Campbell Scientific 227-L Delmhorst Cylindrical Soil Moisture Block User Manual

Page 17

227 Delmhorst Cylindrical Soil Moisture Block

Block Resistance (kOhms)

Soil Water Potential (-Bars)

Typical Values from Table 2
0.1 to 10 Bar Polynomial Fit

Typical Values from TABLE 7-3

Block Resistance (kohms)

FIGURE 7-2. Polynomial fit to typical block resistance vs. water

potential

TABLE 7-4. Polynomial Coefficients for Converting Sensor Resistance to Bars

BARS = C0 + C1(Rs) +C2(Rs)2+C3(Rs)3+C4(Rs)4+C5(Rs)5

BARS MULT.

(R1)

0.1 to 10

0.1

0.15836

6.1445

–8.4189

9.2493

–3.1685

0.33392

The coefficients used for the 10 bar range require R

to be scaled

down by a factor of 0.1. In Edlog, this multiplier can be entered
in the Bridge Transform Instruction or in Processing
Instruction 37.

NOTE

TABLE 7-5 shows errors between from the least-squares polynomial
approximation and the typical water potential values.

Our manuals used to show a separate polynomial for the 0.1 to 2
bar range that had slightly smaller deviations from the typical
values over the narrower range. However, the variability
between blocks is much greater than the improved fit and does
not warrant the more complicated program.

NOTE