Campbell Scientific CR10 CR10 Measurement and Control - Page 183

Radiation shielding must be provided when

a

junction

box is installed

in

the

field.

Care

must

also

be

taken that

a

thermal

gradient is not

induced by conduction through

the

incoming

wires.

The

CR10

can be

used

to

measure

the

temperature

gradients within the

junction

box.

13.5

BRIDGE RESISTANCE

MEASUREMENTS

There are

6

bridge

measurement

instructions

included

in

the

standard

CR10

software.

Figure

13.5-1 shows the circuits that would

typically

be

measured with

these

instructions.

In

the

diagrams,

the

resistors labeled

R,

would

normally

be

the

sensors and

those

labeled

R1

would normally

be

fixed

resistors.

Circuits

other

than

those

diagrammed

could be

measured,

provided

the

excitation and

type

of

measurements

were

appropriate.

With

the exception of

Instructions 4 and

8,

which apply an excitation voltage then wait a

specified

time before

making

a

measurement,

all

of

the bridge

measurements make one set

of

measurements with

the

excitation as

programmed and

another

set

of

measurements

with the

excitation

polarity

reversed.

The

error

in

the

two

measurements due to

thermal

emfs

can

then

be accounted

for

in

the

processing of

the

measurement

instruction.

The

excitation

is

switched on

450

ps before

the

integration

portion

of

the

measurement starts

and

is

grounded as soon as

the

integration

is

completed.

Figure

13.5-2 shows the excitation

SECTION

13.

CRlO MEASUREMENTS

and

measurement sequence

for

Instruction 6,

a

4 wire

full

bridge.

Excitation is applied

separately for each phase of

a

bridge

measurement.

For

example, in

Instruction 6, as

shown

in

Figure 13.5-2,

excitation

is switched

on for

the

4 integration periods

and

switched off

between integrations.

lnstruction 8

measurement sequence consists

of applying

a

single excitation voltage, delaying

a specified

time, and

making 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

ci

rcu

itry

req

u

i

ring-diff

erential

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 V1

which equals

V,

(R./(R3+R4)

-

R2l(R1+R2)).

(ln

other words,

to

make

the

output

the

same

as

Instruction

6,

use

a

factor of 10004/*

in

the

multiplier.)

13-17

Campbell Scientific CR10 CR10 Measurement and Control - Page 183

Bridge Resistance, Measurements - conductivity program

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