Diluent Discussion From RebreatherWorld:
Last week, Underwaterbear began a great thread about the choice of diluent gas in a rebreather. Over the years, I have witnessed a great change in general practices. In the early days we always chose a diluent to match our planned PO2 at maximum depth. In other words, if I was planning a dive at Ginnie Springs in 100 feet of water, I might select EAN30, if I was planning on a 1.2 set point. This resulted in less oxygen use, easier SCR bailout, and easier buoyancy control since the ADV or manual diluent injections were close to target and would not result in additional solenoid firing.
Today, some people would call this practice dangerous, arguing that you need to have a leaner gas available to quickly lower PO2 in hyperoxic situations. Others would argue that you cannot verify sensors properly at maximum depth. Still others would suggest that it is easy to accidentally exceed MOD. More often these days, I see people choosing 10/50 to use in all of their diving activities, deep or shallow. Personally I believe this presents a much greater risk.
Let’s take each point separately…
Hyperoxia – a diluent flush with any gas will quickly lower PO2. Try it for yourself. I have found that it does not take any more gas to bring the loop back to a safe PO2 if you are using a slightly richer diluent. On the flipside, I feel a little safer recovering from hypoxia with a richer diluent. At max depth, I don’t need to shock my body with rapidly shifting partial pressure, just get it back to the target PO2 and safety.
Sensor Verification – if the sensors are all reading correctly, you are not likely to need a diluent flush for verification. When the sensors are whacky, you will still be able to quickly isolate errant readings with a flush. The bad sensor will stay bad. Alternatively, ascend a little or lower PO2 by breathing down the loop and then make your verification flush. Knowing that the diluent flush is a safe, breathable, target PO2 gives you plenty of time to assess the sensor issue in safety.
10/50 Diluent on Shallow Dives – What’s the point? You should be starting with a full diluent cylinder for any dive, so why not fill it with the best possible mix?
Finally, I just posted to RebreatherWorld that I hate hard and fast rules. Take all this to heart and then make your own choice. Every environment is different. I dive in caves where maximum depth in not likely to be exceeded. You might dive on wrecks where the plan changes as quickly as the weather. Every rebreather is different. They all have Achilles Heels. Some have known faults that result in hypoxia. Others have a greater risk of a solenoid stuck open. You must make your own risk assessment and the best choice for yourself. Rebreathers belong in the hands of thinking people, not those that want hard and fast rules of engagement. Jill Heinerth
