Lab 7: Measurement of Vapor Conductance

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Lab 7 Measurement of Vapor Conductance

• Last week we measured heat conductance
• We intuitively understand heat flux
• This week we measure another common flux
• Water vapor flux
• Major means of energy exchange in some
  environments (latent heat of vaporization)
• Dealt with all the time
• Cooling after a swim
• Transpiration from a leaf surface

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Vapor conductance from an animal or plant

• Again, flux is driven by a concentration
  difference and limited by a conductance to water
  vapor

E evaporation (mol m-2 s-1) gv vapor
conductance (mol m-2 s-1) Cv vapor mol fraction
(mol H2O/mol air)
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Free water in tissue is limited in movement by
barriers Diffusion or convection of vapor in
boundary layer Cuticle or skin at surface
Subscripts t, s, and a are tissue, surface, and
air
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Evaporation from an animal or plant
How can we estimate Cvt?
5
gva - estimating from theoretical values
k is now 0.147 (not the 0.135 for
heat) Estimation is not very accurate
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gva- measuring from complex shapes

• Where is the difficulty
• Calculations of gva from fluid dynamics theory
  lack precision
• Complex shapes are difficult to find a
  characteristic dimension for
• Also, it may be hard to calculate the wind speed
  (e.g. in canopy)
• Solution build physical model
• keep surface wet. Why?
• measure water loss over time (evaporation)

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Model of old system
gvs
gva
Cvs
Cvt
Cva
8
Model of new system

• If we keep surface wet
• Reduce rvs to zero (gvs 8)
• Cvs es(Tsurface)

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gva-measuring from complex shapes
(simplified with assumptions from previous slide)

• What do we need to know to calculate gva?
• Evaporation - weighing or reservoir
• Cvs surface is wet, es(Ts)
• Cva measure ea

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Example If a 0.01 m2 Plaster of Paris frog was
at 20o C, ea 1 kPa and it lost 1.2 gH2O in 1 h.
What is the boundary layer conductance gva?
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Now lets look at gvs
Cvt es(Ttissue) Cva youd better know how to
measure this! gva we can estimate from theory
or measure (previous slide) gvs ??
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gvs - weighing method

• Not practical to calculate surface conductance
• Incomplete theory
• Measure gvs by creating system where we know all
  of the other parameters
• E mass of water lost over time period (obtain
  by weighing)/surface area of object
• Cva measure with psychrometer, etc.
• Cvt es(Tobject)
• What can we do to make rva 0 (gva 8)???

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Governing equation and model of overall system
What happens if gva becomes very large?
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Simplified system
gvs
Cvs Cva if gva is large
Cvt
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gvs direct measurement with diffusion porometers

• Three types
• Dynamic Porometer
• Delta-T Devices, UK
• Measures time it takes to increase humidity 5
• Null Balance Porometer
• Li-COR (discontinued)
• Measures increase in humidity over dry air
• Steady State Porometer
• Decagon
• Uses known conductances in series with gvs to
  measure gvs

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LI-COR Null Balance Porometer
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Using null balance porometer to find gvs
combine
f is flow rate over leaf Cva is vapor mole
fraction coming out of cuvette Cva(in) is vapor
mole fraction coming into cuvette (0) A is
surface area of cuvette
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Delta T dynamic porometer
Increase in vapor concentration over time in
chamber is proportional to stomatal conductance
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Decagon Steady State Porometer
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Using steady state porometer to measure gvs
gd1 and gd2 are strictly a function of the
distance between the two measurements
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Todays lab

• Measure gva from an evaporating surface at three
  wind speeds
• Keep surface wet so gvs is infinity
• Get E by measuring water volume pulled from
  reservoir
• Compare gva measured to gva theoretical
• Measure gvs of plant leaves with diffusion
  porometer(s)
• Look at environmental effects on gvs
• Calculate gvs from given situation