Properties Of Three Belden Lead Wires Used By Campbell Scientific; Model 024A Wind Direction Sensor - Campbell CR510 Operator's Manual

SECTION 13. CR510 MEASUREMENTS
FIGURE 13.3-4. Wire Manufacturers Capacitance Specifications, C
CR510
TABLE 13.3-2. Properties of Three Belden Lead Wires Used by Campbell Scientific
Belden
Wire # Conductors
8641 1 shld. pair
8771 1 shld. 3 cond.
8723 2 shld. pair
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
13-6
FIGURE 13.3-5. Model 024A Wind Direction Sensor
Insulation
polyethylene
polyethylene
polypropylene
AWG (ohms/1000ft.)
24
22
22
conductors in CR510 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
w
ground as represented below.
w
Rl C
w
(pfd/ft.)
23 42
15 41
15 62