Lithium hydroxide canisters

To the left of the LEB (Lower Equipment Bay) is the LHEB (left-hand Equipment Bay), where panel 350 contains two square receptacles, labelled A and B. These receptacles share common inlet and outlet manifolds to the rest of the ECS (Environmental Control System) Pressure Suit Circuit and each contains a neatly fitting filter canister. When the pressure suits are not being used, a valve allows cabin air to enter the circuit where it will be cleansed of particles, cooled, dried and scrubbed of exhaled carbon dioxide (CO2) and odours. The filter canisters contain granules of lithium hydroxide (chemical formula - LiOH) to absorb CO2, and activated charcoal to remove odours. The canisters are changed, one at a time, at roughly twelve hour intervals so that each canister is in situ for around 24 hours. Twenty four canisters are carried in all, two installed for launch and the others stowed in various compartments around the CM cabin. The Flight Plan advises the crew on changes and where to store exhausted canisters. About now, canister 1 is being replaced by number 3, with 1 going into stowage compartment B5.

The Lunar Module also carries LiOH canisters, two installed for launch and replacements kept within the MESA (Modular Experiment Stowage Assembly), a section of the LM Descent Stage which is accessed during the lunar EVAs (Extra Vehicular Activity). The Apollo 13 emergency highlighted the incompatibility between the square canisters used in the CM, and the round ones used in the LM. It also demonstrated that having the LM's spare canisters outside the cabin was unfortunate. As was well illustrated in the movie, Apollo 13, the crew of this extraordinary mission had to build a rig out of available items in the cabin to allow the CM canisters to work in the LM's air circuit.

It is interesting to note that although the two canisters in the Command Module are identical, the two in the LM are incompatible with each other. Of the two, only the cartridge in the "primary" circuit is used, with the "secondary" only for emergencies. The secondary LiOH cartridge is actually interchangeable with the PLSS LiOH canister, allowing 12-14 hours of purification capacity while the PLSS's are onboard.

Scott, from 1998 correspondence - "Maybe this implication becomes the same as in the movie Apollo 13 - whereby it appears to be some form of mistake or oversight. Not so! As with all of Apollo (and to continuously emphasize this point), everything was very carefully planned and very precise in its objective. In this case, as you point out, the secondary cartridge will fit in the PLSS; and the LM was designed for only one primary cartridge - two people, smaller cabin, shorter life, etc., etc. Pretty clever to have the LM secondary and PLSS cartridges interchangeable!" (ap15fj)