Did a dive on Hollis Explorer today

[Dr. Lecter]

SCRs don't have constant PPO2s. Basically, they're all the complexity and much of the risk of a CC rebreather just for the sake of making a smaller tank of nitrox last a really long time. Short of a very specific need like a tiny unit for long distance combat swimming with a need to include depths deeper than one can "safely" push an O2 rebreather (and excursions to 60' are not unheard of for that application), I don't get why people use them. GUE/Halcyon seem to have a real hardon for them, though. See, e.g., the RB-80. I think AJ did the RB-80 course, so he can probably provide a very different and more informed perspective.

 

[darushin]

I am sorry, since I am not a rebreather dive, I want to understand this a little more.

If the gas in this rebreather can be breathed at any depth of the dive, how can the rebreather maintain a constant PPO2? Say you plan to dive to 100ft, you carry 32% (assume 1.3). At 100ft, it is 1.3, but at say 50ft, how can you get 1.3 out of 32%?

The dil. determines the Max PO2. you can't exceed PO2 of your dil in the loop. So in your case at 50ft, the maximum PO2 the SCR could obtain is .80. The explorer uses a algorithm to try and balance your PO2 with resource consumption, so you never really hold a constant PO2. It does seem to level off the PO2 at around 1.1 if can obtain that.

 

[tphelps]

I am sorry, since I am not a rebreather dive, I want to understand this a little more.

If the gas in this rebreather can be breathed at any depth of the dive, how can the rebreather maintain a constant PPO2? Say you plan to dive to 100ft, you carry 32% (assume 1.3). At 100ft, it is 1.3, but at say 50ft, how can you get 1.3 out of 32%?

I'm an Explorer diver as well, I believe you can change the PO2 by controlling the flow rate. The more the unit adds gas, the higher FO2 and higher PO2 (of course PO2 can not exceed maximum MOD of gas). If you dial back the flow rate, less FO2 means you have a lower P02. It's a nice ability to be able to change your flow rate because if you're just cruising at 20ft, you dont need all that gushing gas...you can get by with a lower PO2 so it conserves your gas. Hope that helps!

 

[Dr. Lecter]

I'm an Explorer diver as well, I believe you can change the PO2 by controlling the flow rate. The more the unit adds gas, the higher FO2 and higher PO2 (of course PO2 can not exceed maximum MOD of gas). If you dial back the flow rate, less FO2 means you have a lower P02. It's a nice ability to be able to change your flow rate because if you're just cruising at 20ft, you dont need all that gushing gas...you can get by with a lower PO2 so it conserves your gas. Hope that helps!

To be clear, this won't alter the ppO2 the unit is capable of acheiving at any given depth. It will only alter the ppO2 to the extent that, if you throttle back the inflow enough, you'll be metabolically consuming O2 faster than the inflow of gas adds O2 to the loop, causing pO2 to drop until (1) your metabolism changes, (2) you up the gas flow, and/or (3) you go deeper. Keep it up long enough and you can (depending on any automatic addition safeguards in the unit's design) make the breathing mix go hypoxic. Though I imagine it'd take a while

 

[tphelps]

As with most questions I have, I differ to the manual. Reference page 34 on the Dive Control Parameter.

http://www.hollis.com/media/wysiwyg/HO/manuals/Explorer_Manual_12-4102-r06_6-17-14.pdf

 

[Dr. Lecter]

The EXPLORER is a dynamic PO2 controller. This means it can vary the PO2 that it mantains based on the value of certain resources. A higher DCP (Dive Control Parameter) value means less tissue loading (higher PO2) but more gas usage and a lower PO2 is the opposite. For a set DCP the actual PO2 will vary throughout the dive profile.

Mmm, word salad...my favorite. The manual says what I already said while avoiding any mention of how or why the PO2 will always vary or what, exactly, the Explorer is doing to "vary the PO2". Physics says that it can't raise the PO2 any more than whatever the fO2 in the tank times the current ambient pressure creates--just like OC nitrox--and that the only way it can reduce PO2 without a tank of lower fO2 dil is to let metabolic consumption outpace unit addition of O2 to the loop. Interestingly enough, the rest of the manual never quite addresses how the DCP system "controls" PO2.

 

[eelnoraa]

Sorry, I am totally noob here.

By controlling (limiting) the inflow (diluent injection), since you are consuming O2, so PPO2 drop. But do you also lose loop volumn as CO2 is being absorb by scrubber. Or the amount of CO2 get absorb is negligible volume wise?

 

[Dr. Lecter]

Theoretically, yes, but practically negligble volume wise. Plus, the ADV should take over if for some reason loop volume drops enough (from O2 consumption, lack of other dil/O2 addition, and/or CO2 fixation).

 

[PfcAJ]

SCRs don't have constant PPO2s. Basically, they're all the complexity and much of the risk of a CC rebreather just for the sake of making a smaller tank of nitrox last a really long time. Short of a very specific need like a tiny unit for long distance combat swimming with a need to include depths deeper than one can "safely" push an O2 rebreather (and excursions to 60' are not unheard of for that application), I don't get why people use them. GUE/Halcyon seem to have a real hardon for them, though. See, e.g., the RB-80. I think AJ did the RB-80 course, so he can probably provide a very different and more informed perspective.

I just saw this.

There's a few things that make the rb80 a great unit within the DIR system. Its REALLY simple (pretty much just 2 scubapro r190s, a scrubber, a few one way valves and a drysuit dump. The procedures for using it are 99% the same as with OC, which is the 'foundation' from our perspective. Transitioning from OC to RB80 and back is easy peasy.

From a gas usage perspective, its REALLY good. No, its not CCR good, but its still great. 8:1 over OC doesn't sound like a lot on paper, but it translates to ridiculous dive times on very little gas. So you sacrifice the CCR efficiency for sesame-street level simplicity. No sensors, controllers, solenoids, MAVs, nuthin'. You can pretty much plug in a gas to the unit and go. Yes, the po2 drops a bit, but in real life its like adding 10ft to your depth. Not a big deal, imo. Yes, you have to plug in deco gases, but you'd have to carry those gases with you anyways on a dive for OC bailout, so I don't mind that either.

For long range cave diving, its a tough unit to beat. For ocean stuff or short dives, I think it falls short, but I'd rather use OC for a short thing and I don't ocean dive unless I have to.

All that said, its my understanding that the Hollis is a completely different animal than the RB80.

 

[wedivebc]

The net result of adding offboard gas, be it O2 or Nitrox, is to fool the unit into thinking you have not created as much CO2. The unit assumes that the O2 in the onboard tank is what you have available for metabolism. As you metabolize that O2 you create measurable amount of CO2 and therefore you have a calculable duration for your scrubber. If you bypass the onboard cylinder you are bypassing the pressure monitoring that informs the computer as to the amount of O2 that is used in the calculation. These are the volume calculations that yield the CO2 for fixation. There are other factors in the scrubber duration calculations and eventually they will determine the "filter" time that remains and is displayed on the status screen. The fact that you are deceiving the computer with this method forces the computer to yield somewhat inaccurate information regarding the usage parameters set by the manufacturer. That is your choice, but you damn well better know what the hell you are doing if you want to avoid injury. At this level of off-standard usage, the optional CO2 sensor is an ABSOLUTE necessity. If you are going to push the absorbent farther than you were instructed, you must have the CO2 sensor installed. Doing any of this is at YOUR OWN RISK.
As far as the actual content of the loop, the O2 sensors do track PO2 and therefore provide data regarding decompression/NDL limits accurately as long as the only two gases are O2 and N2.
There are a few of us that have pushed the limits of the Explorer, but we understand that what we are doing is for our knowledge and understanding. If you wanted a full trimix/100m eCCR and bought an Explorer, you made a mistake. That being said, I have both, I instruct for both and love to tinker. The Explorer is a fantastic unit when used within the manufactures specifications

You actually don't know what you're doing. For starters CO2 "filter" duration calculations have nothing to do with tank pressure. Better go study your manual.
Any instructor sanctioning and discussing ways of modifying a rebreather, against manufacturers designs is not only violating standards but acting irresponsibly.
Nothing you have said in your post leads me to believe your have any "knowledge or understanding" of this rebreather and I am actually going to make it my business as a Hollis Explorer Instructor Trainer to bring this post to the attention of training department at Hollis.