Campbell Scientific CR10 CR10 Measurement and Control - Page 93

5K

0,

O.O1%

R5

1OO

O

PRT

7.11

1OO

OHM

PRT IN

4 WIRE FULL

BRIDGE

This example describes obtaining

the

temperature

from a

100

ohm PRT

in

a

4

wire

full

bridge

(lnstruction

6).

The

temperature

being

measured

is in

a

constant

temperature

""

bath and

is

to be

used as

the

input

for

a

control

algorithm.

The

PRT

in

this

case

does

not

adhere to

the

DIN

standard

(alpha

=

0.00385)

used

in

the temperature

calculating

Instruction

16.

Alpha

is defined

as

((R160/Ro)-1y100,

where

R199

and

Rg

are

the

resistances

of

the

PRT

at

100"C and

0"C, respectively.

In

this

PBT

alpha

is

equalto

0.00392.

The

result given

by

Instruction

6

(X)

is

1000

V.ly'*

(where

V.

is

the

measured bridge output

voltage, and

V,

is

the

excitation voltage) which

is:

X

=

1000 (Rs/(R.+R1)-R3/(R2+Rg))

The

resistance

of

the

PRT

(Rs) is

calculated

with

the

Bridge

Transform

Instruction

59:

Rs

=

Rr X7(1-X)

Where

X'=

)01000 +

R3/(R2+R3)

Thus,

to

obtain the

value

Rs/Ro, (Ro

=

Rs

@

0'C)

for the

temperature calculating

Instruction

16,

the

multiplier and

otfset

used

in

lnstruction

6

are 0.001

and

R3/(R2+R3),

respectively. The

multiplier used

in

Instruction 59 to

obtain

Rs/Ro

is

Rt/Ro

(5000/100

=

50).

It

is

desired to

controlthe

temperature

bath at

50'C

with

as

little variation as

possible.

High

resolution is needed so that the control

algorithm

willbe

able

to

respond to minute

changes

in

temperature. The highest

resolution

SECTION

7.

MEASUREMENT PROGRAI'IMING EXAMPLES

Q

,

O.O1%

R3

120

Q,

O.O1%

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

(Vs

=

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

50"C.

The

120 ohm

lixed

resistor balances

the

bridge at approximately

51"C.

The output

voltage

is:

V.

=

V,

[R'(R.+R1)

-

R3(R2+Rj]

=

Vx

[Rs/(Rs+5000)

-

0.023438]

The

temperature

range

to

be

covered

is

+50

r10"C.

At

40'C

R.

is

approximately

115.8

ohms,

or:

Vs

=

-802.24x10-6

V*

Even with an excitation

voltage

(Vr)

equalto

2500

mV,

V,

can

be measured on

the +2.5

mV scale

(40oC

=

115.8 ohms

=

-2.006

mV,

60"C

=

123.6

ohms

=

1.714

mV).

There

is

a

change

of

approximately

2

mV from the output at

40'C

to the

output at

51oC,

or

181

pV/"C.

With a

resolution of

0.33

pV on the

2.5 mV

range,

this

means that

the

temperature

resolution

is

0.001

8"C.

The

5 ppm per

"C

temperature

coefficient

of

the

fixed

resistors

was

chosen so that their 0.01%

accuracy tolerance

would

hold over the desired

temperature

range.

The

relationship between temperature

and

PRT

resistance

is

slightly

nonlinear

one.

Instruction

16

computes

this

relationship for

a

DIN

standard

PRT where

the

nominal

temperature

coetficient

is

0.00385/"C.

The

change

in

TEMPERATURE COIFFICIENT

oF

ALL

F|XED

RESTSTORS

<

5

PPM/'C

FIGURE

7.11-1. Full Bridge Schematic

for

100

ohm

PRT

7-9

Campbell Scientific CR10 CR10 Measurement and Control - Page 93

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