SMI Scrubber

silent running

Steve, all models are flawed, they are models and thus incomplete. Applying the standard incomplete=misleading means no model should ever be used for a point of discussion or anything else as it's not complete and can't be fully trusted. It would be misleading only if data/models show something which is the opposite of what turns out to be true. But I don't see how the solid trend of metal losing heat faster than plastic in the models could somehow be leading us in an opposite direction to the full truth.

Why does it matter that the outlet temp is higher than the inlet? Doesn't that just make it all the more important to slow the heat transfer, as there's more heat to lose?

silent running

Joe, have you ever picked up an active metal scrubber which was cold to the touch? It's always radiating, unless it were in an ambient temp that was higher than it is, which can't ever happen underwater. If you took 2 of the same size scrubbers, 1 made with SS and the other composite, and activated them for the same amout of time, then took them out, the metal scrubber will always be warmer to the touch. Just like my analogy many moons back: a pc of metal put near the fire will heat up and burn you much faster than a same size pc of plastic. Look at the temp gradients in the model, do you notice how the temp is always going down, the closer it gets to the outside/cool water? That means that the material is radiating it away, it should not make any difference where the heat is coming from, scrubber core or no.

As for gas density, the 250ltrs per minute is well within the realm of possiblity at 100ft, and more so as you get deeper. And aren't we at least as concerned with how our scrubbers perform deep under high workloads as we are with their performance when shallow with low RMVs?Are you saying that if we were to slow down the RMV to say 50lts RMV, near the surface, there would be less relative heat loss? Maybe so, but there is still heat transmission and thus heat loss to the colder water. There could be no magic low RMV number where the principle stops working. The 12C drop in the 250lt per minute metal model is a significant amount, so a little difference in the relative heat loss will still be a significant differential. The materials themselves still have the same thermal conductivity at all depths and flow rates.

webslave

Classic Andy! Truely Classic!!!

mverick

Sorry, that's wrong. If you have a plastic scrubber and a metal scrubber basket in your Rebreather. And you pull both of them out both will be the same temp to the touch. Then, you're doing heat transfer of a solid to a solid and guess what. Heat transfer to your finger is fast in plastic and metal.

Plastic will get to the same temp as metal. It's been in the scrubber for an hour or so. Even though it might take longer to absorb heat energy. It still does. Want proof. When I pull the scrubber out of my Meg after a 3hour dive. It's hot to the touch. Without question.

Physics is not a conspiracy. No kidding. The conspiracy is when you don't follow it and try to prove your point.

Then inner basket if made of metal will absorb heat. Without question. But once at equalibrium. It holds heat. Not absorbing or radiating. Then, when temp in the loop drops from the scrubber falling off. It will radiate the stored heat back to the loop. Radiating back most through the easiest path. Which in this case is perferated metal wall to solid sorb material. Some will be lost to the air. But, the best path is solid to solid.

Whether it will help the process or not is the question. But there is no question physics works that way.

That's basic physics. And, you can't bend it to fit your ideas.

I don't think his models are a conspiracy but I also don't think they explain whats being described at all. And, if he read the thread. I think he knew that. If he is an engineer. It's basic physics. I don't think he's stupid at all.

dive2dive2000

Hi group,

This thread is way over my head. I am no enginer, or highly educated, high school with some college. I do know this SMI states and teaches Scrubber 5 hr duration. All PRISM Divers that I dive with (8 total) including me use this as our guideline and no one has had a channling issue yet. 3 of the 8 have thousands of scrubber hrs. the rest of us 100's of scrubber hrs. Who really gives a about metal or plastic material. please kill the already and go do a 5 hr dive

__________________
Safe Diving,
Martin

"but what's the fun of getting what you need, instead of what you want?"

silent running

Hi Marty, you know I'm a warm water dude, so no diving for me until my Nov trip. One person who cares about plastic vs metal in scrubber design is that guy who made the fine CCR you dive finer... Sorry if you're bored, just trying to get to the bottom of this, may not be possible...

silent running

I think you might be assuming that the temp will stay the same at any boundry, when in fact it will always be changing due to the reaction front moving closer or nearer to that boundry. If you think that both scrubbers will be the same temp after 1 hr, then it must not matter what material we make the scrubber or bucket out of. It is agreed that a metal bucket will transfer more heat to the cold water around it than plastic, right? So that means the metal surface of a scrubber would have a higher initial temp on the outside of the bucket than a plastic one, otherwise it couldn't transfer any more heat to the water than a plastic one could. The temp on the outside of the metal bucket would then drop, and faster than a plastic bucket.
So why would this not apply to the outer surface of the 2 different scrubber canisters too? The point is that they would each be losing heat at different rates. In my theoretical example I was thinking that initialy, after coming up to temp, the metal scrubber would be warmer and would then start to drop off faster than the plastic one, which would never get as hot but would drop off more slowly, should have been more specific.



Mverick, how in the world can you say something like this and then keep insisting that the metal scrubber doesn't radiate a lot of heat while it's working? This is entirely consistant with what I've been saying-all that heat is radiaiting away from the scrubber, to your hands after a 3 hr dive or to the cooler air around it in the bucket. When I change the Prism scrubber, the outlet surface is not that hot, even after 10 hrs in 80F water, when it should be at it's highest temp outlet temp, near the end of it's life. If you look at the models, there's a lot less thermal energy inside the metal scrubber and a lower temp inside than in the composite/plastic one. Why? because it is radiating away. This would be consistant with my experience when dumping the Prism scrubber, the inner sorb seems to still have a lot of heat, similar temp to the outlet surface.



Yes, that is your question. But even if the metal does manage to store some heat for release after the temp lowers, I can't see how a pc of perf metal that thin, with that little mass could store much heat at all. If it helps, it won't be for long as the reaction front moves away from it. I don't think you realize how relentlessly a dense medium like coldwater sucks up heat and the process accelerates as the temp drops. That's why people in 50F water without exposure protection can die in a matter of minutes and why even dives in 82F you feel colder the longer you stay in.

I just don't see how the metal can ever even come close to storing up enough energy to offset the constant extra heat loss radiating away from the metal outlet surface, you know the surface that was hot when you touched it. -Andy

mverick

Nevermind,

Prism is the worlds greatest rebreather and no improvements can ever be made.

My meg scrubber by the was is the original plastic. It's hot to the touch when I remove it after 1 hour.

Prism is made of unobtanium plastics.

silent running

Mverick, as you know how hot a scrubber is when measuring with your hands is subjective to say the least. I forgot that you don't actually have the radial metal scrubber, I guess only a few people do at this point. The main question remains: how much heat can be stored in the perf SS, is it enough to offset all the radiated heat loss during the life of the sorb? Of course there's room for improvement in any CCR, Prism included-I wish it had a BOV and a few other things.... I think there's another model coming... -Andy