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built an rmh, update on project, minor problems i have please look and help, appreciated.  RSS feed

 
Posts: 13
Location: Hungary
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Hey everyone , i posted earlier on making a rocket mass heater without the steel pipe but didnt work out for safety reasons, i could find some 9" flue piping and made the 8" system from fire bricks and perlite insulation, my flue pipe is going 6 meters in a straight line from my living room through the bathroom and then goes up 2.3 meters to exit the building, outside it then goes up again 2 meters so the chimneys top is abt 4.3 m in height, thats 14 feet roughly,
the pipe is embedded into a mixture of cob and gravel concrete pieces, the size of the mass is enormeus, till the pipe goes 2.4 meters straight, the mass is already about 3 cubic meters, im wondering if this causes problems because i have lit up the rmh for abt 3-4 hours and the mass did not warm, the cob is still soaking wet, do i have to take away some mass?? it seems that there is a strict calculation for the best efficiency with mass, what is the least and the max mass for a 9" pipe? accross 6 meters of piping
my second concern is, i have backdraft on windy days, yesterday i had a nice rocket effect, today we had 15 mile winds and i could not light up the rmh because it was smoking back to the room. even if i put my hand above the feed tube without being lit , i can feel the gust coming onto my hand. is this normal with every rmh or is this a mistake i made in the design?
what do you guys do in such situation? its getting freezing here so i need help.
Thanks and appreciation for the creator of this forum to exist.
Simon,
 
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Simon Opaski : A late Welcome to Permies and out sister site, richsoil.com , and a Big Welcome to our rocket stove, and wood stove Forum / Threads ! With over
20,000 fellow members world wide, you should be able to come here 24 / 7 and Find someone to talk to about what you want to talk about.

We also have our own Cloud the 'pClould' that can be reached from the Permies Toolbox at the top of this page by clicking on Search, and then typing in your area to Search
and select the Google Search engine, and press the Permies button to do a Search within Permies only, and that is available 24 / 7 !

Going from an 8'' system to 9'' exhaust Is a problem, as is your failure to report a strong rocket sound will your Rocket is hot ! This Is most likely the cause of all your other
problems, I expect that the only way now to determine a positive course of action would be to use an exhaust booster fan near your final exit from the building somewhere
in a vertical (?) section, First this will serve as a test, and second a way to run your system while you keep warm this winter !

This presents you with the possibility of being unable to run your rocket mass heater RMH, during a power failure, If you could find a 12-24 volt Dc powered exhaust Fan, say
from a car radiator, Two automobile batteries on trickle charge should carry You for 24-48 hours !

While you are trying to dry out the Cob you have 3 separate sources of water to deal with, besides the water in your Cob, you are also pumping in the water vapor trapped in
the wood you are burning, 20%-ish by weight, and H2O from all of the burning hydrocarbons in your wood! They will condense out because your interior surface Temps are
well below 100*C ! the slow exhaust flow caused by your 9'' pipe in an 8'' system is carrying less water vapor through to the outside then normal !

I am sure that by now you are alarmed by the amount of wood that you are burning, and it will be a lot eventually you will reach an equilibrium as the Cob gets Dryer and Dryer!
Ideally your final exhaust gas temps should be high enough to barely feel warm inside your house near the final vertical chimney ! With a booster fan you should be able to run
your system until spring and then reevaluate !
So- add a booster fan inline after the cob, at a readily accessible place where you can set up a trickle charger and batteries, use a 12-24 volt Dc Car Fan and have an identical
spare car radiator type fan, live with It till spring, or tear into it and replace all the 9'' piping, I Think a test use of a 'Booster fan inline in your Exhaust pipe should be the
First step! For the good of he Craft ! Good Luck, come back and tell us how you made out ! Big Al !
 
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Location: northern Wisconsin
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Hi Allen, I didn't know where to post this question so I will post it here. I have some type M masonry mortar, can I use it someplace in the bench to mortar in rocks instead of cob? Can I mix it with fire clay? I will appreciate your help.
 
allen lumley
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Willie Bost : Welcome to Permies.com , our sister site richsoil.com , and a Big Welcome to the Rocket and Wood stoves Forum/Threads! With over 20,000
Fellow members you can generally come here 24 / 7 and find someone who wants to talk about what you want to talk about ! Browse around I came here to talk about
rockets and now I read and 'post' on many forums, you can too, do not be afraid to start a new thread, in an appropriate forum, we never want to be labeled as one
who Highjacks an others thread !

For the rest of he year we are asking our fellow members to update their My Profile information to include a general location and their climate zone if you know it. It helps
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selected as -well, similar to the subject of this thread !

Assuming that you are planning on using a fire brick chimney wrapped in insulation on its outside, this would be the best place to use your mortar to level and plumb your
Heat riser 1 course of bricks at a time! Remember you are not joining the bricks together you are holding them apart, to make leveling and plumbing easier !

For the good of the Crafts ! Big Al !
 
Simon opaski
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"" Going from an 8'' system to 9'' exhaust Is a problem, as is your failure to report a strong rocket sound will your Rocket is hot ! This Is most likely the cause of all your other
problems, I expect that the only way now to determine a positive course of action would be to use an exhaust booster fan near your final exit from the building somewhere
in a vertical (?) section, First this will serve as a test, and second a way to run your system while you keep warm this winter ! ""

Spent $700 for the pipe, its expensive stuffm any other way of correcting my failure? modifying the burn chamber or something?? bit dissapointed, how an 1" can make a difference, surprising...
 
allen lumley
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Simon Opaski : My answer was probably affected by the near zero Centigrade temps we are getting now! with cold weather seemingly here to stay, I did not think that
you would want the tear everything apart to rebuild your Feed tube, Fire tunnel and Heat riser all to a larger size possibly requiring a modification of your insulation and
/or barrel !

Think of a rocket mass heater as a stream running fast and full bank to bank, dark, cold, scary Add just one more inch and the water just stands there a lake on the land!
Still and unmoving (actually this is more dangerous because you do not know where the deep water is !)

You may be able to get all of your materials lined up to tear your Rocket burner and base apart, and wait for good weather, however this is not drying your cob and until
the cob is dry, you are still not getting the benefit of the Energy you are using so freely !

My best advice is to get an inline booster fan or make one and see if your problems go away, yes you can get in there an reduce the Wettest cob off of the far end, if you
seal the material in plastic bags you may find months from now it is still wet and waiting to be reapplied, however it would be easy for you to really damage your
Horizontal chimneys stove pipe if you are not careful ! I am sure you know how likely a late november early December break in the weather is, better than me !

Around here the older farmers say 'Climate is what you expect to get, weather is what you do get !' For the good of the Craft ! PYRO - Logically Big AL
 
Simon opaski
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My best advice is to get an inline booster fan or make one and see if your problems go away, yes you can get in there an reduce the Wettest cob off of the far end, if you
seal the material in plastic bags you may find months from now it is still wet and waiting to be reapplied, however it would be easy for you to really damage your
Horizontal chimneys stove pipe if you are not careful ! I am sure you know how likely a late november early December break in the weather is, better than me !

i thought of something to correct the inner diameter of my tube, i drew my pipe system in 2d
, the pipe is just going in a straight line but half way there is a slight angle then goes straight again
would filling in something to reduce inner cm3 help?

image of drawing: http://kepfeltoltes.hu/view/131126/pipe_www.kepfeltoltes.hu_.png
 
allen lumley
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Simon : First let me say as a person who can reads and speak in two or more languages you exceed my barely acceptable attempts to communicate in one !

Great thinking outside the box ! You can go to 'the engineers toolbox' and look up flow rates of air through various shapes of ducting here you want information
relating to turbulence and Laminar flow rather than on Cm3 per unit of time !

we have not discussed laminar flow, which simply means the hottest gasses are moving the fastest in the exact center, and as the column of flowing air moves
out toward the walls it slows down and its temperature drops, finally the full effect of the Laminar Flow takes place and you have a layer of still air, by increasing
pipe size you ^ the % of surface area affected !

Short of pouring in a clay slurry ( with some kind of a as yet unspecified binder ) there is probably not a good way to introduce a dense material that would pass
the heat of your hot exhaust gases through itself to your original Cob with out slowing the rate of, and total temperatures the exchanged heat energy !

Using a clay slurry will simply add more water to your system when you are trying to decrease it! If you have an idea for a liner with higher Heat energy pass
through, great but I am blocked and will probably not come up with anything better !

In the meantime you still need to dry out your cob, what other plans do you have ! For the Craft ! Big AL !
 
Simon opaski
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allen lumley wrote:

Using a clay slurry will simply add more water to your system when you are trying to decrease it! If you have an idea for a liner with higher Heat energy pass
through, great but I am blocked and will probably not come up with anything better !



i have quartz river pebbles in abundance, they have a smooth surface and are more conductive than cob, this need no binding material and is dry, but because of each rock has a different shape it could slow down the flow of air yes>?
 
allen lumley
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Simon Opaski : Physics is not my best subject, Consider any air no matter its size, as insualtive, and no matter its shape as though it we a perfect' bubble'. To pass heat energy thru
an air pocket is easy, BUT at every boundry layer, both at entrance into, and then on exit out of the bubbles surface the flow of heat energy will be slowed, this again is I Think part
of the Laminar Flow Effect. Because a bucket of sand, weighing more than a bucket of pebbles has more trapped 'air bubbles' it will be more insulating than the pebbles! But the
pebbles too will have these 'air bubbles', just fewer of them and bigger! and areas of Laminar ( Low) flow> so they to will be insulateing .


They Will also add to the turbulence, but I expect that they will slow done even further your drying of the Cob. with dry Cob this might be a project worth considering ! what else
have you got ! For the good of the craft! Big AL

P.S. Cob being an admixture of clay and sand, when dry, has fewer air pockets between the sand grains because the clay naturally gravitates there due to its smaller size ! A.L.
 
Simon opaski
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allen lumley wrote:

They Will also add to the turbulence, but I expect that they will slow done even further your drying of the Cob. with dry Cob this might be a project worth considering ! what else
have you got ! For the good of the craft! Big AL



i have looked over the forum and found that many people have blowbacks, well some of them said they had put a wind turbine cowl on the chimney and it sucked the air out even on dead calm days.
what about that?
 
allen lumley
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Simon Opaski : Because of the number of people following this barely day old thread, I am pointing out that a little more time here at Permies Dot Com would have sent you to
rocketstoves.com , to pick up a PDF Copy $15U.S. of Ianto Evans'> "rocket mass heaters" , and now we are working harder to fix this problem !

Only two things can make a wind turbine spin, a column of warm-er air rising in the exterior vertical chimney, or wind blowing past the Wind turbine , I have Zero Objections to
you spending money on it and it may help !

Lets review the numbers again I Believe You stated that your Total Horizontal run was 6 meters then up 2.3 meters vertically but your sketch shows it as a very shallow incline
if shallow it must be added onto the horizontal ( you also show more vertical after this,in your sketch)!

The best case Scenario I can make is you have a minimum of 6 meters of horizontal run, and ~3meters~ of vertical lift ! we need to count the Elbows, counting the Elbows of
the shallow rise as equal to 1 elbow how many elbows or 'T's do you have now including the one in out doors !

With exterior chimneys you have two measurements to make, One the length of the External vertical chimney, as 15' of 'good' vertical chimney will correct for 10' of marginal
horizontal chimney ! Also we need the exterior chimney to be Taller than the nearest obstruction in the area, usually regarded as 5'-6' over the peak of the roof!

If you wind turbine is by itself 95% effective in stoping 'smoke back' you will only have about 18 days a year when you will have this problem ! Which way does the prevailing
winds blow during heating season, if your stove pipe is on the lee side of the building you have a slight negative pressure, usually a good thing If your stove pipe is on the
'storm' side you will have positive pressure and the only way to break that is to have a high enough chimney*, our cheat and use a booster fan !

Because of your multiple problems, a booster fan is becoming a re-occuring theme, and your cob is not getting any dryer ! Lets get a final clearer picture of the layout of your
horizontal and vertical pipe and the location of any more elbows and 'T's, and location of the high point of your chimney and house !For the craft ! Big Al

* I can not speak for your parents, but your grandparents would have automatically known about the height and location of all vertical chimneys !
 
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Just an idea, rebuild the rocket itself with a nine inch cross section toilet match the exhaust flue.
 
allen lumley
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William Bronson : Let me speak quickly for Simon Opaski : and then get a second opinion, Simon lives in Hungary and materials are not as easy to come by as Stateside!
And that is accepting the limitations of the system as built IF the deployment of an inline fan will boost air flow enough to give him his needed High Flow rates to assure
adequate combustion. I am going out on a limb here to make a suggestion that I thought I would never make!

I Thinkthat a booster fan will allow him to find out if that will give him a functioning rocket NOW, and allow him to get through to warmer weather, I have Advised him
his really only other choice is ripping everything out and starting over, but If I was in his shoes, I would do almost anything to avoid such a challenging situation so late in
the year !

Now that I have played Devil's Advocate, what is your best opinion Now ! As always I welcome all comments and questions, and both solicit them and hope for them ! Big AL !
 
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Simon,

I have less than a week of experience with rocket stoves, and zero experience with mass heaters attached to rocket stoves.

Please take what I have to say as coming from someone with less experience than you with this exact problem..but i have been dealing with the physics of airflow a couple ways for some years now.

For one thing, the physics of a mass heater looks like the equations will be very similar to those of a particular kind of subwoofer speaker system called a transmission line array, but that isn't pertinent here.

It also looks to me like a mass heater should follow some of the same rules that apply to automobile exhaust systems.

Before I dive in, I encourage you to get the US$15 .pdf by Ianto Evans and Leslie Jackson. Probably about Euro$12 or so. Once you have it, my comments about your system are with that .pdf open to page 33, and from by background in other similar physics problems.

I personally _think_ your nine inch flue should help the draft of your eight inch system.

Look at two cycle engines used by serious racers for instance, most visible on motorcycles. In that instance, when the exhaust gas leaves the thick metal walls of the engine block into the thin tubing of the exhaust system it is going to cool rapidly - and slow down. By putting that cone shape in the pipe that gets wider and wider as it gets further from the engine, the slug of cooling gas is allowed to move more and more slowly - but into wider and wider piping so back pressure at the exhaust valve exiting the engine block is reduced. Less back pressure, better air/fuel charge on the next engine cycle, more power to the wheels, hopefully more trophies.

Like this one, just any old google hit on an image: http://www.motosport.com/dirtbike/FMF-FATTY-PIPE-POWERCORE-2-SILENCER-COMBO

Another example is stepped exhaust headers on larger displacement four cycle engines:

decent technical discussion: http://www.team-integra.net/forum/blogs/michaeldelaney/2-header-exhaust-design-effects-engine-power.html

and image alone of simple concept: http://www.team-integra.net/forum/blogs/michaeldelaney/2-header-exhaust-design-effects-engine-power.html notice four small pipes merging into two medium pipes merging in to one pipe even larger still.

The simple concept is increasing the diameter should help keeping gasses flow as they cool. I would estimate at the low temperatures, low velocities and relatively large diameters in your system the effect is nearly negligible; however putting an 8" flue on a 9" burn box would probably be quite noticeable. In that case the cooling slower moving gasses would also have to be forced into a smaller pipe, greatly increasing the risk of stall and smoke blow back. Again, see Evans and Brown, page 33.

So awesome, thanks Michael, what does it mean?

I am not sure. But I have a few questions. Supposedly tall stacks give the gas inside more time to cool, and the gasses move more slowly. On cars, longer exhaust systems- all other things being equal- make for quieter exhaust sounds.

1. Do you know or can you measure your exhaust gas temperature while the rocket is running full speed? I am guessing less than 150C/350F.

2. What happens if you turn the elbow outdoors 180 degrees, so the stack exit is pointing down instead of up?

3. What happens if you take any old sheet of scrap metal say 1 meter square, bend it into a 9 inch cylinder and make your existing stack 1 meter taller without spending a lot of money to find out?

4. Can you add the dimensions of the roof line of your home to the drawing linked above?

From Evans and Jackson, page 34, K is the diameter of the piping in the mass:

Evans and Jackson wrote:K is the exhaust, the horizontal flue that carries hot gas through your floor, heated bench, bed etc. This should be at least the size of the heat riser (8" to 10" diameter) or it could be made of more than one duct, totaling a much bigger cross sectional area


 
Michael Scott
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Thinking about it more I wonder if maybe your stack isn't too tall instead of too short. More curious to see your roof line added to the drawing now.

Also, from Zero's rocket .pdf:

Zero wrote:Everything else about the internal dimensions is all about ratios and cross-sectional areas. In a nutshell,
the cross-sectional area that the hot gases must pass through should be the same or increase slightly at
every point along the path with the exception of the area between the riser and inner tank wall which is
often much greater. So, starting with my riser of 3.5” x 3.5”:
• Horizontal burn tube area should be < or = the riser. Mine is 3.5W x 3.25H (11.375 sq-in)
• Firebox opening also should be < or = riser but not < than the burn tube. Mine is 3.5” x 3.25”
• Area at the top of the riser should be between 1.5x to 2x the area of the riser itself.
• Area of the exhaust port should not be < area of the riser. Mine of 4” round or 12.56 sq-in.



http://alt-nrg.org/rocket.html
 
Michael Scott
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Simon,

I thought of one other inexpensive option. How about you go find a piece of kiln dried lumber and run your new stove on that for a little while, even an hour or two?

I mean no offense, I am new to rockets but not new to heating with wood. Many many people who I think should know better are wildly optimistic about the actual moisture content of their wood piles. Kiln dried lumber from a building supply store or lumber yard should have a moisture content around 7-10%. I have a difficult time getting my wood pile below about 15%, even allowing two years for drying.

I remember some items in the EU are very expensive compared to the US, others very inexpensive; I hope ten or twenty Euros would buy enough kiln dried lumber to run your stove for an hour or two.

If your rocket runs well on kiln dried, the next question is what is the actual moisture content of your wood pile?
 
Simon opaski
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Hey everyone, im sorry not to reply but i had so many things to do, I have rethinked and reobserved everything about the problems, it seems that its not even the pipe.. yesterday it was a windy day, and the rocket mass heater worked like a charm, it sucked air so perfectly with strong rocket sound and everything, i even lifted the burning wood up above 15 the feed tube and the wood was burning downwards, which means good drafting, i dont know if this days atmospheric pressure caused this but it was working nicely, The wood i burn is black locust, Black locust is in abundance here and it burns amazingly hot, even if te wood is wet..
please note that i do not have a wind cap on the chimney and tis might be the simple problem i have, because i observed that wind direction affects the rmh, as a member said 9" inch piping should help turbulence may be he was right, once the rmh is hot enough it drafts like crazy and i even have blue flames. which i know is more oxygen gets drafted. and the wood burns hotter.

One thing im not for sure is why i have to burn so much wood and the mass doesnt warm, its wet but water conducts heat better than cob. Can it be that te mass is too thick and the heat cant get through?
Question: How come i need to burn so much wood with a wet cob, may be i wasnt listening but i dont understand whats the difference between dry and wet cob when it comes to heat dissipation.
 
allen lumley
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Simon Opaski : By now you should be ready to use the information that your last observation gave you ! A strong breeze blowing over and past your vertical chimney will create
an area of low pressure, this is the way the venturi pipe works to make a carburetor a carburetor ! A booster fan located in or near your final vertical chimney will have a similar
effect !

When you are not trying to actively burn wood, shut the fan off and cap the top of the Feed Tube !

You are right, Black Locust does burn hot- however the wood that you are burning is adding in (2) two more additional streams of Water into your fire, there is the water content
from not burning dryer wood and can approach 20% by weight! Also, there is all of the newly released hydrogen that was bound up in the wood fuels hydrocarbons that now is
turning to water H2O ! I will promise that You are moving in the right direction all the time, and eventually your cob will get drier and your wood consumption will drop markedly !

For the Good of the Craft ! Big AL
 
Simon opaski
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allen lumley wrote:
When you are not trying to actively burn wood, shut the fan off and cap the top of the Feed Tube !



i do not use a fan. the rmh is running perfectly right now as well. very rockety. satisfied with burning, and the pipes are burning hot.
does anyone know whats the efficiency compared to a masonry heater?
 
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does anyone know whats the efficiency compared to a masonry heater?



Which masonry heater?

Efficiency is the amount of heat extracted from the wood burned. The first part of that is how completely the wood is burned; the second part is how much of the heat created goes up the chimney without warming the dwelling. Masonry heaters have various fireboxes which may differ on how completely the wood is burned. They are all supposed to do well on the second part which is storing the heat created.

I am a partner in Dragon Heaters. We sell a very efficient cast refractory combustion system, which is the first part. We have used a emissions test device to confirm this. We have also designed plans and kits of the specialty materials for you to create an heater which stores the heat created. We like to have the exhaust leave the heater at 140°F which is sufficient to drive most of the moisture up the chimney.
 
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Simon, with the wet cob, you are using loads of energy to push water out of the cob. It won't act to conduct or store heat, instead it will evaporate. That evaporation takes tremendous energy.

My RMH is very new and steaming as it runs, driving water out of the cob. I expect it will be doing this for some time.
 
Simon opaski
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Peter Ellis wrote:Simon, with the wet cob, you are using loads of energy to push water out of the cob. It won't act to conduct or store heat, instead it will evaporate. That evaporation takes tremendous energy.

My RMH is very new and steaming as it runs, driving water out of the cob. I expect it will be doing this for some time.



thanks, so it might take a hole winter time? i hope not... i see it steaming strongly aswell. next time im building a rocket mass heater i will not use cob, i see cob as a poor conductor and poor heat storing material, the best thing is Definately river pebbles, not because i have it in abundance but because its Quartz, and quartz crystals are very compact glassy structures, another good one would be broken pieces of lead glass, A1 for mass..!!
 
Peter Ellis
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Simon, there are trade offs. Cob is structural and sculptural. You can build forms with it, such as the ubiquitous cob bench. It's monolithic structure makes for good heat transfer and storage, even if it is not an optimal material for that job. River pebbles will not pack solidly, so your mass will be loaded with air pockets. Air pockets equal insulation, and that is not what you want in your thermal mass.

My point is that there are lots of options, each with positives and negatives. The "best" solution is likely to be different from situation to situation.

In my case, I am using cob, rammed earth, and lots of packed sand. My soil is all sand, I have to buy clay, so I am using the smallest amount of cob and rammearth I can manage, with straight sand for almost all of my thermal mass.

Sand is pretty dense and the small particles can be packed tightly, so I am hoping it will serve well. And I have lots of it for the cost of digging
 
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