Campbell 21X Manual do Operador descarregar pdf (Página 147)

polynomial

approximation

of

the

NBS

TC

calibrations.

l

13.4.1 ERROR ANALYSIS

The

error

in

lhe

measurement

of

a

thermocouple

temperature

is the sum of the

errors in the

feference

junction

temperature,

the

thermocouplb

output

(deviation from

standards

published

in

Monograph

125),

the

voltage

measurement, and

the

polynomial

(difference

between NBS

standard an{

21X

polynomial

approximations).

The

of

errors

which

follows is limited

to these errdrs

in calibration

and

measurement

and does

not include

errors

in installation or

matching thq

sensor

to the

environment

being

measured.

REFERENCE

JUNCTION

TEMPERATURE

WITH

21X

PANEL

The

21X

parfeltemperature is measured

with a

Fenwal Electronics

UUT51

J1

thermistor

mounted

in

fre

center

of the

analog input

terminal stri$.

The accuracy

of

this

measuremerirt

is a combination

of Fenwal's

interchange{bility

specification,

the

precision

of

the

bridge

rQsistors,

and

the

polynomial

error.

ln

a

"worst

c!se"

example,

allerrors

add to

yield

a *0.3oC

err[r

in the

range

of

-35oC

to

+sooO.

lt

o.8

o-6

o-4

o

-2

o

-o-2

-o-4

-o

-

6

-o-a

-1

-L-2

-L-4

-L-6

SECTION

13.

21X

MEASUREMET{TS

is

emphasized

that

this

is the

worst case.

In

Campbell Scientific's

experience,

the

overall

accuracy

is typically

better

than

t0.2oC.

The

major error component

in the

-35oC

to

+50oC

range is the +0.2oC thermistor

specification.

When a21X is outside

of

this temperature

range, the

polynomialerror becomes

much

worse

(Figure

13.4-1), and

may

necessitate the

use

of an external

reference

junction

to

improve

the accuracy.

Any difference

in temperature

between

the

thermistor and

the actual

reference

junction

(the

channelto

which the

thermocouple

is attached)

will become an error

in the

measurement.

The

terminal strip

cover

is designed

to

reduce

the

gradient

from

one end

of the terminalstrip

to

the

other. ln a test

the 21X

was subjected

to

the

severe thermal

gradient

caused

by bringing

the

21X from equilibrium

at

-25oC

into an

ambient temperature

of approximately

20oC.

After.100 minutes

the

gradient

between the

batteries

and

air temperature

was still

15oC.

The time rate of change

of temperature

in

a

natural

environment

would

seldom

cause even

this

great

a

gradient.

The

maximum

error at

this time, approximately

O.3oC,

is used as an

estimate of

the error

which could

occur

in a field

situation

with

proper

radiation shielding.

The

error

is lowest on channel

4 because

it is

directly over

the thermistor.

LINEARIZATION

ERROR

OF

2]-X PNEL

TEMP

I

,1

n

h

U

)

I

q

tI]

tr

lr

H

E

0

L)

0

lrl

/.-.

-._-:-

.--.--'-\..

,/--_--\

-4c -30-20-10

0

ao 20 30

40 50

ACTUAL

TEMPE%TURE

FIGURE 13.4-1.

Thermistor

Polynomial

Error

60

13-1

1

1 2 ... 142 143 144 145 146 147 148 149 150 151 152 ... 190 191

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