Thermal Conductivity of Soil Measurement
Name: Nicole M.
I'm doing a science project and I am measuring the
thermal conductivity of soil and I wanted to know if there was a better
tool to use other than a thermometer. I also need information on muck and peat.
Sorry about the delay in getting back to you; I've been out of town at a
technical conference since last Saturday and just got back. I sent your
message to one of my former professors who is much more knowledgeable than I
on the topic of thermal conductivity of soil and also an authority on
engineering instrumentation. (As well as an all-around swell guy, as you
can see by his response.)
Don't get too wrapped up in the units of the numbers he's given below. The
first number is in US Customary units and the second number is in SI or
metric. Just look at the relative difference in the numbers. Note that
soil is more conductive than air (assuming that the air is at the same
starting temperature and sealed up), but hundreds of times less conductive
than copper. Most importantly, the moisture content of the soil is very
important. I would try to work with soils that are fully saturated (that
is, completely wet) to take that factor out of your experiment. Rather than
using air as a reference, I would suggest using water.
Also, it looks like using a thermometer is your best choice, but try to
embed it as far down as possible in the sample and do the experiment in the
shade and protected from the wind.
If you cannot get clay out of the ground where you are, try calling a local
geotechnical engineer (that's a civil engineer who specializes in soils) or
well driller and ask if they could spare a very small amount of bentonite in
powder or pellet form. This is a pure clay used to seal wells and other
soil borings to prevent contaminants from getting in the hole. It's very
cheap (a few pennies worth, I imagine they'll just give you some) and you'll
only need a small amount. You'll find it's very gooey when mixed with
water. People who do pottery also use a lot of clay. That will probably be
your best choice for home-made much.
Best of luck with your science experiment! If I find any more information
or if Professor Ray learns anything from his geologist friend, I'll let you
Herb Cohen is a world expert on peat. He's in the geology department
here. I'll see what he thinks
As far as I can remember, the thermometer sounds like a good idea. I am
wondering however just what the experimental set-up will be. If the
thermometer is directly exposed to radiant head (like sticking it in the
ground a little ways on a hot, sunny day, the thermometer may heat up and
conduct heat to its own system). Thermistors from say, Radio shack, with the
little hobby circuit, may also be a good choice, but require calibration
(ice and steam) or thermometer, again.
The thermal conductivity of most soils is a big complicated due to moisture.
Check Jim Mitchell's book, Fundamentals of Soil Behavior for a discussion,
or Yoder and Witzak discussing frost penetration.
Table 12.3 from Mitchell
Air 0.014 BTU/hr/ft^2/deg. F/ft
0.024 W/m/deg. Kelvin
Water 0.30 0.60
Ice 1.30 2.25
Shale 0.90 1.56
Granite 1.60 2.76
Copper 225 389
Soil 0.15-1.5 say, 1.0
0.25-2.7 say 1.7
Looking at other figures in the same chapter, variation as a function of
water content is about a factor of 2 or 3
times more conductive when saturated as when dry
Could you give me a brief description of your experiment? I need to
understand what you are measuring a little better.
Peat is a term referring to material made up almost exclusively of
decomposed organic material. Muck, on the other hand, is not really an
"official" soil term. Perhaps what you are referring to is a highly organic
clay. This is similar to peat, but also has non-organic soil mixed in.
Andy Johnson, Ph.D., P.E.
Click here to return to the General Topics Archives
Update: June 2012