Counterlung Pressure Decay Testing

The counterlung is a pretty important part of a rebreather- for the design I am pursuing, it is doubly so- not only is it a single counterlung, providing both buoyancy and breathing gas, but it houses the scrubber canister. In addition to housing the scrubber, gas addition and gas dump are located on the counterlung.

Obviously, water should be kept out of the counterlung, and gas should be prevented from escaping the counterlung. There are some qualitative tests that can be done to tell if something is leaking- spraying it with soapy water to see bubbles, or holding the counterlung underwater can show if gas is coming out. Or, the counterlung could be filled with water and I could look for water leaking out. However, both of these tests require careful observation and it is not always easy to see where the leak is coming from- for examples bubbles or water can leak from one area, get trapped, and appear to come from somewhere else. Its also hard to quantify how bad a small leak is. For a rebreather, there should be no bubbles at all.

For a more sensitive and quantitative assessment of leakage, a pressure decay test can be used. This is easier than measuring extremely low gas flow rates. The concept and execution are simple- fill the counterlung up to a pressure, and observe it the pressure goes down. This gives you an idea of how leaky something is, and its used on everything from respirator masks to space craft parts. The leak decay is much better than pressing the bag by hand, since the test can be preformed at the operating pressure of the bag. Ideally, its never more than ~20″ of water, from roughly the mask of the user to the bottom of the counterlung. Not coincidentally, 16″ of water is 40 millibar, and is the EN-14143 specification for the cracking pressure of the overpressure valve.

Magnahelic 650-20 Indicating transmitter, in the background is the schrader valve for adding gas and in the foreground is the gas tap plug that goes to the transmitter

Here is the pressure gauge I’m using. Its a transmitting gauge, which means at some point I’ll hook it up to some kind of data acquisition device to get some time series data, but for the moment I am just reading off the gauge face. It is also extremely sensitive- each division is .018 PSI or ~125 pascals.

counterlung in the vertical position- metal bar is on the bottom

To give an idea of the sensitivity of the gauge, I was pleased and surprised that it can indicate a change in pressure when the sealing bar on the bottom loads the walls of the bag (by lifting the bar with my hand, it takes the weight off the fabric structure). This is important because hopefully the leak is very small and very slow.

Issues with data collection

There are a few issues with collecting good data. One is that temperature plays a huge role in pressure, and so a room full of windows and a struggling AC (temperature fluctuation) and storms rolling in and out (air pressure fluctuation), its hard to measure a small pressure differential. I have actually seen the pressure go up, which is the opposite of what you would expect in a leak decay test.

To resolve this, I might do one of two things- either test during a calm night when the solar effects and weather effects are minimized, or add a temperature sensor and automatic data logging to try to correct for the temperature swings.

Another annoying detail is that my hose is a hair too small, and sometimes it starts to slip off the very delicate barbs (which occasionally snap). So I may need to characterize the leak of these barb connections to make sure I don’t accidentally characterize that leak as the leak of my system, if the leak is significantly large.

Preliminary Results

Two important things were done- first, qualitative submersion test to look for gross leaks. A few were identified around the sealing bar, which is by far the sketchiest seal. The leak always occurred right in the middle of the bar, where the seam from the bag was. Replacing the hand-tightened nuts with nyloc nuts and cranking down with a socket solved this issue.

I also ran a few leak decay tests, which turned up the aforementioned pressure problems. However, the pressure did hold for a few hours at only seemingly a loss of a few inches of water, which seems acceptable but I think further testing will be important to do.

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