SECTION 7. MEASUREMENT PROGRAMMING EXAMPLES

The average is used, instead of a sample, in order to cancel out effects of wind loading on the lysimeter.

PROGRAM

01:P9 Full BR w/Compensation

01:

1

Rep

02:25 2500 mV 60 Hz rejection EX Range

03:22 7.5 mV 60 Hz rejection BR Range

04:

1

IN Chan

05:1 Excite all reps w/EXchan 1

06: 2500

mV Excitation

07:1 Loc [:RAW mm ]

08:

46.583

Mult

09:

0

Offset

02:

P34

Z=X+F

01:1 X Loc RAW mm

02: 266

F

03:2 Z Loc [:mm H20 ]

7.14227 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 option is used. The output of Instruction 5 is the ratio of the output 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 -10 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 -10 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 polynomial coefficients.

In this example, we wish to make measurements on six 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 1-6 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 7-12 by changing the value in Parameter 3 to 7 in Instruction 55. Section 8.3 gives an example using the AM416 Multiplexer to measure 16 Soil Moisture Blocks.

FIGURE 7.14-1. 6 227 Gypsum Blocks Connected to the CR10

7-13