Campbell Scientific 227-L Delmhorst Cylindrical Soil Moisture Block - Manual (Page 16)

227 Delmhorst Cylindrical Soil Moisture Block
TABLE 7-3. Typical Soil Water
Potential, R
s
and V
s
/ V
x
BARS R
s
(kohms) V
s
/V
x
0.1 0.060 0.0566
0.2 0.130 0.1150
0.3 0.260 0.2063
0.4 0.370 0.2701
0.5 0.540 0.3506
0.6 0.750 0.4286
0.7 0.860 0.4624
0.8 1.100 0.5238
0.9 1.400 0.5833
1.0 1.700 0.6296
1.5 3.400 0.7727
1.8 4.000 0.8000
2.0 5.000 0.8333
3.0 7.200 0.8780
6.0 12.500 0.9259
10.0 17.000 0.9444
11.0 22.200 0.9569
12.0 22.400 0.9573
13.0 30.000 0.9677
14.0 32.500 0.9701
15.0 35.000 0.9722
For the typical resistance values listed in TABLE 7-3, soil water potential
(bars) is calculated from sensor resistance (R
s
) using a 5th order polynomial
(FIGURE 7-2 and TABLE 7-4). The nonlinear relationship of R
s
to bars rules
out averaging R
s
directly.
The polynomial is entered as an expression in CRBasic or by Edlog instruction
Polynomial (P55). The polynomial to calculate soil water potential is fit to the
0.1 to 10 bar range using a least square fit. TABLE 7-4 lists the coefficients
and equation for the 0.1 to 10 bar polynomial.
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