Hi Dennis,
am just catching up with the forum here now. My thoughts on your questions:
I'm completely clueless when it come to rebreathers. I'm curious about shallow water use-20-35 ft.
Shallow water use is fairly uncommon with rebreathers. For truly shallow diving O2 rebreathers are used, like the above mentioned LAR V from Dräger. Mostly by combat swimmers (SEALs etc), to a lesser extend by researchers or UW monitoring (CA Fish & Game has a few for example), and a few by civilian divers. They're for most part limited to 6m/18ft depth since O2 becomes toxic at higher pressures.
If you want to venture deeper than that you need to dilute the O2, either with pre-mixed gas as is done on semi-closed units, or with a diluent tank like mixed-gas CCRs use. So for the range you're asking about, either one of these would do.
Since at that depth gas usage on OC isn't all that high few people would justify the expense and effort that rebreathers take, unless there's a particular requirement.
If you are deeper the PPO2 would require less O2 addition to keep your desired O2 level-I think ?.
If you are diving shallow, you would have to add more 02?
No, the O2 consumption is based solely on your metabolism. The harder you work, the more O2 you will need and the more CO2 you will generate. This is where rebreathers shine, CCRs in particular. You only need a small amount of O2 molecules, and only those are replenished in the loop. You'll need the same amount at the surface as you do at 50 ft or 250 ft., hence the enormous gas savings when diving deep. And the money savings on helium when diving deep.
The general average that is used is about 1 ltr O2/ min, so a small 13 cf rebreather tank (that's 2 liters x 200 bar = 400 liters) can literally last you several hours, independent of depth!
When diving a CCR, the most important thing is the partial pressure of O2, the pO2.
It has to be in a range that can sustain life, between 0.17 and 1.6 ata (air at surface is 0.21 ata, O2 at surface is 1.0 ata). These are the extreme ends, the pO2 is usually kept between 0.7 ata and 1.4 ata.
The partial pressure is based on the fraction of O2 in the gas and the ambient pressure, the get multiplied. In other words, at 18ft/6m O2 has 1.6 ata, hence the depth limit on O2 rebreathers. Air would have about 0,34 ata.
So when you ascend with a CCR, you have to add O2 to maintain the pO2 (while venting gas from the loop as you do on your BC
).
Diving shallow doesn't use more or less O2, changing depth or working level does.
You swim hard against a current, you breathe hard, you use more O2.
Is a 4-5 hour dive time in 20-35 ft in the normal range with existing gas and sodasorb setup.
Duration depends on a bunch of factors, CO2 already mentioned, ambient pressure, ambient temperature, kind of scrubber and scrubber media.
Only very few CCRs have ratings in the 4-5 hr range, the MK15.5 and the PRISM come to mind. However, duration ratings are usually based on worst case scenarios, very low temperatures (4-4.5 deg C/39-40 deg F) and high CO2 addition, 1.3 liters/min by the US Navy, 1.6 lpm by European CE standards.
Since CCRs use very little gas the supply is usually not the limiting factor. Scrubber duration, deco consideration and bailout gas supply are.
