Campbell 21X Manuel de l'opérateur télécharger pdf (Page 142)

L=500 ft.

The

values

in Table

13.3-3 show that

significant

error occurs

at

large

direction values for leads

in

excess of 500 feet. Instruction 4,

Excite,

Delay, and Measure,

should

be

used to

eliminate errors in

these types of situations.

Using

a 1Oms

delay,

settling

errors

are

eliminated up to lengths

that

exceed

the drive

capability of the excitation channel

(=2000

ft.).

13.3.3

TRANSIENTS TNDUCED BY

SWITCHED

EXCITATION

Figure

13.3-6

shows

a typical

half

bridge

resistive sensor,

such as Campbell

Scientific's

Model 107 Temperature

Probe, connected

to

SECTION

13. 21X

MEASUREMENTS

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:

Cr

=

3'3nfd

C*

=

41pfd/ft.,

Belden

#8771

wire

t

=

450tts

TABLE 13.3-3.

Settling Error

(Degrees)

for

024A

Wind

Direction

Sensor vs. Lead

Length

the 21X.

The lead

wire is

a single-shielded

pair,

used for

conducting the

excitation

(V,)

and

signal

(Vr)

voltages.

When

V,

is

switched

on, a

transient is capacitively induced in V",

the

signal

voltage. lf the

peak

transient level, V"o, is

less

than

the true

signal, Vro,

the

transient

has no

effect on the

measurement.

lf

V"o is

greater

than Vro,

it

must

settle to

the correct

signal

voltage

to

avoid

errors.

SHIELD

FIGURE

13.3-6.

Resistive

Half

Bridge

Connected

to

Single-Ended

21X Input

The

size of

the

peak

transient

is

linearly related

to the excitation

voltage

and increases as the

bridge resistor,

Rs,

increases.

Table

'13.3-4

shows measured

levels

of V"o for

1000 foot

lengths

of

three Belden

wires used

in

Campbell

Scientific sensors.

Values

are

given

for

R1

equalto 1

kohm

and 10 kohm.

Table

13.3-4 is

meant

only

to

provide

estimates

of

the

excitation transients encountered:

the

exact

magnitude

will

depend upon

the

specific

sensor

configuration.

Wind

Direction

3600

2700

1 800

9oo

Error-----

L=1000

ft.

470

310

120

10

go

c

10

00

TABLE 13.3-4.

Measured Peak

Excitation

Transients for 1000 Foot Lengths of Three Belden

Wires

Used by CampbellScientific

-

V"o(mV)

Vx(mV)

5000

4000

3000

2000

1 000

Rr=l

kohm

#

8771

200

165

130

100

65

R1=10

kohm

#

8771

320

260

200

140

90

#

8641

125

100

75

50

25

#

8723

130

110

90

60

40

#

8641

215

180

140

100

60

#

8723

180

150

110

80

40

NOTE:

Excitation

transients are eliminated

if an option exists

to contain

excitation

in

a shield

independent

from the signal

leads.

13-6

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