Campbell Manufacturing CR10 manual 187

SECTION 13. CR10 MEASUREMENTS

TABLE 13.3-4. Measured Peak Excitation Transients for 1000 Foot Lengths of Three Belden

Lead Wires Used by Campbell Scientific

Vx(mV)		Rf=1-----------------------kohm		Veo(mV) -----------------------	Rf=10 kohm
	#			#		#	#
		#	#
	8641	8771	8723	8641	8771	8723
2000	50	100	60	100	140	80
1000	25	65	40	60	90	40

NOTE: Excitation transients are eliminated if excitation leads are contained in a shield independent from the signal leads.

The size of the peak transient is linearly related to the excitation voltage and increases as the bridge resistor, Rf, increases. Table 13.3-4 shows measured levels of Veo for 1000 foot lengths of three Belden wires used in Campbell Scientific sensors. Values are given for Rf equal to 1 kohm and 10 kohm. Table 13.3-4 is meant only to provide estimates of the size of excitation transients encountered; the exact level will depend upon the specific sensor configuration.

Equation 13.3-7 can be solved for the maximum lead length, L, permitted to maintain a specified error limit. Combining Equations

13.3-7 and 13.3-4 and solving for L gives:

L = -(RoCf+ (t/ln(Ve/Veo)))/RoCw [13.3-15]

where Ve is the measurement error limit.

EXAMPLE LEAD LENGTH CALCULATION FOR 107 TEMPERATURE SENSOR

Assume a limit of 0.05°C over a 0°C to +40°C range is established for the transient settling error. This limit is a reasonable choice since it approximates the linearization error over that range. The output signal from the thermistor bridge varies nonlinearly with temperature ranging from about 100 µV/=°C at 0°C to 50 µV/°C at 40°C. Taking the most conservative figure yields an error limit of Ve = 2.5 µV. The other values needed to calculate the maximum lead length are summarized in Table 13.3-5 and listed below:

1)Veo ~ 50 mV, peak transient at 2 V excitation

2)Ve ~ 2.5 µV, allowable measurement error

3)t = 450 µs, CR10 input settling time

4)Ro = 1 kohm, 107 probe source resistance

5)Cf = 3.3 nfd, CR10 input capacitance

6)Cw ~ 42 pfd/ft., lead wire capacitance

Solving Equation 13.3-15 gives a maximum lead length of:

L ~ 1003 ft., error ~ 0.05°C

Setting the allowable error at 0.1°C or approximately 5 µV, the maximum lead length increases to:

L~ 1085 ft., error ~ 0.1°C

13.3.4SUMMARY OF SETTLING ERRORS FOR CAMPBELL SCIENTIFIC RESISTIVE SENSORS

Table 13.3-5 summarizes the data required to estimate the effect of lead length on settling errors for Campbell Scientific's resistive sensors. Comparing the transient level, Veo, to the input range, one suspects that transient errors are the most likely limitation for the 107 sensor. The sensors in the WVU-7 are the same as in the Model 107 (the lead wire is different), but the signal leads for the WVU-7 wet- and dry-bulbs are not subject to excitation transients because they are shielded independently from the excitation.

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