"Build Your Own Diving Lung", Revisited in 2018 -2022

[antique diver]

I recently acquired the makings of Bendix Lung conversion #3. Doesn't look like much in this photo, but I have hopes for it to be a good one. It will be a "back to the basics" diving lung, utilizing the slightly modified/simplified internal first stage. More photos soon, when it starts to look recognizeable.

Here's the #3 with all the all the important inhalation adjustments in working condition at 1000 psi tank pressure. The cracking pressure of 0.4" of water column is shown on the Magnehelic. It can be adjusted to lower effort, but may get a bit out of control in actual use. I'll have to experiment with that when I take it diving. It stays very close to same at all usable pressures, but gets slightly lower at 300 psi due to upstream second stage. I haven't decided on the exhalation side configuration yet, but that will be the next to dream up.


Breathing effort stays comfortably low as shown in the following video clip. It can adjust lower by removing a short restriction tube that easily unscrewed and removed with the inhalation hose disconnected. Diaphragm cover is removed to show it in action with normal to fairly heavy inhalations.

I didn't have luck trying to get directly to the 9 second video, but it is accessible with this link:
https://i.imgur.com/Ff532Xz.mp4

[antique diver]

This is the original appearance of the first stage pressure sensor in the original two stage Bendix Oxygen diluter unit (circa 1942). Changes in ambient pressure adjusts the clearance between the small 0.067" HP inlet orifice and the tapered end of the HP seat via the lever. As flying altitude increases the HP seat retracts from the orifice to send more Oxygen percentage to the aviator.



Unfortunately, diving with the Bendix "diving lung" makes this mechanism work in an opposite manner than designed, as it gradually reduces the seat to orifice clearance as diver descends. So it took a lot of adjusting and tinkering to get this system breathing ok below 15-20'.

We had been diving it with 1800 psi tanks, which seemed to work ok, but the life of the HP seat tip was taking a beating and needed removal for refinishing way too often. Three years later I found out that the Bendix was originally designed to work off a supply pressure of no more than 500 psi. The HP seat just wasn't up to that, so I added an in-line reg to drop the tank pressure to about 450, and leaving the original first and second stages intact while keeping the unit operating within its design parameters.

Apparently the abuse of diving for a few years with the 1800 psi had been hard on the bellows (as predicted by Phil!), and it finally gave up during a dive. It wasted some air and ascent was required.

Photo of the failed/blown first stage bellows shows how it cleanly separated. Phil was right about metal fatigue.





Well that was the start of a whole new approach to converting these beasts, and I have had much better luck with my recent two (and final) "diving lungs".