Campbell Scientific CR10 CR10 Measurement and Control - Page 179

13.4.1

ERROR

ANALYSIS

The

error

in

the

measurement of a

thermocouple

temperature

is

the

sum

of

the

errors

in

the

reference

junction

temperature,

the

thermocouple

output

(deviation

from

standards

published in NBS

Monograph

125),

the

thermocouple

voltage

measurement, and

the

polynomial

error

(difference between NBS

standard and CR10

polynomial

approximations). The

discussion of errors

which

follows

is limited to these

errors

in

calibration

and

measurement and

does

not

include

errors in

installation

or

matching

the

sensor to

the environment

being measured.

REFERENCE

JUNCTION TEMPERATURE

The

junction

which

is

created when a

thermocouple

is

wired to the wiring

panel

is

referred

to

as

the

reference

junction.

The

temperature

of

the

reference

junction

must be

known in order to calculate the absolute

temperature of

the

measuring

junction.

The

CR1

OTCR

Thermocouple

Reference

Temperature (CR10TCR)

is used

to

rneasure

the

temperature

of

the

reference

junction

(terminal strips).

The

CRl0TCR

uses

a

thermistor to

measure

the

relative

temperature

of

the terminal

strips.

The

accuracy of

this

measurement is

a

combination of

the

thermistor's

interchangeability specification,

the

precision

of

the

bridge resistors, and

the

polynomial

SECTION

13.

CRlO MEASUREMENTS

error.

In

a

"worst case" example, all errors add to

yiefd

a

t0.4'C

error

in

the

range

ol

-24C

to

+48oC

and

*0.9"C

over

the

range of

-38oC

to

+53"C.

lt

is

emphasized that

this

is

ihe

worst

case.

Campbell

Scientific's experience shows that

the

overall

accuracy

is

typically

better

than

t0.2oC.

The

major

error

component

in

the

-0"c

to +50oc

range

is

the

t0.2"

C

thermistor

inte rchan geabi

lity

specif

ication.

When a

CR10 is outside

of

this

temperature

range,

the

polynomial

error becomes

much

worse

(Figure

13.4-1),

and

may necessitate

the

use of an external

reference

junction

to

improve

the

accuracy.

lf

the

terminalthat

the

thermocouple

is wired into

is at a different temperature than

the CRlOTCR

thermistor,

this difference

in

temperature

becomes

an error

in

the

thermocouple

temperature measurement.

With

the

CR10

in

one of

the

enclosure

options

(Section

14)

this

error will generally

be

less

than

0.3"C.

THERMOCOUPLE LIMITS OF

ERROR

The standard

reference

which

lists

thermocouple

output voltage as

a

function

of

temperature

(reference

junction

at

0'C)

is

the

NationalBureau

of Standards Monograph

125

(19741.

The

American

National Standards

lnstitute

has

established

limits of error on

thermocouple wire

which

is

accepted

as an

industry standard

(ANSIMC

96.1, 1975).

Table

13.4-1 gives the

ANSI

limits of error

for

standard and special grade thermocouple wire

of

the types

accommodated

by

the

CR10.

TABLE 13.4-1. Limits

of

Error for Thermocouple

Wire (Reference

Junction

at 0"C)

Limits

of

Error

(Whichever is greater)

Standard

Special

t

1.0"C

or

1.5%

t

1.0oC

or

0.75o/o

t

0.5'C

or

O.4Yo

Thermocouple

Type

J

E

Temperature

Range

oC

-200 to

0

0

to

350

0

to

750

-200 to

0

0

to

900

-200 to

0

0

to

1250

x

2.2oC

or

0.75/"

t

1.7"C

or

1.Oo/o

l

1.7oC

or 0.5%

x

2.2oC

or

2.Oo/o

x

2.2"C

or

0.75o/o

t

1

.1oC

or

Q.4"/o

t

1.0'C

or

O.4o/"

t

1.1oC

or

O.4"h

13-13

Campbell Scientific CR10 CR10 Measurement and Control - Page 179

Standard, 75o/o

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