Campbell Scientific CR10 CR10 Measurement and Control - Page 175

SECTION 13. CR1O MEASUREMENTS TABLE 13.3-5. Summary of Input Settling Data For Campbell Scientific Resistive Sensors Sensor Model # Belden Wire # Ro Cw a' (kohms) (ptdlft.) (us) Input Range(mV) Vr(mV) V"o(mV)** 107 207(RH) WVU-7 227 237 o24A 8641 8771 8723 8641 8641 8771 1 42 1 41 1 62 0.1-1 42 1 42 1-6 41 45 7.5 44 250 65 7.5 5-45 250 45 25 1-222 250 2000 1 500 2000 250 2500 500 50 85 0 0 65 0-90 Estimated time constants are for 1000 foot lead lengths and include 3.3nfd CR10 input capacitance. Measured peak transients for 1000 foot lead lengths at corresponding eXS{q!!q1.V* TABLE 13.3-6. Maximum Lead Length vs. Error for Campbell Scientific Resistive Sensors Sensor Model# Error Range Maximum V"(uv) Length(ft.) 107 2A7(RH) WVU-7 o24A 227 237 0.05'c 1o/"RH 0.05'c 3o 10 kohm OoC to 40"C 2oo/o to 9oo/o OoC to 40oC @ 360' 20k to 300k 5 250 5 2083 1000 10001 2oo03 8522 3802 2ooo3 20003 1 based on transient settling 2 based on signal rise time 3 limit of excitation drive The comparatively smalltransient yet large source resistance of the 024A sensor indicates that signal rise time may be the most important limitation. The analysis in Section 13.3.2 confirms this. The Model 227 Soil Moisture Block has a relatively short time constant and essentially no transient. Lead lengths in excess of 2000 feet produce less than a 0.1 bar (0-10 bar range) input settling error. With this sensor, the drive capability of the excitation channel limits the lead length. lf the capacitive load 0.1 pfd and the resistive load is negligible, V, will oscillate about its control point. lf the capacitive load is 0.1 or less, V, will settle to within 0.1% of its correct value 150 ps. A lead length of 2000 feet is permitted for the Model 227 betore approaching the drive limitation. Table 13.3-6 summarizes maximum lead lengths for corresponding error limits in six Campbell Scientific sensors. Since the first three sensors are nonlinear, the voltage error, V", is the most conservative value corresponding to the error over the range shown. MINIMIZING SETTLING ERRORS IN NON' CAMPBELL SCIENTIFIC SENSORS When long lead lengths are mandatory in sensors configured by the user, the following general practices can be used to minimize or measure settling errors: 1. When measurement speed is not a prime consideration, lnstruction 4, Excite, Delay, and Measure, can be used to insure ample settling time for half bridge, single-ended sensors. An additional low value bridge resistor can be added to decrease the source resistance, Ro. For example, assume a YSI nonlinear thermistor such as the model 44032 is used with a 30 kohm bridge resistor, R1. A typical configuration is shown in Figure 13.3-7A. The disadvantage with this configuration is the high source resistance shown in column 3 of Table 13.3-7. Adding another 1 K resistor, R1, as shown in Figure 13.3-TB,lowers the source resistance of the CR10 input. This offers no improvement over configuration A because Rt stillcombines with the lead capacitance to slow the signal response at point P" The source resistance at point P (column 5) is