Dil flush - whats the story ?

edster

I was idly wondering to myself - when you do a dil flush, how much of the loop is initially flushed? The fresh dil is coming from the inhale side, so it can come into the inhale counterlung, through the DSV, into the exhale lung and bingo bongo, out the dump valve (assuming you're holding it open). Alternatively, it could also go back through the T-piece, back into the scrubber canister, through the sorb, out the exhale hose and - bingo bongo - out the dump valve. That would be in the 'wrong' direction. Question is - does it go that route as much as it does via the DSV route? Either route would leave most of the exhale lung still full of used gas. It will dilute the 'old' gas in the loop as you breathe it round, but how much does the sorb limit the flow of gas before that? How much of the loop is flushed before you breathe it around? Any thoughts?

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Deep Thought

Sounds like you are describing an Inspiration loop, as the concept varies slightly depending on which rig you are flushing, so I’ll take it as such. As an example the meg has an opposite loop flow to inspiration, the KISS classic has back lungs without push button diluent injector, etc…

The question you ask: “when you do a dil flush” the reasons for doing a diluent flush are if you are in doubt of the integrity of the gas in the loop, or you are flushing a know gas past sensors to validate the display.

I have seen this skill taught with different techniques, and I have seen divers demonstrate this skill in fashions that I feel are inadequate. As the technique varies from rig to rig, I will be discussing an inspiration loop.

If the goal is to rid the loop of unknown gas and replace it with a known breathable diluent so you may start problem assessment, then the act should be vigorous. The goal is to move the gas as you describe from the inhale CL (point of injection), past the DSV, onward toward the exhale counterlung, through the canister, around again, exiting at the most advantageous point. At some point you must allow gas to exit the loop or you just over-inflate the CL and go buoyant. If your technique is correct you will see a readily identifiable change in the display, if the display lags or you see very little change you are not using the correct technique.

Example where I have seen divers flushing with ineffective techniques include; pushing diluent (holding dump valve in) but allowing the dil to flush straight out the mouthpiece. I have watched the divers P02 and see little or no change…think about it- the gas goes in and out without pushing through the loop past the sensors, then you know it is not adequate. Not to say that you cannot allow some dil to bypass your lips when the loop is flushed, it is the technique.

Your body position, how you may squeeze the CL, pull the dump cord, length of dil flush, etc. all contribute to effective “Flush” . All too often I see little wimpy flushes…not effective to rid the loop but only to dilute what was there.

Instructors often tell their students to practice a skill now and then to stay proficient and keep motor muscles memory fresh. I highly recommend that divers do dil flush to ensure that they really see movement on the PO2 display, and definitely do a dil flush to see how much gas is required to validate a PO2 display with a know diluent.

Ron

Please Flush Correctly

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