1 ac excitation – Campbell Scientific CR800 and CR850 Measurement and Control Systems User Manual

Page 293

Section 8. Operation

293

Table 61. Resistive-Bridge Circuits with Voltage Excitation

Resistive-Bridge Type and

Circuit Diagram

CRBasic Instruction and

Fundamental Relationship

Relationships

Key: V

= excitation voltage; V

, V

= sensor return voltages; R

= "fixed", "bridge" or "completion" resistor; R

= "variable" or

"sensing" resistor.

Where X = result of the CRBasic bridge measurement instruction with a multiplier of 1 and an offset of 0.

See the appendix Resistive Bridge Modules

(p. 539)

for a list of available terminal input modules to facilitate this measurement.

CRBasic Example 64. Four‐Wire Full‐Bridge Measurement and Processing

'Declare Variables

Public

'Main Program

BeginProg

R2 = 1000

'Resistance of R2

R3 = 1000

'Resistance of R3

R4 = 1000

'Resistance of R4

Scan

(500,mSec,1,0)

'Full Bridge Measurement:

BrFull

(X,1,mV2500,1,1,1,2500,True,True,0,_60Hz,1.0,0.0)

X1 = ((-1 * X) / 1000) + (R3 / (R3 + R4))
R1 = (R2 * (1 - X1)) / X1

NextScan

EndProg

8.1.3.1 ac Excitation

Some resistive sensors require ac excitation. These include electrolytic tilt
sensors, soil moisture blocks, water conductivity sensors, and wetness sensing
grids. The use of dc excitation with these sensors can result in polarization, which
will cause erroneous measurement, shift calibration, or lead to rapid sensor decay.