On a price basis co2 cylinders are much less viable for retestingI got told for people wanting testing on co2 (albeit co2 is becoming less and less popular) can use fire extinguisher service and refill stations not sure on how much it costs tho
What size are exempt cos I've bought an old phantom with 3 co2 tanks seem small a lot smaller than the 20Oz and don't need big tanks to run a pump will post pictures in a bitOn a price basis co2 cylinders are much less viable for retesting
Small cylinders may be exempt from testing
Unless you have specific circumstances for co2 I'd recommend switching to a 3000psi steel/aluminium cylinder