SECTION 13. CR10 MEASUREMENTS

Before proceeding with examples of the effect of long lead lengths on the measurement, a

discussion on obtaining the source resistance, Ro, and lead capacitance, CwL, is necessary.

FIGURE 13.3-2. Typical Resistive Half BridgeFIGURE 13.3-3. Source Resistance Model for Half Bridge Connected to the CR10
DETERMINING SOURCE RESISTANCE

The source resistance for several Campbell

The source resistance used to estimate the

Scientific sensors are given in column 3 of

Table 13.3-5.

settling time constant is the resistance the

 

CR10 input "sees" looking out at the sensor.

DETERMINING LEAD CAPACITANCE

For our purposes the source resistance can be

 

defined as the resistance from the CR10 input

Wire manufacturers typically provide two

through all external paths back to the CR10.

capacitance specifications: 1) the capacitance

Figure 13.3-2 shows a typical resistive sensor,

between the two leads with the shield floating,

(e.g., a thermistor) configured as a half bridge.

and 2) the capacitance between the two leads

Figure 13.3-3 shows Figure 13.3-2 re-drawn in

with the shield tied to one lead. Since the input

terms of the resistive paths determining the

lead and the shield are tied to ground (often

source resistance Ro, is given by the parallel

through a bridge resistor, Rf) in single-ended

resistance of Rs and Rf, as shown in Equation

measurements such as Figure 13.3-2, the

13.3-8.

 

second specification is used in determining lead

Ro = RsRf/(Rs+Rf)

[13.3-8]

capacitance. Figure 13.3-4 is a representation

of this capacitance, Cw, usually specified as

If Rf is much smaller, equal to or much greater

pfd/ft. Cw is actually the sum of capacitance

than Rs, the source resistance can be

 

between the two conductors and the

approximated by Equations 13.3-9 through

capacitance between the top conductor and the

13.3-11, respectively.

 

shield. Capacitance for 3 Belden lead wires

Ro ~ Rf, Rf<<Rs

[13.3-9]

used in Campbell Scientific sensors is shown in

column 6 of Table 13.3-2.

Ro = Rf/2, Rf=Rs

[13.3-10]

 

Ro ~ Rs, Rf>>Rs

[13.3-11]

 

13-5