Oxygen cylinder and regulator assembly leakage rates

ATA 35

By: Nicolas Audet - Aircraft Systems, Technical Services, Global Series

The leakage rates exhibited by elastomeric seals used on certain cylinder assemblies can be more challenging to evaluate than that of traditional metallic seals. This is due to the permeability of elastomeric seals. The purpose of this article is to provide guidance in evaluating the actual leakage rate versus the allowable leakage rate using leak detection compound.

Once leak detection compound is applied to the valve/cylinder, observe the area for a stream of bubbles greater than Ø.062 in. (1.6 mm) originating from a single point or two or more bubbles that grow and then release (Reference: ASTM E515). Verify that bubbles are continuing to form and release.

If there are no bubbles greater than Ø.062 in. (1.6 mm), or if foam appears but bubbles do not propagate, the equipment is considered to have “no evidence of leakage.” (See Figure 1). If leakage is present, estimate the diameter of the bubbles (See Figure 2). Quantify the number of bubbles per minute (or over 15 sec. and multiply by 4). Use Table 1 to identify leakage rate and compare with the maximum allowable leakage rate. Dry the external surface of the equipment tested using clean, dry air (that is oil-free). If there is doubt or the result is inconclusive, repeat the procedure.

When performing leak checks, it is important to remember that MIL-PRF-25567 is the only leak detection fluid acceptable for use on aircraft and other related oxygen systems. A leak rate of 5cc/hr or less is acceptable on the Oxygen Cylinder and Regulator assembly.