Exit Bag and Inert Gas Basics —A very, very long post, I know, but I feel like I've fielded a lot of questions about this, so I figured I'd spell out everything I know. Others on the forum should please add to this thread whatever reliable technical information they feel is appropriate for someone attempting this method to have.
The idea behind using an exit bag with inert gas is to create an atmosphere around your head that is both free of life-sustaining O2 and can carry away the exhaled CO2 that would activate your hypercapnic alarm.
The Gas:
You will need to keep the inert gas flowing at 15 liters per minute (Lpm) for 40 minutes to be confident of ending your life —in other words, you'll need a minimum of 600 liters of inert gas. In the past this would have been helium (He), but due to the uncertain availability of genuinely pure He, the best current options are nitrogen (N2) and argon (Ar). Both are reliably available in pure form (no air contamination) from stores that supply welders. N2 is also available from some brewery supply houses, but the purity of the gas should be confirmed to your satisfaction. Both N2 and Ar should work to ctb and are similar enough in their properties to be treated identically for use with an exit bag.
Pressurized gas cylinder sizes are not standardized across the industry, nor internationally, and it can be difficult, if not impossible, to tell how much gas they contain from a photograph on a website.
In the US, a 20 cubic foot (cf) cylinder is the smallest you should use for ctb; a 40cf will allow some margin for flinching, practicing, etc. "A 20cf" and "a 40cf", or "a 20" and "a 40" is nomenclature US welding supply houses will recognize, so asking for either should get you the desired product. A 20cf cylinder is quite small for industrial use; I've had clerks tell me, "well, we have a 40, but we'll need to special order a 20." It's up to you what you do in that situation. Personally, I have a 40cf cylinder. It has allowed me to flinch and abort my attempt three times now, and I don't need to worry about refilling it. I bought my N2 at AirGas, a national company here in the US. They do not demand any sort of professional certification for purchasing inert gas, and no more than the usual forms of ID depending on payment method.
I know nothing of gas cylinders outside the US, so if you are using other than US-typical cylinders, you'll need to call and ask to make sure whatever tank you're contemplating holds 600L of compressed gas. Hopefully knowledgeable forum members will add cylinder sizing information to this knowledge base.
Cover story: N2 and Ar are both used for welding. N2 is used in beer brewing. In my day job, I use both N2 and Ar to flood partial cans of expensive paint, to displace the O2 in the cans and prevent the paint from skinning over and going bad. It's a cost saving measure. The clerks I've bought gas from have never heard of that trick, which has meant they don't know enough to interrogate me on my motives or to try and quiz me on my welding knowledge. They have merely said, "huh! That's a new one!"
The store clerk may ask how you're carrying the pressurized cylinder "back to your shop," since pressurized cylinders can be very hazardous to transport (the valve is vulnerable to damage in an accident). I haven't had any trouble when I have told the clerk, "in the back footwell of my car, and very carefully padded!" What they really want to know is that you are aware that a pressurized cylinder is a missile waiting to explode if anything damages it, and that you know enough to not let that happen.
