Lab 8: Measurement of Soil Thermal Properties

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Lab 8 Measurement of Soil Thermal Properties

• Many biophysical questions can only be answered
  by measuring or modeling heat flow in soil
• How fast does the soil heat in spring? (crop
  planting and seed germination)
• How deep will the soil freeze?
• How much heat is stored in the soil?
• Surface energy balance Rn H LE G

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Heat flow and thermal properties of solids (soils)

• Thermal Conductivity k (W m-1 K-1)
• Soils ability to transfer (conduct) heat
• Specific Heat cs (J kg-1 K-1)
• Amount of energy needed to heat 1kg of soil 1
• ?s density of soil (kg/m3)
• cs?s volumetric heat capacity (J m-3 K-1)
• Thermal Diffusivity ? (m2/s)
• Speed of soil response to thermal change
• k?cs?s

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Soil Heat Flux Density

• Atmospheric Heat Fluxes Convection, diffusion
• Heat fluxes through solids Conduction
• Soils are porous media diffusion and latent
  heat transfer come into play, but we will ignore

(Fouriers Law)
Where G is soil heat flux density (W m-2)
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Measurement of soil heat flux
Heat Flux Plates
Epoxy or glass core (known k)
dz
Thermopile (measures dT)
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Combination Method
Gsurface Gplate storage
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Physical barrier to heat and fluid flow
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ksoil
?T/?z
ksoil is measured with the single-needle heat
pulse method (Shiozawa and Campbell 1990)
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Measurement of Heat Capacity - Calculation
Volumetric specific heat can be calculated from
sum of specific heats for each soil constituent
?s cs ?mcmxm ?wcwxw ?acaxa ?ocoxo
Organics
x volume fraction of each constituent mineral
(m), water (w), air (a), and organics (o) (values
for ? and c given in lab 7, Table 1)
Air
Water
Minerals
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Measurement of Heat Capacity Calorimeter

• Heat known mass of soil to 100 C
• Add known mass of water _at_ room temperature (we
  know the heat capacity of water)
• From final temperature of mixture, we can
  calculate heat capacity of soil

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Measurement of Heat Capacity - dual needle heat
pulse

• Dual needle probe inserted into soil
• One needle heated, and temperature response
  measured at other needle

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Dual needle heat pulse theory
heating curve
cooling curve
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Measurement of Heat Capacity - dual needle heat
pulse
q is heat input per unit length of needle
(J/m) r is the distance between the needles ?Tm
is the maximum temperature increase at the second
needle tm is the time at which ?Tm occurs
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Measurement of Thermal Conductivity single
needle heat pulse

• Single needle inserted into soil and heated
• Temperature response measured at same needle
• Same set of equations as dual needle technique

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Measurement of Thermal Conductivity single
needle heat pulse
q is the heat input to the probe (W m-1) k is
thermal conductivity (W m-1K-1) T is temperature
t is time (s) C is a constant
What do we notice about this equation?
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Measurement of Thermal Conductivity single
needle heat pulse
Linear relationship!! (y mx b)
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Todays Lab

• Measure heat capacity with calorimeter
• Measure thermal conductivity with single needle
  heat pulse approach
• Use soil heat flux plate