What is the primary objective of pre-breathing a manual CCR?

[Mr_C]

I pre breath until I see the scrubber sensors start to report its guess at scrubber duration. It is my final check that all the parts of the system is working. Ironically, because my rEvo is a hybrid system, I have to turn the O2 off until the dive or the leaky valve will slowly drain the tank. So the pre-breathing check to make sure your O2 is on - is negated. And yes - there have been several dives where I have discovered this when I cannot manually pump up my PO2

 

[davehicks]

During pre-breathe on my Spirit I'm just verifying that PO2 is stable and the unit is doing what it's supposed to (no sensor/monitor issues and no unexpected loop volume/gas pressure loss). Since it's mechanical there's very few things to monitor while doing the pre-breathe.

I'm fairly new (100 hours range), but I agree with other points already mentioned (5-minute won't reliably catch scrubber issues, tests prior to a pre-breathe will catch pretty much everything else, etc.).

I've been diving the Sprit and its predecessor (Sport Kiss) for nearly 20 years. I don't pre-breathe anymore but I do perform checks of O2 flow (watch and listen), O2 injection (see all 3 sensors register), ADV function, and BOV function. This takes about 10-20 seconds. I've never seen any benefits to pre-warming the scrubber even in very cold water. This is in addition to pre-dive pos/neg tests and a full check at assembly time.

The only issues I have experienced (1 each in 20 years) were:

• Quick disconnect to the off-board diluent failed and would not pass gas (never got in the water)
• The O2 orifice clogged and barely flowed, but the MAV injection still worked (discovered this watching the PO2 degrade during a dive in unusual ways)
• The MAV o-rings got worn and was leaking excess O2 into the loop (discovered watching PO2 keep rising in unusual ways)

I resolved all of these issues by doing a New Years service in January of all three components. I carry extra Quick Disconnects as well because they do sometimes start to bubble or get sticky at random. I also carry a spare MAV+Orifice from an older unit I salvaged for parts.

I see these examples as a strong selling point for mCCR units. Using your own brain as the master controller you will see these anomalies quickly and adjust in ways that an eCCR controller would not.

I did get a CO2 hit once years ago on the Sport KISS, but none of these checks or a 10-minute pre-breath would have caught it. (2nd dive on a fresh scrubber 24 hours after the first with 4 hours of driving in between) The Sprit scrubber is much better than the Sport and I think a similar occurrence would be highly unlikely.

 

[trimix1050]

I do the pre-breathe for about 3-5 minutes on every dive. For the manual system to make sure the O2 is flowing and for the electronic system to make sure the solenoid is firing. It also helps to warm up the scrubber before you start the dive. It also gives me a few minutes to think through my dive plan, most of my dives are cave dives.