Campbell Scientific CR6 CR6 Measurement and Control System - Page 221

Section 7. Installation 7.9.12.5.4 FieldCal() Slope (Opt 3) Example Most CRBasic measurement instructions have a multiplier and offset parameter. FieldCal() Option 3 adjusts the multiplier argument such that the output of the sensor being calibrated is set to a value consistent with the linear relationship that intersects two known points sequentially entered in the FieldCal() KnownVar parameter. Subsequent measurements are scaled with the same multiplier. FieldCal() Option 3 does not affect offset. Some measurement applications do not require determination of offset. Wave form analysis, for example, may only require relative data to characterize change. Case: A soil-water sensor is to be used to detect a pulse of water moving through soil. A pulse of soil water can be detected with an offset, but sensitivity to the pulse is important, so an accurate multiplier is essential. To adjust the sensitivity of the sensor, two soil samples, with volumetric water contents of 10% and 35%, will provide two known points. Table 39. Calibration Report for Water Content Sensor At Deployment SimulatedWaterContentSignal 175 mV KnownWC 10 % SimulatedWaterContentSignal 700 mV KnownWC 35 % WCMultiplier 0.0476 %/mV The following procedure sets the sensitivity of a simulated soil water-content sensor. 1. Send CRBasic example FieldCal() Multiplier (p. 221) to the CR6. 2. To simulate the soil-water sensor signal, place a jumper wire between terminals U1 and U2. 3. Simulate deployment-calibration conditions in two stages as follows: a. Set variable SimulatedWaterContentSignal to 175. Set variable KnownWC to 10.0. b. Start the calibration by setting variable CalMode = 1. c. When CalMode increments to 3, set variable SimulatedWaterContentSignal to 700. Set variable KnownWC to 35. d. Resume the calibration by setting variable CalMode = 4 4. When variable CalMode increments to 6, the calibration is complete. Calibrated multiplier is 0.0476. 221