Campbell Manufacturing CR10 AWG, Wire #, (pfd/ft.), Conductors, Insulation, (ohms/1000ft.), DIELECTRIC ABSORPTION, SECTION 13. CR10 MEASUREMENTS

SECTION 13. CR10 MEASUREMENTS

FIGURE 13.3-4. Wire Manufacturers Capacitance Specifications, Cw

	TABLE 13.3-2. Properties of Three Belden Lead Wires Used by Campbell Scientific
Belden	Conductors	Insulation	AWG	Rl	Cw
Wire #				(ohms/1000ft.)	(pfd/ft.)
8641	1 shld. pair	polyethylene	24	23	42
8771	1 shld. 3 cond.	polyethylene	22	15	41
8723	2 shld. pair	polypropylene	22	15	62

DIELECTRIC ABSORPTION

The dielectric absorption of insulation surrounding individual conductors can seriously affect the settling waveform by increasing the time required to settle as compared to a simple exponential. Dielectric absorption is difficult to quantify, but it can have a serious effect on low level measurements (i.e., 50 mV or less). The primary rule to follow in minimizing dielectric absorption is: Avoid PVC insulation around conductors. PVC cable jackets are permissible since the jackets don't contribute to the lead capacitance because the jacket is outside the shield. Campbell Scientific uses only polyethylene and polypropylene insulated

conductors in CR10 sensors (see Table 13.3-2) since these materials have negligible dielectric absorption. Teflon insulation is also very good but quite expensive.

13.3.2EFFECT OF LEAD LENGTH ON SIGNAL RISE TIME

In the 024A Wind Vane, a potentiometer sensor, the peak transient voltage is much less than the true signal voltage (see Table 13.3-5). This means the signal rise time is the major source of error and the time constant is the same as if Cw were between the signal lead and ground as represented below.

FIGURE 13.3-5. Model 024A Wind Direction Sensor

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