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SECTION 13. CR7 MEASUREMENTS

13-5

TABLE 13.3-2. Properties of Three Belden Lead Wires Used by Campbell Scientific

Belden R

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

FIGURE 13.3-4. Wire Manufacturers

Capacitance Specifications, C

DIELECTRIC ABSORPTION

The dielectric absorption of insulation

surrounding individual conductors can seriously

effect 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, for example 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 CR7 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 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

were

between the signal lead and ground as

represented below.

FIGURE 13.3-5. Model 024A Wind Direction

Sensor

, the source resistance, is not constant

because R

varies from 0 to 10 kohms over the

0 to 360 degree wind direction range. The

source resistance is given by:

= R

-R

)/(R

) = R

(20k-R

)/20k

[13.3-12]

Note that at 360

, R

is at a maximum of 5k

=10k) and at 0

, R

is 0 (R

=0). It follows

that settling errors are less at lower direction

values.

The value of R

for any direction D (degrees) is

given by:

(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) =

−+

tCCL

of w

/(R ( ))

[13.3-14]

Equation 13.3-12, -13 and -14 can be combined

to estimate the error directly in degrees at

various directions and lead lengths (Table 13.3-

3). Constants used in the calculations are given

below:

= 0.01 ufd

= 41 pfd/ft, Belden #8771 wire

t = 0.5 ms

1 2 ... 124 125 126 127 128 129 130 131 132 133 134 ... 185 186

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