#11  
Old 12-13-2011, 07:35 PM
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Default Re: Rocket cob oven

What makes you say that finer grade vermiculite is a poorer insulator? They all seem to be the same density to me. If you pick up a bag of fine stuff it weighs the same as a bag of the course stuff.Although the spaces between the grains are smaller for the finer stuff their total volume will be the same. A bucket full of large marbles will weigh the same as a bucket full of small marbles. This would indicate that there would be no difference in the insulation value between small and large grain size. Correct me if I'm wrong. Is there any data on this?
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  #12  
Old 12-13-2011, 09:19 PM
Peasant
 
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Default Re: Rocket cob oven

Interesting. Small marbles would have smaller gaps between them, glass is denser than air. A volume of small marbles would be denser than the same volume of large marbles.

I am thinking of a bucket of sand and a bucket of bowling balls rather than marbles to illustrate the size difference. You can get more grains of sand in a bucket and less air, whereas a bucket of bowling balls will have maybe two balls in it and loads of air.

However, marbles don't contain air in them, whereas vermiculite does.

Anyway, I am most likely to use perlite. Rocks that float on water are hard to beat, in any situation!

Unless I am mistaken, I forgot about the existance of convection currents a moment ago.

Edit: of course the proportions would be the same, with the same number of contact points around the spheres regardless of the size of the sphere. If the spheres are all the same size. If they are varying sizes then there will be some differences and also where the marbles come into contact with the sides of the containers, smaller marbles will have smaller air gaps between their surface and that of the bucket, whereas larger spheres will have large gaps around the surface of the bucket.

Also smaller particles can be compacted driving out more air space. Not marbles though. I take it that the insulation material is not going to be compacted but loose filled.

I was going to mix up some concrete and perlite but it might be better just to use loose perlite as it need not have any structural purpose.

Last edited by jislizard; 12-13-2011 at 09:28 PM. Reason: Second guessing myself
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  #13  
Old 12-13-2011, 09:42 PM
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Default Re: Rocket cob oven

Perlite is a better insulator than vermiculite, but as soon as you add cement to it there is not much difference. Use it loose if you can because it will insulate better and you won't have the problem of removing the water.
A 10;1 vermicrete mix has about the same mass of cement in it as mass of vermiculite. and for every 10 L of vermiculite you add about 3 L water.
I've also found (and I can't explain why) that a 50/50 mix of perlite and vermiculite mixes up with cement and water way better than either of them alone. It might be like mixing alcoholic drinks where 1+1=3
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Old 12-15-2011, 01:06 PM
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Default Re: Rocket cob oven

Mark203
Could you describe your rocket stove, perhaps a diagram. Once your oven comes to temperature do you keep the rocket burning to help sustain the temp?
rob
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Old 12-15-2011, 06:01 PM
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Default Re: Rocket cob oven

Quote:
Originally Posted by david s View Post
What makes you say that finer grade vermiculite is a poorer insulator? They all seem to be the same density to me. If you pick up a bag of fine stuff it weighs the same as a bag of the course stuff.Although the spaces between the grains are smaller for the finer stuff their total volume will be the same. A bucket full of large marbles will weigh the same as a bucket full of small marbles. This would indicate that there would be no difference in the insulation value between small and large grain size. Correct me if I'm wrong. Is there any data on this?
I am a newbie to the whole concept of pizza ovens and vermiculite as an insulator. Very sorry if my post has mislead anyone about insulation efficiency. But, my searching this sight and trying to find an ecomomical way to build my oven has came across many post's that recomend course vermiculite over fine vermiculite.
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Old 12-15-2011, 06:48 PM
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Default Re: Rocket cob oven

Quote:
Originally Posted by david s View Post
A bucket full of large marbles will weigh the same as a bucket full of small marbles.
I am breaking my own personal rule of posting after three drinks, but what the hey:

My 9th grade science teacher presented our class with a container full to the brim with large rocks.

He asked us "if the container was full"? We all agreed that it was.

He then, transfered the contents to a larger container and poured in some pea gravel. He shook it up and poured all of the contents back into the original container and again it was full to the brim.

Same question was posed and we all agreed that it was full.

He then poured all of the contents back into the larger container and then added some sand, He again shook it up and then poured the contents back into the original container and again, all of it's contents filled the container to the brim with none left over.

He asked the same question again. By this time nobody would utter a word.

He then poured in some water.

I may have missed something from what he was attempting to drive in to our youthfull heads, and I will admit that we did not weigh the container after each fill but, I bet that a bucket of small mables weighs more than a bucket of large marbles.

Just Sayin"
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  #17  
Old 12-15-2011, 07:48 PM
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Default Re: Rocket cob oven

The large rocks in the jar and then the gravel poured in to fill the spaces between is exactly the principle used for making concrete where the large aggregate leaves spaces between which are filled with sand and the spaces between the sand are filled with cement so the resultant total volume is much the same as the volume of the heavy aggregate alone. In the case of vermiculite the course stuff I get has both course medium and fine particles, which makes its total density much the same as the fine grade. (Both bags weigh the same) If you could get a course grade that didn't have the fine grains in it as well then it probably would be a better insulator. Maybe this is why perlite is a better insulator than vermiculite because most of the grains are of a more uniform size (at least the ones I get are)

A bucket of small marbles still weighs the same as a bucket of large marbles.
Any physics experts out there to confirm this?
I'll shout you a drink if I'm wrong.
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Old 12-16-2011, 07:44 PM
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Default Re: Rocket cob oven

Well, I have been told many times that I have lost my "marbles" and will nolo contendere on that subject I have no "data" or "physists" to consult. However, I came to the understanding that course vermiculite was the better insulator compared to fine by reading many threads on this site. It seems to be the consensus. So far I have only installed a 4"-5to1 portland vermicrete floor under my dome. For the top 1/2" I used horticuture grade (fine vermiculite) only because I ran out of the course grade. I later sought out and was given enough 2" CalSil to correct what I thought was an inferior 1/2 inch of insulator. Do you have any data to confirm that their is no differnce? I am refering only to vermicrete, not loose fill.
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  #19  
Old 12-16-2011, 10:44 PM
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Default Re: Rocket cob oven

No, I don't have any data on this that's why I asked "is there any data on this?" I've tried both fine and course vermiculite to make vermicrete and it seems to perform the same to me aand as the bags of fine and course weigh the same they therefore have the same densityand should provide the same insulation value,lthough I've not conducted any tests, that's why I was asking.
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  #20  
Old 12-29-2011, 06:37 PM
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Default Re: Rocket cob oven - back to topic

Beautiful oven there mark. I've attached my design for my cob oven mk II. Let me know what you think. Nothing is to scale, just to give an idea of general layout.

Does anyone know about optimization of a rocket in terms of height to diameter? There will be a tradeoff whereby increased height will allow more complete burning, but more heat to escape.

Also, I'd like to find an alternative to stainless. It will be expensive to get that fabricated. I'm already using mud because I'm too poor to buy bricks.

I couldn't figure out how to do it in my diagram without doing a full set of orthographic drawings but I'll try to explain it here; I'm thinking I'll make the hearth component (metal plate and fire bricks) flush with the cob walls on the front and maybe the sides too, so all the hot air from the rocket will be coming up the walls at the back of the oven and have to travel all the way forward to get out the door, heating the oven etc as it goes. This also means I don't have an annoying gap at the front.

I'm aware that there are proper plans out there for rocket ovens but I haven't managed to find any that use cob with rocket technology. Any thoughts or suggestions on the matter will be much appreciated.
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