Dave Talks About Rebreathers – The Basics, Part 1
David Snyder Dec 10, 2010
David Snyder diving a
Megalodon Rebreather
Todays’s post comes from David Snyder, Technical Rebreather Instructor. David answers some questions about rebreather diving which show the basics of rebreather diving.
Q: Do you need a specific class for a specific type of rebreather?
A: Yes, the physics and physiology is the same for all units but each rebreather is different, hence specific classes.
Q: How do you know when it is time to change your scrubber material? Is this a problem with divers trying to get more out of the material then they are suppose to?
A: You track the dive time on the scrubber media and change it per the manufacturer’s recommendation.
Q: What I am now curious about is, you said on a CCR, at 60ft your PO2 is 1.3. Wouldn’t that figure limit your depth, or do you have the ability through out the dive to adjust that percentage? And if that is the case, is it that ability that gets divers into trouble?
A: You dial the set point in and that is then constant, independent of depth. You asked, if you descend won’t the PO2 spike? It would except that because of Boyle’s Law the volume in the loop will decrease as the pressure increases and you will need to add diluent to the loop. This has the effect of negating the spike. As you ascend you vent gas from the loop and O2 is added to maintain the set point.
Q: Is it better (more flexible) and safer to be able to manually inject O2 right into the breathing loop, or is it better to have it done automatically?
A: Since you retain the ability to manually add gas in all units, some divers will adjust the set point lower than their target and then “fly the unit manually” and use the eCCR controller as a backup. With some units this is easier than others. All eCCR training covers this procedure.
Q: Okay, how the heck did you come up with .59 PO2 at 60 feet? If pressure at sea level is 14.7, and the air we breath is roughly 80% nitrogen, and 20% O2, if I then multiply 14.7 x .2= 2.94, would that be the PPO2 at the surface,
A: Now let us remember back to our Nitrox class. Dalton’s Law, the sum of the partial pressures equals the total pressure. At the surface PO2 = .21 and PN2 = .79 for a total of 1.0. This is then 1 ATA. Therefor at 33fsw ATA would be 2. Because of Dalton’s Law, PO2 would be .42 and PN2 would be 1.58. At 66fsw the ATA would be 3, there for PO2 would be .63 and PN2 would be 2.37. The formula to convert depth to ATA is (Depth / 33) + 1.
Now for 60ft the ATA would be (60/33)+1 or 2.8. Since the ratio of O2 to N2 is the same no matter the depth PO2 would be (2.8 * .21) or .59. It is easiest to convert depth to ATA then calculate the PO2 and PN2 as needed.
