Campbell Scientific CR10 CR10 Measurement and Control - Page 91

E1

H2

CR1

O

L2

H1

L1

AG

VX

10K

0

v2

a

!

v1

R,

100

0

TEMPERATURE COEFFICIENTS

<

1O

PP

SECTION

7.

MEASUREMENT PROGRAMMING EXAMPLES

R.

RS

'100

0

PRT

I,A

/'C

The

10

ppm/'C temperature coefficient

of

the

fixed

resistor

will

limit the error due

to

its change

in

resistance with temperature

to

less than

0.15"C

over

the

specified

temperature

range.

Because

the measurement

is ratiometric

(RJR|,the

properties

of

the

10 kohm resistor

do

not affect

the

result.

The

result

of

Instruction

9

when the

first

differential measurement

(V1)

is not made on

the 2.5

V

range

is

equivalent

to

Rr/ft.

Instruction

16

computes the

temperature

("C)

for a DIN

43760

standard

PRT from

the

ratio of

the

PRT resistance

at

the

temperature

being

measured

to

its resistance at

0"C

(RJR9).

Thus,

a

multiplier

of

Ry'Rg

is used

in

Instruction

9

to obtain

the

desired intermediate,

Rs/Ro

(=Rs/ft

x

Ry'Ro).

lf

R.

and

Re

were

each

exactly'100

ohms,

the multiplierwould be

1.

However,

neither

resistance is likely

to

be exact.

The

correct

multiplier

is

found

by connecting

the

PRT to

the

CR10 and

entering

lnstruction

9

with

a

multiplier

of

1.

The

PRT

is

then

placed

in

an

ice bath (@

0'C;

R.=Re), and

the

result

of

the

bridge measurement

is

read using

the

*6

Mode.

The

reading is

Rr/R1,

which

is

equalto

Ro/R1

since Rr=Ro.

The correct value of

the

multiplier,

Ry'Ro, is

the reciprocalof

this

reading. The

initial reading assumed for this

example

was

0.9890.

The

correct

multiplier

is:

Ry'Ro

=

1/0.9890

=

1.01

1

1.

The

fixed

1OO

ohm

resistor must

be

thermally

stable.

lts precision is not important because

the

exact resistance is incorporated, along with

that

of

the

PRT, into

the calibrated

multiplier.

FIGURE

7.9-1. Wiring

Diagram

for

PRT

in

4

Wire

Half Bridge

01:

P9

01:

1

02:

23

PROGRAM

Full

BR

w/Compensation

Rep

25

mV

60

Hz rejection EX

Range

25

mV

60

Hz rejection

BR

Range

lN Chan

Excite

all

reps

w/EXchan

1

mV Excitation

Loc

[:Rs/Ro

]

Mult (Rf/Ro)

Offset

03:

o4:

05:

06:

o7:

08:

09:

02:

01:

o2:

03:

04:

05:

23

1

1

2100

1

1.0111

0

P16

Temperature

RTD

1

Rep

1

RlRo

Loc

Rs/Ro

2

Loc

[:TEMP

C

]

1

Mult

0

Offset

7-7

Campbell Scientific CR10 CR10 Measurement and Control - Page 91

RJR|,the, Rr/ft.

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