SECTION 13. CR10 MEASUREMENTS

source resistance at point P (column 5) is essentially the same as the input source resistance of configuration A. Moving Rf' out to the thermistor as shown in Figure 13.3-7C optimizes the signal settling time because it becomes a function of Rf and Cw only.

Columns 4 and 7 list the signal voltages as a function of temperature using a 2000 mV excitation for configurations A and C, respectively. Although configuration A has a higher output signal (2500 mV input range), it does not yield any higher resolution than configuration C which uses the ±250 mV input range.

NOTE: Since Rf' attenuates the signal in configuration B and C, one might consider eliminating it altogether. However, its inclusion "flattens" the non-linearity of the thermistor, allowing more accurate curve fitting over a broader temperature range.

3.Where possible, run excitation leads and signal leads in separate shields to minimize transients.

4.Avoid PVC-insulated conductors to minimize the effect of dielectric absorption on input settling time.

5.Use the CR10 to measure the input settling error associated with a given configuration. For example, assume long leads are required but the lead capacitance, Cw, is unknown. Configure Rf on a length of cable similar to the measurement. Leave the sensor end open as shown in Figure 13.3-8 and measure the result using the

same instruction parameters to be used with the sensor. The measured deviation from 0V is the input settling error.

6.Most Campbell Scientific sensors are

configured with a small bridge resistor, Rf, (typically 1 kohm) to minimize the source resistance. If the lead length of a Campbell Scientific sensor is extended by connecting to the pigtails directly, the effect of the lead

resistance, Rl, on the signal must be considered. Figure 13.3-9 shows a Campbell Scientific Model 107 sensor with 500 feet of extension lead connected directly to the pigtails. Normally the signal

voltage is proportional to Rf/(Rs+Rb+Rf), but when the pigtails are extended, the signal is proportional to

(Rf+Rl)/(Rs+Rb+Rf+Rl). Rl is much smaller than the other terms in the denominator and can be discarded. The effect on the signal can be analyzed by taking the ratio

of the signal with extended leads, Vsl to the normal signal, Vs:

Vsl/Vs = (Rf+Rl)/Rf

Plugging in values of Rf=1k and Rl=.012k (500' at 23 ohms/1000', Table 13.3-2) gives an approximate 1% error in the signal with extended leads. Converting the error to °C gives approximately a 0.33=°C error at 0°C, 0.53°C error at 20°C, and a 0.66°C error at 40°C. The error can be avoided by maintaining the pigtails on the CR10 end of the extended leads because Rl does not add to the bridge completion resistor, Rf, and its influence on the thermistor resistance is negligible.

TABLE 13.3-7. Source Resistances and Signal Levels for YSI #44032 Thermistor Configurations

Shown in Figure 13.3-7 (2V Excitation)

 

 

--------A--------

-----B-----

-------C-------

T

Rs

Ro

Vs(mV)

Ro@P

Ro

Vs(mV)

 

(kohms)

(kohms)

 

(kohms)

(kohms)

 

-40

884.6

29.0

66

30.0

1

2.2

-20

271.2

27

200

27.8

1

6.6

0

94.98

22.8

480

23.4

1

15.9

+25

30.00

15.0

1000

15.2

1

32.8

+40

16.15

10.5

1300

10.6

1

42.4

+60

7.60

6.1

1596

6.1

1

51.8

13-10