CR200/CR200X Series Dataloggers - Campbell Scientific
CR200/CR200X Series Dataloggers - Campbell Scientific
CR200/CR200X Series Dataloggers - Campbell Scientific
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Section 4. Sensor Support<br />
4.2.5 Self-Calibration<br />
A calibration measurement to measure the ground offset is made at the<br />
beginning of each measurement instruction that includes a voltage<br />
measurement. This calibration takes about 400 microseconds. Only one<br />
calibration measurement is made per instruction regardless of the number of<br />
reps.<br />
The battery voltage is checked every 8 seconds to ensure it is within the<br />
allowable range.<br />
4.3 Bridge Resistance Measurements<br />
Many sensors detect phenomena by way of change in a resistive circuit.<br />
Thermistors, strain gages, and position potentiometers are examples. Resistance<br />
measurements are special case voltage measurements. By supplying a precise,<br />
known voltage to a resistive circuit, then measuring the returning voltage,<br />
resistance can be calculated.<br />
Two bridge measurement instructions are included in the <strong>CR200</strong>(X), ExDelSE<br />
() and Therm109 (). ExDelSE () is used with sensors that have a simple half<br />
bridge circuit. Therm109 () is used with <strong>Campbell</strong> <strong>Scientific</strong>’s 109-L thermistor<br />
probe. Sensors with bridge circuits that require a differential voltage<br />
measurement, such as full bridge or 3-wire half bridge, cannot be measured with<br />
the <strong>CR200</strong>(X).<br />
FIGURE. Half Bridge Circuit Used with ExDelSE (p. 41) shows the circuit that<br />
is typically measured with ExDelSE (). In the diagram. Rs is normally the<br />
sensor and Rf is normally a precision fixed (static) resistor. Vx is the excitation<br />
voltage (either 2500 or 5000 mV) and V1 is the voltage (mV) measured by the<br />
analog input channel.<br />
Calculating the resistance of a sensor that is one of the legs of a resistive bridge<br />
requires additional processing following the bridge measurement instruction.<br />
FIGURE. Half Bridge Circuit Used with ExDelSE (p. 41) lists the schematics of<br />
a typical half bridge configuration and the calculations necessary to compute the<br />
resistance of any single resistor, provided the value of the other resistor in the<br />
bridge circuit is known.<br />
Figure 28: Voltage Excitation Bridge Circuit<br />
41