Campbell 21X Manuel de l'opérateur télécharger pdf (Page 150)

SECTION 13.

21X

MEASUREMENTS

built-in thermistor.

Errors in

the thermocouple

and

reference

temperature

polynomials

are

extremely small,

and

error

in

the voltage

measurement

is negligible.

To

illustrate

the relative

magnitude

of

these

errors in

the environmental

range, we

willtake a

worst case

situation

where all errors

are

maximum

and additive. A

temperature

of 45oC

is measured

with

a type

T

(copper-constantan)

thermocouple,

using

the

*SmV

range.

The

nominal accuracy

on

this

range is

2.5pV

(0.05%

of SmV), which

at 45oC

changes the

temperature by

0.06"C.

The

RTD is

25oC

but

is

indicating 25.3oC,

and

the

terminal

that the

thermocouple is connected to is

O.3oC

cooler

than

the

RTD.

TABLE

13.4-4.

Example of Errors in

Thermocouple

Temperature

Source Error

oC

o/"

of Total Error

loc

1ol"

Slope

Error Error

13.4.2

USE

OF

EXTERNAL

REFERENCE

JUNCTION

OR JUNCTION

BOX

An

externaljunction box is

often

used

to

facilitate

connections and to reduce the

expense

of

thermocouple

wire when the

temperature measurements

are

to

be

made

at

distance from the 21X.

In most situations, it is

preferable

to

make

the

box the

reference

junction,

in which case its

temperature

is

measured and used

as the reference

for

the

thermocouples; copper wires

are

run from

the

box

to the

21X

(Section

7.4).

Alternatively, the

junction

box can be used

to couple

extension

grade

thermocouple wire

to the

thermocouples

being

used

for measurement,

and

the 21X

panel

used as the

reference

junction.

Extension

grade

thermocouple

wire

has a

smaller temperature range than standard

thermocouple

wire,

but meets the same limits

error within that range. The

only

situation

it would

be

necessary

to use extension

grade

wire instead of a external measuring

junction

is

where

the

junction

box

temperature

is outside

the range of reference

junction

compensation

provided

by the

21X. This

is only

a factor

when

Lsing type

K

thermocouples, where

the

upper

limit

of the

reference

compensation

li

is

100oC and the upper limit of the

extension

grade

wire

is

200oC. With

the other

types of

thermocouples,

the reference

compensation

range

equals or

is

greater

than the

extension

wire

range.

In

any case, errors

can

arise

if

temperature

gradients

exist within the

junction

box.

Figure 13.4-3 illustrates

a typical

junction

box.

Terminal

strips

will

be a different

metalthan

the

thermocouple wire. Thus, if

a temperature

gradient

exists

between A

and A'or

B

and

B',

the

junction

box will

act as another

thermocouple in series,

creating

an error in

the

voltage

measured by the 21X. This

thermoelectric offset

voltage

is

a

factor whether

or not the

junction

box is used for the reference.

It can

be

minimized by making

the

thermal

conduction between

the two

points

large

and

the distance small. The best

solution

in the

case where extension

grade

wire

is

being

connected to thermocouple

wire

would

be to

use

connectors

which

clamped

the

two

wires

in

contact with each other.

Reference Temp.

TC

Output

ANSI

0.01

x 20oC

Voltage

Measurement

Reference

Linearization

Output

Linearization

36.1

69.6

60.1

23.2

0.6

1.0

0.2

0.06

0.001

0.001

7.03.6

0.1

0.'l

0.1

0.1

Total Error

With ANSI

error

Assuming 17o

slope error

100

100

1.662

0.862

13-14

1 2 ... 145 146 147 148 149 150 151 152 153 154 155 ... 190 191

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