Campbell 21X Instrukcja Operatora Pobierz pdf (Strona 141)

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

1 2 ... 136 137 138 139 140 141 142 143 144 145 146 ... 190 191

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