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SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES

7-13

02: P34 Z=X+F

01: 1 X Loc mm RAW

02: 266 F

03: 2 Z Loc [:mm CORECT]

7.16 227 GYPSUM SOIL MOISTURE

BLOCK

Soil moisture is measured with a gypsum block

by relating the change in moisture to the change

in resistance of the block. An AC Half Bridge

(Instruction 5) is used to determine the

resistance of the gypsum block. Rapid reversal

of the excitation voltage inhibits polarization of

the sensor. Polarization creates an error in the

output so the fast integration time is used. The

output of Instruction 5 is the ratio of the mid-

bridge voltage to the excitation voltage, this

output is converted to gypsum block resistance

with Instruction 59, Bridge Transform.

The Campbell Scientific 227 Soil Moisture Block

uses a Delmhorst gypsum block with a 1 kohm

bridge completion resistor. Using data supplied

by Delmhorst, Campbell Scientific has

computed coefficients for a 5th order polynomial

to convert block resistance to water potential in

bars. There are two polynomials, one to

optimize the range from -0.1 to -2 bars and one

to cover the range from -0.1 to -15 bars (the

minus sign is omitted in the output). The -0.1 to

-2 bar polynomial requires a multiplier of 1 in the

Bridge Transform Instruction (result in Kohms)

and the -0.1 to -15 bar polynomial requires a

multiplier of 0.1 (result in 10,000s of ohms).

The multiplier is a scaling factor to maintain the

maximum number of significant digits in the

coefficients of the polynomial.

In this example, we wish to make

measurements on 12 gypsum blocks and output

the final data in bars. The soil where the

moisture measurements are to be made is quite

wet at the time the data logging is initiated, but

is expected to dry beyond the -2 bar limit of the

wet range polynomial. The dry range

polynomial is used, so a multiplier of 0.1 is

entered in the bridge transform instruction.

When the water potential is computed it is

written over the resistance value. The

potentials are stored in Input locations 1-12

where they may be accessed for output to Final

Storage. If it was desired to retain the

resistance values the potential measurements

could be stored in locations 13-24 by changing

parameter 3 in Instruction 55 to 13.

FIGURE 7.16-1. 12 Gypsum Blocks

Connected to the CR7

The first 6 blocks are excited by excitation

channel 1 and the last 6 by channel 2. Thus, 6

is entered for the number of measurements per

excitation channel in Parameter 7 of Instruction

PROGRAM

01: P5 AC Half Bridge

01: 12 Reps

02: 16 500 mV fast Range

03: 1 IN Card

04: 1 IN Chan

05: 1 EX Card

06: 6 EX Chan

07: 6 Meas/EX

08: 500 mV Excitation

09: 1 Loc [:POTEN #1 ]

10: 1 Mult

11: 0 Offset

02: P59 BR Transform Rf[X/(1-X)]

01: 12 Reps

02: 1 Loc [:POTEN #1 ]

03: .1 Multiplier (Rf)

03: P55 Polynomial

01: 12 Reps

02: 1 X Loc POTEN #1

03: 1 F(X) Loc [:POTEN #1 ]

04: .15836 C0

05: 6.1445 C1

06: -8.4139 C2

07: 9.2493 C3

08: -3.1685 C4

09: .33392 C5

1 2 ... 76 77 78 79 80 81 82 83 84 85 86 ... 185 186

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