Campbell Scientific 229 Water Matric Potential Sensor and CE4/CE8 - Manual (Page 23)

229 Heat Dissipation Matric Water Potential Sensor
22: Sample (P70) ;Sample 16 initial soil temperature readings
1: 16 Reps
2: 3 Loc [ Tinit_1 ]
23: Sample (P70)
;Sample 16 delta T readings
1: 16 Reps
2: 51 Loc [ dT_1 ]
24: Do (P86)
1: 21
Set Flag 1 Low
6. Calibration
6.1 General
The heat transfer properties of a 229 sensor depend both on the thermal
properties of the various sensor materials and on the interfaces between the
different materials. Heat transfer between the stainless steel needle containing
the heating element and thermocouple and the ceramic material depends on the
density of points-of-contact between two different materials. Heat transfer also
depends on the arrangement of the wires in the hypodermic needle and the
amount of contact between the needle and the ceramic. The uncontrollable
variability in heat transfer properties warrants individual calibration of the 229
sensors.
The calibration used to relate temperature increase and the soil water potential
is strictly empirical, and the functional expression of the relationship can take
several forms. The most commonly used function is:
) * exp( β α ψ + Δ = T
[1]
with ψ the soil water potential, exp the exponential function, ΔT the
temperature increase during the chosen heating period of time, α the slope and
β the intercept.
The relationship between the natural logarithm of soil water tension and the
temperature increase is linear which simplifies derivation of the calibration
function.
β α ψ + Δ = T * |) ln(| [2]
(Soil water potential is a negative value and becomes more negative as soil
dries.) Figure 6-1 is a typical calibration and the data set is easily described
with linear regression. A power function works well in applications when
calibration is needed for -500 kPa
≤ matric potential ≤ -10 kPa. A power
function calibration has the form | ψ| = a*(ΔT) b with the multiplier, a, and
exponent, b, as fitted parameters.
A variety of calibration methods are suitable. The sole requirement is that the
water potential of the medium surrounding the sensor must be known. Either
the applied potential can be controlled at a specified value or the water
potential can be independently measured. Hanging water columns and
pressure plate extractors are typically used. Several data values which
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