If you go back to one of the earliest posts found on this website, you can read about the importance of water to the Apollo Moon missions. The story below relates to a device that was developed for the Apollo backpack, but never included in the final versions.
Just as a quick reminder, you may remember that water was boiled off into the vacuum of space by the Sublimator in the Apollo backpack. Since the water quantity left in the backpack reservoir was then a crucial issue, we thought that adding a water quantity sensor would be a good idea. That way an astronaut using the backpack could tell how much time he had left before he had to return to the Lunar Module from walking on the Moon’s surface. So, one of our designers designed one, and it was built and tested. It worked as planned, but…
When tested, it always overestimated the amount of water used by quite a bit. That came as a surprise, a very disappointing surprise, and one that took a while to figure out. And then one day it dawned on one of our engineers just why it happened. Unfortunately, it made perfect sense, and there was nothing we could do about it.
If you remember, there was nitrogen dissolved in the water held in the Reservoir of the backpack. And, as the water was released for use in the Sublimater, the pressure of the water dropped dramatically as this happened, and the nitrogen would come out of solution and form bubbles in the water, much like the formation of bubbles when you open a soda. Well, that now happened inside the newly designed Water Quantity Sensor. The Water Quantity Sensor then counted the volume of bubbles as it did the volume of water that passed through it. As a result, the sensor was always wrong on the high side. Given that sort of reading, an astronaut on the Moon’s surface would always stop his moon walk quite early. That wasn’t acceptable, so the wonderful little mechanical Water Quantity Sensor was never used in the final design of the backpack. The astronauts then had to limit their walks by timing them.
Another great idea down the drain, if you’ll excuse the pun!