Campbell Scientific CR23X Micrologger - Manual (Page 207)

SECTION 13. CR23X MEASUREMENTS
13-7
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 CR23X 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 034A 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
w
were between the signal lead and
ground as represented below.
FIGURE 13.3-5. Model 034A Wind Direction Sensor
FIGURE 13.3-6. Resistive Half Bridge Connected to Single-Ended CR23X Input