9 volt batteries to test with. One battery read just under 1 amp on the meter, the other just under 2 amps. I had made 4 matches and first tested them all on the low capacity battery, non of the matches fired. Then I tested them on the 2 amp battery, in turn each one fired with a nice crack sound and sent some sparks flying. Honestly, I was a little disappointed. I had hoped these would fire with less than 1 amp.
Test Batch 2:
5 parts PC, 4 parts ATS and 1 part lamp black.
I made four more matches, as indicated in the table below.
| # | Process | Result | Fire Current |
| 1 | Single dip, NCL uncoated. | Quick burn | 2 amp |
| 2 | Single dip, NCL coated | Medium pop | 2 amp |
| 3 | Double dip, NCL uncoated | Quick pop | 2 amp |
| 4 | Double dip, NCL coated | Quick pop | 2 amp |
The double dip simply meant I applied 2 coats of the same mixture to give a thicker coat on the match head. NCL Coated means I did a final dip in plain NCL to seal the match after they had dried. I always allow the match heads to dry for at least 1 hour before the second dip or NCL coating.
Again, I tested all 4 matches on the under 1 amp battery first, and none fired.
Test Batch 3:
Primer Coat: 3 parts PC and 3 parts ATS.
Burn Coat: 3 parts PC and 1 part lamp black.
For this test I made six matches, using a primer coat only on 2 matches, then using a second dip in what I call a burn coat on the other 4 matches. Half of the matches were given a last seal dip in NCL.
| # | Process | Result | Fir Current |
| 1 | Primer coat, NCL uncoated | Moderate Burn | 1 amp |
| 2 | Primer coat, NCL coated | Moderate Burn | 1 amp |
| 3 | Primer with burn coat, NCL uncoated | Pop and Burn | 2 amps |
| 4 | Primer with burn coat, NCL uncoated | Snap and Burn | 2 amps |
| 5 | Primer with burn coat, NCL coated | Snap and Burn | 1 amp |
| 6 | Primer with burn coat, NCL coated | Snap and Burn | 1 amp |
What's interesting here is that the primer coat only tests fired on the under 1 amp battery, didn't fire on the 1 amp battery with the burn coat, but did fire again on the 1 amp battery when I added the NCL coat over the other 2 coatings. As for my results column, that's a pretty subjective thing. I can only say I think all of the matches would set off a deployment charge, but the snap and burn would seem to be ideal. The pop and burn is almost like a small fire cracker going off, and may be too explosive for what we are trying to do. The moderate burn I suppose is fine too, but seems better suited to an igniter than an e-match.
At this point it seems test batch 3 is the best, with the process on matches 5 and 6 the best of batch 3. This isn't a huge surprise since this is the most common formula I found for this type of e-match. It does seem to indicate the multi dip process that I really didn't want to do is best, but frankly it just doesn't take that much more time or effort to multi dip the matches.
The next set of tests will include the primer and burn coats of test batch 3, all will be seal coated with NCL as it seems to increase the sensitivity and allowing lower current firing. This set of tests will now move vacuum chamber testing.
Test Batch 4:
Primer Coat: 3 parts PC; 3 parts ATS
Burn Coat: 3 parts PC; 1 part lamp black
| # | Process | Altitude | Result | Current |
| 1 | Primer, Burn Coat, NCL Coat | 42,000' | Slow Burn | 2 amps |
| 2 | Primer, Burn Coat, NCL Coat | 42,000' | Slow burn 1 side only | 2 amps |
| 3 | Primer, Burn Coat, NCL Coat | 0' | Snap and fast burn | 2 amps |
| 4 | Primer, Burn Coat, NCL Coat | 42,000' | Snap no burn | 3.5 amps only |
| 5 | Primer, NCL Coat | 42,000' | Snap no burn | 2 amps |
| 6 | Primer, NCL Coat | 42,000' | Slow burn | 2 amps |
These tests confirm what my previous tests had indicated, what a huge difference pressure makes (or lack of it). Test sample 3 was done at ambient pressure, just to make sure nothing was wrong with the e-matches. Sample 4 was fired using a fresher battery, to see if more current helped. It didn't appear to help at all. None of the matches fired with the under 1 amp battery, while all fired with the 2 amp battery expect sample 4 which was only tested with the 3.5 amp battery.
When a result indicates a snap and no burn, what happens is the end of the match covering the bridge wire blows off, the remaining pyrogen on the match head does not burn. Sample 1 blew the tip off as well, but the remaining pyrogen did burn. Sample 6 did not blow off at the tip, but rather started a very weak burn that took several seconds to complete.
I'm not sure where this leaves me, it's certainly better results than the black powder green meal e-matches that didn't burn at all. I'm concerned the snap and no burn results might not be enough to get a black powder deployment charge initiated. I'm also at the limit for my old vacuum pump, it's been getting weaker this last year or so, and the pressure equivalent of 42,000' is the most I can pull. I may be looking into a new vacuum pump in the near future. I'm sure at even lower pressure the e-matches are going to be even harder to get get lit.
I may have to sensitize my pyrogen even more, sulfur comes to mind, but that's really playing with fire (pun intended). I'll do some more research on what chemicals may work best. I may also try firing some small deployment charges in the vacuum chamber to see if these matches would work reliably at these pressures.
April 2011:
I've been doing a few more e-match tests for an upcoming launch. Not that I'll be flying high enough to worry about the super sensitive e-matches, but I had to make a few so I did a couple of more tests.
Test batch 5:
4 parts PC; 3 parts ATS; 1 part sulfur in NCL binder
I've done a few tests of this mixture and it seems to be doing well just as a one coat mix. I'll do more tests and publish the data, but for now this is a mixture I'm using. I did one test at 42,000' with my old vacuum pump, then another test with a newly purchased pump at about 109,000'. The matches have worked fine, but...
This was a 1 gram charge at about 109,000' in my vacuum chamber. Notice the e-match worked fine and the black powder blew the latex tube apart. But all those black specs on the paper are unburned black powder. Another nail in the coffin for latex tube as charge holders at high altitude. While I didn't weigh the remaining powder, it was clear the vast majority didn't burn. I'd estimate only 20% burned.