Campbell 21X Instrukcja Operatora Pobierz pdf (Strona 149)

of 0.05%

error of

appropriate

the

voltage

of a

degree.

is 25pV which is a temperature

O.6oC.

In

the

environmental

with

voltage

measured

on

an

error

in temperature due

to

rements

is a

few hundredths

SECTION

13.

21X

MEASUREMEI{TS

junction

temperature

is

outside

of the

polynomial

range.

The ranges

covered

by

these

polynomials

include the

21X

environmental

operating range, so

there

is no

problem

when the

21X is

used

as the

reference

junction.

External

reference

junction

boxes,

however, must also be within these

temperature

ranges.

Temperature

difference

measurements

made outside of

the reference temperature

range

should

be

made by obtaining

the actual

temperatures

referenced

to

a

junction

within the

reference temperature

range and subtracting.

Table

13.4-3

gives

the

reference

temperature

ranges

covered

and the limits of

error

in the

polynomials

within

these

ranges.

Two

sources

of

error arise when

the reference

temperature

is out of range. The

most

significant error

is in

the calculated

compensation voltage; however, error

is also

created in

the

temperature difference

calculated

from

the thermocouple output.

For example,

suppose

the

reference

temperature

for a

measurement

on

a type T

thermocouple

is

300oC. The compensation

voltage calculated

by

the

21X corresponds to a temperature

of

272.6oC,

a

-27.4oC

error. The type

T

thermocouple

with

the measuring

junction

at

29OoC

and

reference at 3O0oC

would output

-

578.71tY;

using

the

reference

temperature

of

272.6oC,

the

21X

calculates a temperature

difference of

-10.2oC,

a

-O.2oC

error.

The

temperature calculated by the

21X would be

262.4oC,27.6oC low.

TABLE

13.4-3. Reference Temperature

Compensation

Range

and

Polynomial

Error

Relative

to

NBS Standards

THERMOCQUPLE

POLYNOMIALS:

Voltage

to Temperafure

NBS Monogfaph

125

gives

high

order

polynomials

for computing the output

voltage

of

a

given

therinocouple

type over

a broad range

of temperatqres.

In order to

speed

processing

and accomnhodate

the

21X's math

and

storage

capabilities,

4

separate 6th order

polynomials

TC

Type

T

are used

to from

volts to temperature

over the

covered by each

thermocouple

type.

Table

13.4-2

gives

error

limits for

the

polynomials.

TABLE

13.4-2. Limits of

Error on 21X

Thermocorlple

Polynomials

(Relative

to NBS

Standards)

Ra4ge

oC

Limits

of

Error

oC

-27q

-27q

-2AA

-10q

100

-150

-100

760

300

1000

-130

200

1000

*0.008

*0.002

r0.4

*0.005

t0.02

to 400

to

-200

+18@

-270

to

-100

+0.08

to

100

+0.001

to 400

+0.015

E

.24Q

-24Q

_130

20Q

TC

Type

T

J

E

K

to

to

to

to

to

to

to

to

to

1372

950

+0.01

1372

t0.04

JUNCTION COMPENSATION:

to

Voltage

The

polynoinials

used

for reference

junction

compensatipn

(converting

reference

temperaturE

to equivalent

TC output voltage) do

not cover tl'p

entire thermocouple

range.

Substantial

lerrors

will result

if the

reference

ERROR

SUMMARY

The

magnitude

of the

errors

described

in the

previous

sections

illustrate that the

greatest

sources of error

in a thermocouple temperature

measurement are

likely

to be

due to the

limits

of

error

on the

thermocouple

wire and in the

reference

ternperature determined

with

the

Range

oC

-100

to

100

-150

to

296

-150

to

206

-50

to 100

Limits

of Error

oC

r

0.001

r 0.005

t 0.005

+ 0.01

REFERE

13-13

1 2 ... 144 145 146 147 148 149 150 151 152 153 154 ... 190 191

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