[Louby Lou]

Hi Everyone

Pete Readey of SMI has kindly written an article about the PRISM scrubber and it can be found in the Rebreatherworld library HERE.

Please use this thread for any feedback, questions or comments you may have for Shas and Pete and as always please use the article to leave reputation

Thanks again to SMI for the article

Cheers

Miss Lou

[rigdiver]

Thanks Pete and Shas. Ya'll are a wealth of info. Please continue to share.

[silent running]

Yes, thank you Pete and Sharon for the info. Always good to have something in print to reference. I'm curious about something else, any idea what the difference in volume is between the inner tube of the scrubber and the outer space all around the scrubber and bucket? -Andy

[mverick]

Ok,
Saying thermal conductivity of the scrubber is a reason for it being plastic? I understand a little. But, if it's metal. Won't it come to temp and stay there? You're already loosing heat through the bucket. All the time. So, loosing heat to metal of a scrubber and then holding it there might be a good thing. That Thermal Mass thing. Also, when you want to hold the temp there. The metal would hold more heat to release to the scrubber material? If the ends of the scrubber are plastic and the mesh is metal. No direct transfer of heat to the bucket. Or, not much.

Also wondering about compaction from the plastic screen material. I understand the foam doing that. But how flexible is the screen material? Most I've seen isn't flexible. I would understand if you can pack the scrubber so the wall bowed out. Don't know if that's how your packing it. But I think the walls bowing out would be from the thin plastic supports flexing. Not the screen material flexing.

Not trying to start anything. Just wondering out loud.

[Jim Brown]

Greetings:

Thermal coefficient. Metal conducts heat rapidly, plastic less rapidly, and gasses even less. Scrubber surrounded (insulated) by gas that is really quite warm having just passed through the reaction. Moisture condenses on housing wall, resulting in drier (than before, loop always at least 100% humidity) gas to pass over sensors that being electronics, don't really like water.

It's a question of performance. Why would one intentionally remove heat from the scrubber? Cooler scrubber equals lower duration.

Peter also mentioned other reasons...

Regards, Jim Brown

[mverick]

Quote: (Originally Posted by Jim Brown)

Greetings:

Thermal coefficient. Metal conducts heat rapidly, plastic less rapidly, and gasses even less. Scrubber surrounded (insulated) by gas that is really quite warm having just passed through the reaction. Moisture condenses on housing wall, resulting in drier (than before, loop always at least 100% humidity) gas to pass over sensors that being electronics, don't really like water.

It's a question of performance. Why would one intentionally remove heat from the scrubber? Cooler scrubber equals lower duration.

Peter also mentioned other reasons...

Regards, Jim Brown

Ah, but lower duration is at the end. When the scrubber and your breathing has already heated the Mesh. So, you're not loosing heat to the mesh right? It's already heated up. Just like the scrubber itself coming up to temp by pre breathing it.

And, to be honest. The thermal conductivity of the plastic being lower will dictate it takes longer for the plastic to come up to scrubber temps. While the Steel will absorb heat quickly. The big thing is Thermal Mass. How much mass is in either one dictates how much heat they will hold. There is less Thermal mass in the Plastic scubber. For sure. So, I'm thinking it would heat up as quick as the metal one.

I'm not talking about what the Bucket or Can is made of. I'm just wondering if you loose anything to the scubber being plastic or metal. Once the metal is heated. I don't think so. And your duration is at the end. So, Metal mesh is already heated at that point and holding heat. It is not transferring heat to the gas. As you stated. Heat transfer through a gas is minimal.

I know he mentioned other reasons. Just wondering about this reason.

He said the increase in scubber performance was worth going to plastic for the initial investment.

The compaction from a flexible scrubber makes more sense.

[silent running]

Quote: (Originally Posted by mverick)

Ok,
Saying thermal conductivity of the scrubber is a reason for it being plastic? I understand a little. But, if it's metal. Won't it come to temp and stay there? You're already loosing heat through the bucket. All the time. So, loosing heat to metal of a scrubber and then holding it there might be a good thing. That Thermal Mass thing. Also, when you want to hold the temp there. The metal would hold more heat to release to the scrubber material? If the ends of the scrubber are plastic and the mesh is metal. No direct transfer of heat to the bucket. Or, not much.

Also wondering about compaction from the plastic screen material. I understand the foam doing that. But how flexible is the screen material? Most I've seen isn't flexible. I would understand if you can pack the scrubber so the wall bowed out. Don't know if that's how your packing it. But I think the walls bowing out would be from the thin plastic supports flexing. Not the screen material flexing.

Not trying to start anything. Just wondering out loud.

Hi mverick, the screen is a bit flexable and does bow out if packed really tight. I think the plastic structure is not flexing much in that situation. I would guess that it's designed that way as the scrubber needs to stay a certain height to maintain a seal at the top/inlet and the head.

As for thermal conductivity, it seems to me that you'd have to heat up a piece of metal to a higher temperature just to get it to hold a lower temp. In other words, I don't think the scrubber housing ever gets up to the temp of the gas in the jacket, no matter what it's made of. It's more a matter of which material loses heat at the slowest rate than retaining it... -Andy

[mverick]

Quote: (Originally Posted by silent running)

Hi mverick, the screen is a bit flexable and does bow out if packed really tight. I think the plastic structure is not flexing much in that situation. I would guess that it's designed that way as the scrubber needs to stay a certain height to maintain a seal at the top/inlet and the head.

As for thermal conductivity, it seems to me that you'd have to heat up a piece of metal to a higher temperature just to get it to hold a lower temp. In other words, I don't think the scrubber housing ever gets up to the temp of the gas in the jacket, no matter what it's made of. It's more a matter of which material loses heat at the slowest rate than retaining it... -Andy

You have 2 heat sources. The person breathing and the scrubber. Scrubber being hotter.

Be intresting to test loop temps.

The plastic will get up to temp. Whatever the loop temp is plus some. On metal it will quicker. Because it holds the scubber material it has direct contact.

Don't have to heat up metal to a higher temp. And, the scrubber is Hot. Without question. That's why all the condensation on the bucket. But, between the bucket and the scrubber you have an airspace. Which is a great insulator. Thing taking heat away is the air flow through the scrubber. Which is heated through the scrubber.

In to out will be more efficient. Because your not pulling the exhale heat out on the bucket wall before scrubber. I just don't think the scrubber material matters. Out to in you pull moisture out. Then run it through the heated scrubber where it heats back up so it will not condense as easy. Which would be why not noticeable on the cells. You're re heating the air. You do loose some scrubber efficiency though.

You want to loose heat in a rebreather but not loose it in the scrubber. Just want to do it where you want. Otherwise, It would be cooking you from the inside out.

[silent running]

Quote: (Originally Posted by mverick)

Ah, but lower duration is at the end.

Or if you overbreathe it close to the end of it's duration. I want the scrubber to work at peak or as close to it as possible for the whole dive, not just the begining. -Andy

[silent running]

Quote: (Originally Posted by mverick)

You have 2 heat sources. The person breathing and the scrubber. Scrubber being hotter.

Be intresting to test loop temps.

The plastic will get up to temp. Whatever the loop temp is plus some. On metal it will quicker. Because it holds the scubber material it has direct contact.

Don't have to heat up metal to a higher temp. And, the scrubber is Hot. Without question. That's why all the condensation on the bucket. But, between the bucket and the scrubber you have an airspace. Which is a great insulator. Thing taking heat away is the air flow through the scrubber. Which is heated through the scrubber.

In to out will be more efficient. Because your not pulling the exhale heat out on the bucket wall before scrubber. I just don't think the scrubber material matters. Out to in you pull moisture out. Then run it through the heated scrubber where it heats back up so it will not condense as easy. Which would be why not noticeable on the cells. You're re heating the air. You do loose some scrubber efficiency though.

You want to loose heat in a rebreather but not loose it in the scrubber. Just want to do it where you want. Otherwise, It would be cooking you from the
inside out.

mverick, my undersanding of thermodynamics, very limited as it is, is that it's not just the temp differential and conductivity that are taken itnto account, it's also the density and mass of the things transfering the heat. The CCR loop gas is much less dense and there's much less of it than the water surounding it, which would constantly draw the heat away from the loop. The plastic makes this happen more slowly in colder water. And an in to out flow further minimizes this.

As for condesation, the Meg sensors are mounted in the plenum, which directly contacts the outside/water, right? And the first point of condensation is here, right? I think I'd rather have my first point of condensation occur on the scrubber wall-in to out-before it hits the sensors, ala Prism. It sounds like the 1 way drain in the older Megs might have been a good idea, although less simple than a moisture pad... -Andy