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Water versus Sand in the WSM? - Page 2

post #21 of 26
Quote:
Originally Posted by valleypoboy View Post

Water I understand. Sand and clay I dont. Brian, you did a great job of explaining it but why not use something with good thermal conductivity that will absorb the heat then retain it like a big chunk of iron or steel? Sand and clay are terrible thermal conductors. Aluminum is great but will heat up and cool down too fast (imo), copper is excellent, but even faster and expensive. Cold rolled steel is pretty cheap and would work great imho.

 

You don't want a good thermal conductor, you actually want a poor conductor.  The idea behind the sand or clay pot or pizza stone or big rock is that it will absorb the higher temps ( by taking time to heat up ) and release that stored heat back to the smoker as the fire starts to die, or there is a wind gust or something.  Think of it as more of a temperature shock absorber.  Metal would give the heat off too quickly, unless you are going to go for a huge amount of cast iron, but then the sand is cheaper.

Quote:
Originally Posted by bobcats110 View Post
 

Is this talk of sand and thermal conductors new?  I'm actually sitting at my desk thinking you've all split the atom again or something.  Learned more in the last five minutes about my WSM and a new way to use it, so thank you.

 

I get the explanation that Brian gives and understand the theory, and if you take the experiences here that NOT using water DOESN'T result in dry meat, then awesome - I hated the water clean up myself.

 

My question now is - What kind of sand are we talking about?  White Beach, Sandbox that you can buy at Home Depot, etc., etc.

 

Thanks for a great thread,  friends in smoke.....

yeah, any clean sand.  I got a bag a play sand form homer depot for a couple of bucks.  

post #22 of 26

Hmmm... not sure I can agree with the suggestion that using a porous thermal mass inside the cooker is better than one that is highly conductive (of heat).  Consider, for example, that the thermal mass is entirely contained inside the cooker and that any energy that it absorbs or releases is within the cooker as well.  As the temperature in the cooker goes up (spike in charcoal temperature), you want the thermal mass to 'react' quickly, by absorbing that heat.  If the spike in temperature ends and the cooker starts to cool, then you want the thermal mass to release the energy back into the chamber quickly as well.  Porous materials such as clay, sand, or concrete are noted for use where their heat insulation capabilities are important ...like an adobe house that is cool during the day in spite of the rising temperature of the hot sun outside the house.  In that case, you are trying to prevent the temperature differential on opposite sides of the adobe from conducting through the material.  With a BBQ, something like a ceramic or clay BBQ body would help maintain a consistent temperature inside the cooker by preventing sun/wind/rain from conducting heat into/out of the cooker, but a highly conductive mass (say, iron) that is inside the cooker stabilizes the temperature inside the cooker by easily absorbing spikes in energy/heat and easily giving out energy/heat when the inside of the cooker cools - a stabilizing element.  This is why The Big Green Egg (and similar) are made from clay, but no manufacturer of BBQs suggests using sand or clay or bricks inside the cooker ...instead preferring water, both because it conducts heat more easily and also has that 212 F state conversion (liquid to vapor) constant that makes the BBQ more forgiving of too-open vents.

 

Here's some thermal conductivity numbers, followed by an experiment that someone with a remote thermometer should try (source: http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html):

 

Air                                 0.024

Insulating Brick               0.15

Dry Sand                        0.15 - 0.25

Water                             0.58

Building Brick                  0.60 - 1.0

Granite                            1.7 - 4.0

Iron                                80

 

An experiment (hopefully in a somewhat controlled environment, e.g. same outside temperature and no wind or rain):

 

1. Empty bowl (the 'Air' test) - run BBQ at say 250 F and stabilize for 15 minutes, then open vents fully and monitor temperature every 30 seconds (or 1 minute).

 

2. Bowl of water (weighed), steaming/simmering (water plus state change test) - run BBQ at say 250 F so it is stable for at least 15 minutes (water must be steaming/changing liquid to vapor), then open vents fully and monitor temperature as above.

 

3. Repeat with same weight in dry sand as water, same procedure as in #1

 

4. Repeat with same weight in iron, same procedure as in #1

 

Not perfectly scientific, but the range in thermal conductivities is wide enough to at least illustrate a pattern.  The best temperature damper will show the slowest response to opening the vents.  Maybe some weekend, I will try this?

 

 

Brian


Edited by BrazosBrian - 9/25/15 at 11:12am
post #23 of 26
I believe the Columbians during the big cocaine trade used to take big slabs of granite with a fire underneath it to make pretty much a flat top grill to heat up the Coke if it got wet during transport.

I wonder how granite would do inside the pit. The only problem I see with granite is it's extremely heavy and I feel like it would crack over time.
post #24 of 26

I'll bet you can get surplus chunks of granite from your local tombstone maker.  In a prior life, we used to have custom slabs cut at the local tombstone maker for use in high-speed manufacturing to stabilized machines with fast-moving robotics in them.  There were always chunks of granite in odd shapes laying around the tombstone shop... I updated my post above to include the thermal conductivity of granite ...it's good stuff!

 

Brian

post #25 of 26
So is the pan of sand acting like a heat sink does in electronics?
post #26 of 26
Quote:
Originally Posted by bobcats110 View Post
 

Is this talk of sand and thermal conductors new?  I'm actually sitting at my desk thinking you've all split the atom again or something.  Learned more in the last five minutes about my WSM and a new way to use it, so thank you.

 

I get the explanation that Brian gives and understand the theory, and if you take the experiences here that NOT using water DOESN'T result in dry meat, then awesome - I hated the water clean up myself.

 

My question now is - What kind of sand are we talking about?  White Beach, Sandbox that you can buy at Home Depot, etc., etc.

 

Thanks for a great thread,  friends in smoke.....


Amen!

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