Campbell Scientific CR10 CR10 Measurement and Control - Page 92

.i;,:

SECTION

7.

MEASUREMENT

PROGRAMMING

EXAMPLES

E1

cRl

0

H1

L1

Rr

10K O,

1%

TEMPERATURE COEFFICIENT

<

25

A

/-

I<

500

FooT

LEAD

LENGTH,

22

AWG(

.

\

B

PPM/'C

R5

1OO

CI

PRT

ohms

per

1000

feet, which would give each 500

foot

lead wire

a

nominal resistance of 8.3

Two

percent

of 8.3

ohms

is 0.17 ohms.

Assuming that the

greater

resistance is in wire

B,

the

resistance measured for

the

PRT

(Ro

=

100

ohms)

in

the

ice bath would

be

100.17

ohms, and

the

resistance at 40"C

would

be

115.71. The

measured ratio Rr/Ro is 1.1551;

the

actual

ratio

is

1

15.541100

=

1.1554.

The

temperature computed

by

Instruction

16

from

the

measured ratio would be about

0.1oC

lower

than the

actualtemperature

of

the

PRT.

This

source of error

does

not

exist

in

the example

in

Section 7.9, where a

4

wire

half bridge is used

to

measure PRT resistance.

The

advantages

of

the

3

wire

half bridge are

that

it

only requires

3

lead wires going to

the

sensor and takes 2 single-ended input

channels, whereas the

4

wire

half bridge

requires

4

wires

and 2 ditferential channels.

PROGRAM

3

Wire

Half Bridge

Rep

25

mV

6O

Hz rejection

lN Chan

Excite

all

reps

w/EXchan

1

mV Excitation

Loc

[:Rs/Ro

]

Mult

Offset

Temperature

RTD

Rep

R/Ro Loc Rs/Ro

Loc

[:TEMP

C

I

Mult

Otfset

FIGURE

7.1G1.

3

Wire

Half

Bridge

Used

to

Measure

100

ohm

PRT

7.10

1OO

OHM

PRT

IN

3

WIRE HALF

BRIDGE

The

temperature

measurement requirements

in

this example

are

the

same as in Section

7.9.

ln

this case,

a

three

wire

half

bridge,

Instruction

7,

is used

to

measure

the

resistance

of

the

PRT.

The diagram

of

the

PRT

circuit

is shown in

Fig.

7.10-1.

As

in

the

example

in

Section 7.9,

the

excitation

voltage

is

calculated

to

be

the

maximum

possible, yet allow

the

+25 mV measurement

range.

The

10 kohm resistor has

a

tolerance

of

+'l7o;

thus,

the lowest

resistance

to

expect from

it is 9.9

kohms.

We calculate

the

maximum

excitation

voltage

(Vr)

to

keep the voltage drop

across

the

PRT less than

25

mV:

0.025V

t

V"

115.54/(9900+115.54);

vx<2.17

V

The excitation

voltage

used

is

2.1

V.

The

multiplier used

in

Instruction

7

is

determined

in

the same

manner

as

in Section

7.9.

ln this example,

the

multiplier

(Ry'Ro)

is

assumed

to be

100.93.

The

3

wire

half bridge compensates

for

lead

wire

resistance by assuming that

the

resistance

of

wire

A

is

the same as

the

resistance of wire

B.

The

maximum difference

expected

in

wire

resistance

is

2o/o,

but

is more

lkely

to

be

on

the

order

of

1%.

The

resistance

of

R,

calculated

with

Instruction 7, is actually

R.

plus the

ditference

in resistance

of

wires A and

B.

The

average resistance of 22 AWG

wire

is

16.5

01:

01:

02:

03:

o4;

05:

06:

07:

08:

o2:

01:

o2:

03:

04;

05:

P7

1

23

1

1

2100

1

100.93

0

P16

1

1

2

1

0

7-8

Campbell Scientific CR10 CR10 Measurement and Control - Page 92

Rs/ro, Wire Half

Page 92 highlights