Campbell Scientific CR10 CR10 Measurement and Control - Page 172

SECTION 13. CRlO MEASUREMENTS SHIILD FIGURE 13.3-4. Wire Manufacturers Capacitance Specifications, C,n TABLE 13.3-2. Properties of Three Belden Lead Wires Used by Campbell Scientific Belden Wire # Conductors lnsulation Rl cw AWG (ohms/l000ft.) (pfd/ft.) 8641 1 shld. pair polyethylene 24 23 42 8771 1 shld. 3 cond. polyethylene 22 15 41 8723 2 shld. pair 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 ditficult 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. 13.3.2 EFFECT 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 C, were between the signal lead and ground as represented below. cR1 0 SHIELD HI OR LO I Rr: 10K o ) )Rt: 1oK n ) FIGURE 13.3-5. Model 024A Wind Direction Sensor 13-6