Recharging the PLSS

Except for Apollo 11, surface crews had an important item of housekeeping to perfonn once they had returned to the relative safety of the LM’s cabin, namely to replenish their PLSSs if they had another EVA scheduled. Oxygen and water tanks had to be refilled – oxygen for breathing and water for cooling. Batteries for power and lithium hydroxide canisters for carbon dioxide removal had to be replaced. The Apollo 12 crew were the first to carry out these procedures and the engineers saw a chance for a data point.

The early missions tended to be pathfinders for the later extended missions and it was important to know just how– long the consumables in the PLSS had lasted in true lunar conditions as opposed to rehearsals on Earth. The crew could monitor oxygen consumption on their RCU displays and indications of a crewman’s metabolic output could be inferred from the biomedical data that was telemetered to Earth. What was missing was an indication of how much water had been consumed by the sublimator during the EVA to keep a crewman cool, so Conrad and Bean were tasked with weighing the remaining water after their first moonwalk. However, Conrad was none too enamoured with the equipment supplied for the job.

"Houston, you’ll never believe what wfe have been doing for the last 35 minutes.”

“Go ahead,” said Gibson, the Capcom in mission control.

“I am going to take this 35-cent scale that they sent out here to weigh these bags with and break it over somebody’s head!”

NASA had supplied an off-the-shelf spring scale designed for Earth’s gravity with the idea that a lunar measurement would require it merely to be scaled appropriately. But one feature had been overlooked, as Conrad explained after the flight. “The scale ought to have been set to zero. If anybody had thought about it. including myself, the spring tension in the scale itself was never zero in one-sixth g. As I unscrewed it to zero. I unscrewed it all the way, and the screw, the spring and everything disappeared into the bottom of the scale. We had some difficulty putting that baby back together again."