Operator`s manual

SECTION 13. 21X MEASUREMENTS
13-5
FIGURE 13.3-4. Wire Manufacturers
Capacitance Specifications, Cw
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., 50mV 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 21X sensors (see Table 13.3-2)
since these materials have negligible dielectric
absorption. Teflon insulation is also very good
but quite expensive.
TABLE 13.3-2. Properties of Three Belden Lead Wires Used by Campbell Scientific
Belden Rl C
w
Wire # Conductors Insulation AWG (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
13.3.2 EFFECT OF LEAD LENGTH ON SIGNAL
RISE TIME
In the 024A Windvane, a potentiometer sensor,
the peak transient voltage is much less than the
true signal voltage (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 024A Wind Direction
Sensor
R
o
, the source resistance, is not constant
because R
b
varies from 0 to 10 kohms over the
0 to 360 degree wind direction range. The
source resistance is given by:
R
o
= R
b
(R
s
-R
b
+R
f
)/(R
s
+R
f
) = R
b
(20k-R
b
)/20k
[13.3-12]
Note that at 360 degrees, R
o
is at a maximum
of 5k (R
b
=10k) and at 0 degrees, R
o
is 0 (R
b
=0).
It follows that settling errors are less at lower
direction values.
The value of R
b
for any direction D (degrees) is
given by:
R
b
(kohms) = (10k)(D)/360 [13.3-13]
Equation 13.3-6 can be rewritten to yield the
settling error of a rising signal directly in units of
degrees.
()
Error (degrees) =
−− +
De
tt CCL
of w
/(R( )
[13.3-14]