# Measuring Oxygen Quantity

How did the Apollo program measure Oxygen Quantity? The Command Module had a Pressure Gauge and a Quantity Guage for the oxygen bottles. Measuring pressure is simple, but I don't see how quantity is measured in a low gravity environment.

Assuming you are asking about the oxygen in the Service Module tanks - the stuff used as a reactant in the fuel cells - quantity was measured by means of capacitance probes in the tanks.

(Source: Apollo Experience Report - The Cryogenic Storage System - I added the red arrow)

This only worked because the oxygen was stored as a supercritical fluid. If you are not familiar with that, please read this answer:

Why would oxygen be stored as a super critical fluid?

The key takeaway from that answer that applies to the quantity measurement question is that the supercritical fluid is "gas-like" in that it will occupy the entire volume it is contained in.

The gauge measures the density of the fluid. Since the supercritical fluid completely fills the tank, and the tank volume is known, if you have the density, you can calculate the mass of the fluid.

As Wayne Conrad commented, the famous "tank stirring" plays into this. Stirring the tank kept the reactant mixed up so that its properties would be uniform throughout. An unstirred tank in free-fall conditions tends to "stratify" - develop density gradients related to the distance from the heater elements.

More on this stratification effect here:

What does "stirring tanks" mean?

(I thought quantity gauging was covered in one of those "supercritical fluids" questions back in 2019, but I only see it mentioned in comments. If I missed it and this is a duplicate, please VTC.)

• I understand the dialectric changes between the oxygen gas and fluid states, and a capacitance probe can measure the average dialectric. However, the wouldn't the gas/fluid ratio change will pressure & temperature and thus alter the reading? Are these compensated for in the gauge? Apr 24, 2023 at 14:10
• @JamesRix There is no "gas/fluid ratio". The whole point of using the supercritical fluid design is that the entire tank is filled with single phase fluid. Please read the linked question about supercritical fluids. Apr 24, 2023 at 14:18