In this post I would like to discuss the challenge of controlling the flow of the inert gas. The post has dual intent: to get someone to review my calculations and spot mistakes if any, and to help others with theirs.
Sources: post by TiredHorse that started this thread, TPP Handbook, Five Last Acts, Final Exit.
I'll skip the part with constructint the bag, that is covered enough, and will go straight to the gas (Helium/Nitrogen, with Helium being my choice) and tubings.
I'm going to use metric system throughout. My imperial friends can just remember that 1 inch is 2.54 cm, and 1 cu ft is 28 liters.
Recommended flow
Recommended "Letal dose" by varied sources is 125..250 liters. This being inert gas, it isn't letal per se, as it works by pushing away oxygen. So this dose is just amount of inert gas that is enough to maintain big enough flow for big enough time.
How much flow is big enough? 10-15 liters per minute. If it is too little, carbon dioxide may not be evacuated fast enough and panic will happen, or the bag can stick to nose/mouth, causing unpleasant experience.
How much time is long enough? Time to unconsciousness is on average 25 seconds (10-120 seconds), and time to death is on average 13 minutes (2-40 minutes). I did not find where the point of no return is, i.e. whether the flow needs to be stable up until the death, or can be stopped sooner. Sources recommend gas to last for 20 minutes.
From here we can see how important it is to set the right flow: too little, and you are up to very unpeasant experience, too much, and gas runs out too soon, causing you to survive but have permament brain damage (I could not find info on how likely this is, and how that brain damage will manifest itself - info appreciated).
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I'll talk in a moment about how to control gas flow, but first let's think how we can measure it, to ensure we set it up correctly.
Standard small balloon has diameter of 9 inch. This is 0.2286 meters. Radius is 0.114 meters. Volume is (
=4/3*pi*r^3) 6 liters. That means once everything is set up, you can connect balloon to your tubing, and see how quickly it inflates, and calculate you effective flow. I.e. if balloon inflates in 20 seconds, flow is 18 liters per minute. Call it a poor man's flow meter.
Knowing balloon size, you can also calculate tank volume in case they provide it's capacity in balloons. I.e. if they say tank can fill 200 9 inch balloons, you can calculate that it has capacity of 1200 liters. It is safer though to get industrual tank that specifies volume in standard liters.
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Another important consideration is whether the gas is pure. There have been reports of balloon gas sold being not pure Helium, but diluted with air. They do so to save money, and also to prevent suicides. The way I handled it, is by purchasing oxygen meter, inflating a balloon from each of the tanks, and putting oxygen meter sensor inside the balloon. Measurement shows less than 1% oxygen, so gas is pure in my case.
I've got three tanks here, with volumes of ~300, ~900, and ~2400 liters. Let's calculate flow required if using either of these.
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As we see, if I were to use the smallest tank, I need to control flow very precisely, and it is too easy to make it too small, or too large, causing failure. So bigger tank is preferred. With biggest tank I have the most flexibility.
Once I set up flow control, I can measure the flow using a balloon method described earlier.
Next post: how to control the flow.