Campbell 21X Instrukcja Operatora Pobierz pdf (Strona 148)

SECTION 13.

21X

MEASUREMENTS

THERMOCOUPLE

LIMITS

OF ERROR

The

standard

reference

which lists

thermocouple

output voltage

as a function

of

temperature

(reference

junction

at OoC) is the

National Bureau

of

Standards Monograph

125

Thermocouple

Type

T

Temperature

Range

oC

-200

to

0

0 to 350

0 to 750

-200

to 0

0 to 900

-200

to 0

0 to 1250

When

both

junctions

of a thermocouple

are

at

the same temperature,

there

is

no voltage

produced

(law

of

intermediate

metals).

A

consequence of this is

that a thermocouple

cannot

have

an offset

error; any

deviation

from

a standard

(assuming

the

wires

are each

homogeneous and no

secondary

junctions

exist) is

due

to a deviation

in slope. In

light

of

this,

the

fixed temperature

limits

of error

(e.9.,

*1.OoC for type T as

opposed to the

slope

error

of

O.75oh

of the

temperature) in

the table above

are

probably

greater

than one would

experience

when

considering temperatures

in

the

environmental range. In

other words,

the

reference

junction,

at OoC, is relatively

close to

the

temperature being measured,

so the

absolute

error

(the

product

of the

temperature

difference and the slope

error) should

be

closer

to

the

percentage

error

than the fixed

error.

Likewise,

because thermocouple

calibration

error is a

slope

error,

accuracy can

be

increased

when

the reference

junction

temperature

is

close

to the measurement

temperature. For

the same reason,

differential

temperature

measurements,

over

a small

temperature

gradient,

can be extremely

accurate.

(1974).

The

American National

Standards

Institute has

established limits

of error on

thermocouple wire which

is

accepted as

an

industry

standard

(ANSI

MC

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

21X.

TABLE

13.4-1.

Limits

of Error for Thermocouple

Wire

(Reference

Junction

at OoC)

Limits of Error

(Whichever

is

greater)

E

K

Standard

+ 1.OoC

or 1.5o/"

r 1.OoC or

O.75"/o

+,2.2oC

or 0.75"/"

x.1.7oC

or 1.Ooh

t 1.7oC

or 0.5%

*2.2oC or 2.Oo/o

x.2.2oC

or 0.75"/"

Special

+ 0.5oC

or O.4o/o

+ 1.1oC

orO.4o/o

i 1.OoC

or O.4o/o

+

1.1oC or 0.47o

In order

to

quantitatively

evaluate

thermocouple

error when the reference

junction

is not fixed

at

OoC, one needs

limits

of error for the Seebeck

coefficient

(slope

of thermocouple voltage vs.

temperature curue)

for

the

various

thermocouples.

Lacking

this information,

a

reasonable

approach

is

to apply the

percentage

errors, with

perhaps

O.25o/o

added on, to the

difference

in temperature being measured

by

the

thermocouple.

ACCURACY

OF

THE THERMOCOUPLE

VOLTAGE MEASUREMENT

The

accuracy of a

21X voltage

measurement is

specified as 0.1%

(0.05%

0 to 40oC)

of the

full

scale range

being

used

to

make

the

measurement.

The

actual

accuracy may be

better than

this as

it

involves

a slope error

(the

error is

proportionalto

the measurement

being

made

though limited

by

the

resolution).

The

error in

the temperature due to inaccuracy in

measurement

of

the

thermocouple voltage is

worst

at temperature

extremes, where

a

relatively

large

scale

is

necessary

to

read

the

thermocouple

output.

For example,

assume

type K

(chromel-alumel)

thermocouples

are

used

to

measure temperatures

at 1000oC. The

TC

output is on

the

order of 40mV, requiring

the

*50mV input

range.

The

accuracy specification

13-12

1 2 ... 143 144 145 146 147 148 149 150 151 152 153 ... 190 191

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