Campbell Scientific CR1000KD CR1000 Measurement and Control System - Page 297

Table 61. Resistive-Bridge Circuits with Voltage Excitation Resistive-Bridge Type and Circuit Diagram CRBasic Instruction and Fundamental Relationship Section 8. Operation Relationships 1Key: Vx = excitation voltage; V1, V2 = sensor return voltages; Rf = "fixed", "bridge" or "completion" resistor; Rs = "variable" or "sensing" resistor. 2Where X = result of the CRBasic bridge measurement instruction with a multiplier of 1 and an offset of 0. 3See the appendix Resistive Bridge Modules (p. 561) for a list of available terminal input modules to facilitate this measurement. CRBasic Example 65. 'Declare Variables Public X Public X1 Public R1 Public R2 Public R3 Public R4 Four‐Wire Full‐Bridge Measurement and Processing 'Main Program BeginProg R2 = 1000 R3 = 1000 R4 = 1000 Scan(500,mSec,1,0) 'Resistance of R2 'Resistance of R3 'Resistance of R4 '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. 297