Campbell 21X Manual do Operador descarregar pdf (Página 70)

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SECTION

7. MEASUREMENT PROGRAMMING

EXAMPLES

7.11 100

PRT IN 4 WIRE

FULL

BRIDGE

This

exa

describes obtaining

the

from a

100

ohm

PRT in a 4 wire

fullbridge 6).

The temperature

being

is in

a constant

temperature

bath and is

algorithm.

be used

as

the

input for

a control

PRT in this case

does not

adhere to

DIN standard

(alpha

=

0.00385)

used in the

calculating

I nstruction

16. Alpha defined as

(R1e6/Ro-1)/100,

where

are

the resistances

of the PRT

atRloo

and

100oC

and

alpha is eq

),

respectively.

ln

this

PRT,

to 0.00392.

5K

OHM 0.01%

PROGRAM

3

Wire

Half Bridge

Rep

50 mV slow

Range

lN

Chan

Excite all

reps w/EXchan 1

mV Excitation

Loc

[:Rs/R0

]

93 Mult

Otfset

Temperature RTD

Rep

R/Ro Loc

Rs/R0

Loc

[:TEMP

degO]

Mult

Offset

R"

=

Rt X'/(1-X')

Where

X'= )U1000 + R3/(R2+R3)

Thus,

to

obtain the value

R"/Rs,

(Ro

=

R. @

OoC) for the temperature

calculating

Instruction

16,

the multiplier

and

offset

used

in lnstruction 6

are

0.001

and

R3/(R2+R3), respectively. The

multiplier used

in Instruction 59 to obtain Rr/R6

is

Rt/Rs

(5000/100

=

50).

It

is desired

to controlthe temperature bath at

50oC with

as

little

variation

as

possible.

High

resolution is desired

so the

control algorithm

will

be

able to

respond

to

minute changes

in

temperature.

The

highest

resolution

is

obtained

when

the

temperature range

results in

an output

voltage

(V.)

range which fills the

measurement

range

selected

in Instruction 6.

The full

bridge

configuration allows

the bridge

to be

balanced

(V.

=

0V) at or

near

the control

temperature.

Thus,

the output

voltage

can

go

both

positive

and negative

as the

bath

temperature changes,

allowing

the full use of the

measurement range.

The

resistance of the PRT

is

approximately

119.7

ohms at 50oC. The

120 ohm fixed

resistor

balances the bridge at

approximately

51oC. The output voltage

is:

V.

=

V"

[R"/(R.+R1)

-

R3/(R2+R3)]

=

V*

[Rs/(Rs+5000)

-

0.023438]

The temperature

range

to

be covered is 50

t10oC.

At

40oC R.

is approximately

115.8

ohms,

or:

Ys=

-802.24x10-6

Vx

even

with an excitation

voltage

(V")

equalto

5000mV,

V, can be measured on

the

t5mV

scale

(40oC

=

1 15.8 Cl

=

-4.01

mV, 60oC

=

123.6

C2

=

3.428mV).

There is a change

of

approximately

4mV

from the output at

40oC to

the output at

51oC,

or 364pV/oC.

With

a

resolution of 0.33pV

on

the SmV

range,

this

means that the

temperature resolution is

0.00090c.

The 5

ppm

per

oC

temperature

coefficient of the

fixed

resistors was

chosen

so that

their

0.01%

accuracy

tolerance would

hold over

the desired

temperature

range.

RJ

120

oHM

O.0lZ

OF

ALL

FIXED

RESISTORS

<

5 PPM/'C

FIGURE

?.11-1.

Full

Bridge

Schematic for

100

ohm

PRT

The result

([)

given

by lnstruction

6

is

1000

V.A/*

(wheile

V, is the measured

bridge

output

voltage,

an{

V, is the

excitation

voltage)which

is:

X

=

1000

(R"/(R"+81)-R./(R2+R.))

The

resistahce

of the PRT

(Rs)

is calculated

with

the

Briflge

Transform Instruction 59:

7-7

1 2 ... 65 66 67 68 69 70 71 72 73 74 75 ... 190 191

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