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SHIELD
FIGURE
13.3-4. Wire Manufacturers
Capqcitance Specifications, Cw
SECTION
13.
21X MEASUREMENTS
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
lt
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.
AWG
24
RI
(ohms/l000ft.)
23
c*
(pfd/ft.)
22
15
0
to 360 degree
wind direction
range.
The
source resistance
is
given by:
Ro
= Ru(R.-Rn+R)/(R"+Rr) =
Ru(2Ok-R
b)lz0k
[13.3-12]
Note that at 360
degrees,
Ro
is at
a maximum
of
5k
(Ro=10k)
and at 0
degrees,
Ro
is
0
(Ru=O).
It
follows that settling errors
are less at lower
direction values.
The value
of
R5
for any
direction D (degrees)
is
given
by:
Rs(kohms)
=
(10kXD)i360
[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 (degreesl
=
ps-t/(-t(Ro
(c1+c*L))
t',
a.a-',
ol
13.3-2. Properties of Three Belden Lead Wires Used by CampbellScientific
Belden
Wire #
8641
8771
Conductors
1
shld. pair
1
shld.
3
cond.
Insulation
polyethylene
polyethylene
42
41
8723 j
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
signalVoltage (Table
13.3-5). This
means
the signal
ri$e
time
is
the major source of error
and the
time constant
is
the same as if C* were
between
thQ
signal lead and ground as
representedi
below.
21X
SHTELD
Rr=
10K
OHM
)
IRs
=
1oK
oHM
)
FIGURE
13.3-5. Model024A Wind Direction
Sensor
l
Ro,
the
sourJce
resistance, is not
constant
because
R6lvaries
from
0
to
10 kohms over
the
13-5
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