Campbell Manufacturing CR10 manual 188

SECTION 13. CR10 MEASUREMENTS

TABLE 13.3-5. Summary of Input Settling Data For Campbell Scientific Resistive Sensors

Sensor	Belden	Ro	Cw	τ*	Input	Vx(mV)	Veo(mV)**
Model #	Wire #	(kohms) (pfd/ft.) (us)			Range(mV)
107	8641	1	42	45	7.5	2000	50
207(RH)	8771	1	41	44	250	1500	85
WVU-7	8723	1	62	65	7.5	2000	0
227	8641	0.1-1	42	5-45	250	250	0
237	8641	1	42	45	25	2500	65
024A	8771	1-6	41	1-222	250	500	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 excitation, Vx.

TABLE 13.3-6. Maximum Lead Length vs. Error for Campbell Scientific Resistive Sensors

Sensor					Maximum
Model #	Error	Range		Ve(µV)	Length(ft.)
107	0.05°C	0°C to	40°C	5	10001
207(RH)	1%RH	20% to 90%		250	20003
WVU-7	0.05°C	0°C to	40°C	5	8522
024A	3o	@ 360°		2083	3802
227	-	-		-	20003
237	10 kohm	20k to	300k	1000	20003

1based on transient settling

2based on signal rise time

3limit of excitation drive

The comparatively small transient 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. If the capacitive load 0.1 µfd and the resistive load is negligible, Vx will oscillate about its control point. If the capacitive load is

0.1or less, Vx will settle to within 0.1% of its correct value 150 µs. A lead length of 2000 feet is permitted for the Model 227 before 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, Ve, 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, Instruction 4, Excite, Delay, and Measure, can be used to insure ample settling time for half bridge, single-ended sensors.

2.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, Rf'. 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, Rf, as shown in Figure 13.3-7B, lowers the source resistance of the CR10 input. This offers no improvement over configuration A because Rf' still combines with the lead capacitance to slow the signal response at point P. The

13-9