Campbell Scientific CR1000KD CR1000 Measurement and Control System - Page 295

Section 8. Operation 1A/D (analog-to-digital) conversion time = 15 µs 2Reps/No Reps -- If Reps > 1 (i.e., multiple measurements by a single instruction), no additional time is required. If Reps = 1 in consecutive voltage instructions, add 15 µs per instruction. 8.1.3 Resistance Measurements Many sensors detect phenomena by way of change in a resistive circuit. Thermistors, strain gages, and position potentiometers are examples. Resistance measurements are special-case voltage measurements. By supplying a precise, known voltage to a resistive circuit, and then measuring the returning voltage, resistance can be calculated. Read More! Available resistive bridge completion modules are listed in the appendix Signal Conditioners (p. 561). Five bridge measurement instructions are features of the CR1000. Table Resistive Bridge Circuits -- Voltage Excitation (p. 296) show circuits that are typically measured with these instructions. In the diagrams, resistors labeled Rs are normally the sensors and those labeled Rf are normally precision fixed (static) resistors. Circuits other than those diagrammed can be measured, provided the excitation and type of measurements are appropriate. CRBasic example Four-wire Full-bridge Measurement (p. 297) shows CR1000 code for measuring and processing four-wire full-bridge circuits. All bridge measurements have the parameter RevEx, which has an option to make one set of measurements with the excitation as programmed and another set of measurements with the excitation polarity reversed. The offset error in the two measurements due to thermal EMFs can then be accounted for in the processing of the measurement instruction. The excitation channel maintains the excitation voltage or current until the hold for the analog to digital conversion is completed. When more than one measurement per sensor is necessary (four-wire half-bridge, three-wire half-bridge, six-wire full-bridge), excitation is applied separately for each measurement. For example, in the four-wire half-bridge, when the excitation is reversed, the differential measurement of the voltage drop across the sensor is made with the excitation at both polarities and then excitation is again applied and reversed for the measurement of the voltage drop across the fixed resistor. Calculating the resistance of a sensor that is one of the legs of a resistive bridge requires additional processing following the bridge measurement instruction. The table Resistive-Bridge Circuits with Voltage Excitation (p. 296) lists the schematics of bridge configurations and related resistance equations. 295