Campbell 21X Manual do Operador descarregar pdf (Página 146)

SECTION

13.

21X MEASUREMENTS

=

r\ |

-

Jvt\ vtilvl

A)

Ro

=

RsRf/(Rs+Rf), Vs

=

VxRf/(Rs+Rf)

B)

Ro@P

=

ns(Rf+Rf)/(Rs+Rf+Rf)

=

JOK OHM

:

| \l

C) Ro

=

Rf, Vs

=

VxRf/(Rs+Rf+Rf)

FIGURE

13.3-7.

Half Bridge

Configuration

for

YSf #44032

Thermistor

Connected

to 21X

Showing:

A) Large source resistance,

B)

large source

resistance at

point

P,

and

C)

configuration

optimized for input

settling

21X

SHIELD

FIGURE

13.3-8.

Measuring Input

Settling

Error

with

the

21X

13-10

FIGURE 13.3-9. Incorrect

Lead Wire

Extension on Model

107 Temperature

13.4 THERMOCOUPLE

MEASUREMENTS

A

thermocouple consists of two wires, each of a

different metal or alloy, which

are

joined

together

at

each end. lf

the two

junctions

are at

different

temperatures, a voltage

proportional

to

the difference in temperatures

is induced

in

the

wires. When a thermocouple

is used

for

temperature measurement,

the wires are

soldered or

welded

together at

the

measuring

junction.

The second

junction,

which becomes

the reference

junction,

is

formed where the

other

ends

of the wires

are connected to the

measuring device.

(With

the connectors at the

same

temperature,

the

chemical dissimilarity

between the thermocouple wire

and

the

connector

does

not induce

any voltage.) When

the

temperature of the reference

junction

is

known,

the

temperature

of the measuring

junction

can

be determined

by measuring the

thermocouple voltage

and adding

the

corresponding temperature

difference to the

reference

temperature.

The 21X

determines thermocouple

temperatures using

the following

sequence.

First,

the temperature of

the

reference

junction

is measured. lf

the

reference

junction

is the

21X

panel,

the

temperature

is measured

with

the built-in

thermistor

(lnstruction

17).

The

reference

junction

temperature in

oC

is stored in

an input

location

which

is accessed by the

thermocouple measurement

instruction

(lnstruction

13 or 14). The

21X calculates the

voltage

that

a

thermocouple

of the type

specified

would output at the reference

junction

temperature if its reference

junction

were

at

OoC, and

adds

this voltage

to the measured

thermocouple voltage.

The temperature of

the

measuring

junction

is then

calculated

from

a

SHITLD

HI

OR LO

VX

V5

1.,

l-q

R

Cry

1 2 ... 141 142 143 144 145 146 147 148 149 150 151 ... 190 191

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