Ejection Canisters

iter said:

Tony, you need to figure out which terminal in your flight computer is + and which is -. As you're saying, with an e-match, you can plug it either way and it works, like a light bulb. Tantalum capacitors are like LEDs, the only work one way, so you need to make sure where the positive and the negative terminals are.

Ari.

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Could you use two caps in parallel with opposite polarity? It might take a few diodes to prevent the altimeter from charging the correct polarity cap instead of bursting the other one, but that shouldn't be too hard. This would eliminate the need to figure out the polarity of the altimeter and trace that to the capacitor, which seems like it'd be a likely failure mode. It'd increase the cost a bit, but it'd probably still be <$1.

Sure, you can do that. Or you can use a voltmeter if you have one :=)

Every altimeter I know operates with DC, never switching polarity of the current. You only need to figure it out once. A simple way to figure it out without a voltmeter is to connect a capacitor one way and see if it blows up when altimeter applies current. If it doesn't, plug it in the other way and repeat :=)

Ari.

cwbullet said:

I have seen people use plastic wrap, but that make me worry of static charge. Is that me overthinking or is it a possible problem?

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Plastic wraps don't use static to cling to other objects. If it was true, the wrap wouldn't tend to stick to itself as it does (like charges repel each other). So, I don't think static is a problem.

brianc said:

I've been using plastic wrap for about 4 years now. No static problems
even during Florida dry winters.

Here's how to make the plastic wrap charge holders - https://www.info-central.org/?article=302
(scroll down near the bottom).

Click to expand...

I use the same method and in the same Florida environment. It's worked well for me. One advantage it has is that the same method works for small ejection charges and large ones.

-- Roger

Testing:

I tested some canisters this week.

1. plastic wrap with BP - Good results.
2. Plastic Wrap with Pyrodex - poor results. Not very energetic.
3. Surgical tubing with BP - Very good results.
4. Surgical tubing with Pyrodex - poor results. Not very energetic.
5. 12 gauge copper shotgun shell with BP: Great results and very energetic.
6. 12 gauge copper shotgun shell with Pyrodex: Good results and energetic.
7. Straw with BP - Great results and very energetic.
8. Straw with Pyrodex - FAIR results.
9. Tape with BP - worked very well at containing the charge with good energy.
10. Tape with pyrodex - Poor results.

I did charges in each type and based it upon separation. I would have preferred to have a pressure catheter. In my opinion, pyrodex will work, but I will not use it. It is just not as energetic but if you must, use good containment. Tape will work for BP but even it seems to be more energetic with a firm containments like a 12 gauge shell.

This not scientific and it more of myth busters test, but it will work for me. I may start using more tape, plastic, glove fingers once I used up my plastic canisters.