SurgiVet Enviro-Pure Charcoal Canisters. SurgiVet Enviro-Pure charcoal canisters can be used on their own as a conventional charcoal canister or with a SurgiVet Pure-Guard® monitor. Canisters need to be weighed to estimate expiration of the charcoal. Disposable, simply dispose of expired canister and replace with a new canister. 6 canisters per box. Mounting bracket also available, sold separately.