Campbell 21X Manuel de l'opérateur télécharger pdf (Page 151)

FIGURE

tg.4-2. Diagram of Junction Box

An external

reference

junction

box

must

be

constructed

so that the entire terminal area

is

very close tO the

same

temperature.

This

is

necessary sO that a valid

reference temperature

can

be measured. and to avoid

a thermoelectric

otfset

voltagB

which

will be

induced

if

the

terminals

at

Which

the

thermocouple

leads

are

connected

(points

A and

B in Figure

13.4-3)

are

at different

tpmperatures. The box should

contain

elernents

of

high thermal

conductivity,

which will adt to

rapidly remove any thermal

gradients

to

which

the box

is subjected. lt is not

necessary tQ design a constant

temperature

box;

it is de$irable

that

the box

respond

slowly

to external

temperatu re f luctuations.

Radiation

shielding

m{st be

provided

when a

junction

box

is installed

ifi the

field.

Care must

also

be taken

that

a thermpl

gradient

is

not induced by

conduction

frrough

the

incoming wires. The

21X

can

be

Used

to measure the

temperature

gradients

wifhin the

junction

box.

13.5 BRIDGE

RESISTANCE

MEASUREMENTS

There are 6

bridge measurement

instructions

included

in

the

standard 21X software. Figure

13.5-1 shovr4s

the circuits

that

would typically be

measured with

these instructions.

In

the

diagrams,

t(e

resistors labeled R"

would

normally be the sensors and those

labeled R1

would

normhlly be

fixed

resistors. Circuits other

than those

{iagrammed

could be

measured,

provided

th{

excitation

and

type of

measu

remefrts

were

appropriate.

With the exOeption of

Instructions

4

and 8,

which apply an

excitation voltage then wait a

specified

tirne before making

a measurement,

SECTION

13. 21X MEASUREIT,IENTS

allof the bridge

measurements make

one

set of

measurements with the

excitation

as

programmed

and

another

sel

of measurements

with

the

excitation

polarity

reversed.

The

error

in

the two

measurements due to

thermal emfs

is

then accounted

for in

the

processing

of

the

measurement instruction. The excitation

is

switched on

450ps before

the

integration

portion

of

the

measuremenl

stads

and

is

grounded

as

soon

as the

integration is

completed.

Figure

13.5-2

shows

the excitation and

measurement

sequence for

Instruction

6, a

4 wire full bridge.

When

more than

one

measurement

per

sensor

is

necessary

(lnstructions

7

and

9), excitation is

applied

separately

for

each

measurement.

(For

example, in Instruction

9

(6

wire full bridge),lhe

differential measurement

of

the

voltage drop

across the sensor

is

made with the

excitation

at

both

polarities;

excitation

is

again

applied

and

reversed

for the

measurement

of the

outpul

voltage.)

Instruction

8 applies an excitation

voltage,

delays a specified time, and

makes a differential

voltage

measurement, lf a

delay

of 0

is

specified,

the

inputs for

the differential

measurement are not

switched

for a second

integration

as is

normally

the case

(Section

13.2). The result stored is

the

voltage

measured.

Instruction

8

does not

have as

good

resolution

or common mode rejection

as the

ratiometric

bridge

measurement instructions" lt

does

provide

a very

rapid

means of making

bridge

measurements as

well as supplying

excitation

to circuitry

requiring differential

measurements. This

instruction

does

not

reverse

excitation.

A

1 before the excitation

channel number

(1X)

causes

the channel to be

incremented

with each repetition.

The

output of

lnstruction

8 is simply

the

voltage

measurement. When

8

is

used

to measure

a

full

bridge

(same

connections as

Instruction

6 in

Figure

13.5-1), the

result is Vl which equals

V,

(Rs/(Rs+Ra)

R2l(R1+R2)).

(ln

other

words, to

make

the output

the same

as

Instruction 6, use a

factor

of 10004/,

in the

multiplier.)

13-15

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