Best way to pre-kill?

[Badgemash]

death by CO2 has nothing to do with death by loss of oxygen.

its true, mammals cannot detect a drop in blood oxygen levels or atmospheric oxygen levels, mammals dont really know whats going on and then they pass out.

BUT MAMMALS ARE AWARE OF BLOOD CO2 LEVELS AND FREAK OUT COMPLETELY WHEN A RISE IN BLOOD CO2 IS DETECTED.

thats like suffocating in a room without fresh air. its also half of the reason why drowning is so terrible. you do feel that the air is getting bad, unbreathable, you get terrified, and suffocate in agony. that is all the response to blood CO2 level increase. its sneaky and undetectible only when you have a drop in blood oxygen levels WITHOUT corresponding CO2 buildup, like when a pressurized cabin leaks or on mount everest.

with CO2, its not like getting drowsy at low pressure or high altitude. you can have that for your rhodents if you use a noble gas or nitrogen. you cannot have that when CO2 is used. if you want to grant mammals a peaceful death by an undetectible drop in oxygen so that they peaceful pass out, you can use basically anything other than CO2 to displace the oxygen. hydrogen, nitrogen, helium, argon, whatever.

just inhale some CO2 already and tell me it feels just like one of these military pressure drop experiments, i dare you. the fact is it doesnt. you get that light-headedness drowsyness without fear and panic if you overdo it with breathing helium for funny voices. CO2 is totally different and just dreadful.

I do understand what you are saying, and agree to some extent with one key point. What you are talking about is acute hypercapnia, which you would induce by precharging the chamber with CO2 or pouring water on dry ice. I agree, that would be painful and inhumane. I am talking about inducing hypoxia through oxygen displacement and and slowly increasing blood CO2. You are correct that you could also induce hypoxia with other gases, and if you chose to use and have access other gases that have backing from the veterinary community I fully support your decision to do so.