Campbell 21X Manuel de l'opérateur télécharger pdf (Page 145)

error

For e

Use the

1X

to measure

the

input settling

with a

given

configuration.

assume

long leads

are

but the lead capacitance,

C*,

is

Configure

Rl on a

length of cable

SECTION

13.

21X

MEASUHEMENTS

voltage

is

propottionalto

Ry'(R'+Rs+R),

but

when the

pigtails

are

extended,

the signal

is

proportional

to

(Rr+Rr)/(Rs+R6+R1+R1). R;

is

much smaller

than the other

terms

in

the

denominator

and can be discarded.

The

effect on

the signal

can be analyzed

by

taking

the

ratio of the signal

with extended

leads,

Vslto the

normalsignal,

Vs:

Vr/V.

=

(R1+R1)/R1

Plugging

in values

of R61k and

Rr=.012k

(500'at

23 ohms/1000',

Table 13.3-2)

gives

an

approximate

1o/o

error

in the signal

with

extended

leads. Converting the

error

to

oC

gives

approximately a

0.3oC error

at OoC,

0.6oC

error at

2OoC,

and

a 1.5oC

error at

40oC.

The error

can

be

avoided

by

maintaining

the

pigtails

on the

21X end

of

the

extended

leads because

Rr does

not

add to

the bridge

completion

resistor,

Rr,

and

its influence

on

the

thermistor

resistance

is negligible.

similar

td the measurement.

Leave

the

sensor

{nd

open as shown

in Figure 13.3-8

and

me{sure

the

result using

the same

instructiOn

parameters

to be

used with the

sensor.

The

measured

deviation

from

0V

is

the

input

settling

error.

Most Cqmpbell Scientific

sensors are

configurFd

with

a small

bridge

resistor, R1,

(typically

1 kohm)

to

minimize

the

source

resistanpe.

lf

the lead

length

of a

Campbell

Scientifip

sensor is extended

by

connecting

to the

piltails

directly,

the

effect

of

the

lead

resistanfe,

R;,

on the

signal

must

be

considefed.

Figure

13.3-9

shows

a

Campb(ll

Scientific

Model

107 sensor

with

500

feet

of extension lead

connected

directly

fo

the

pigtails.

Normally

the

signal

TABLE

13.8-7. Source

Resistances and

Signal

Levels for YSI

*44032

Thermistor Gonfigurations

Shown in Figure

13.3-7

(2V

Excitation)

T

Rs

(kohms)

884.6

271.2

94.98

30.00

16.15

7.60

Vs(mV)

164

498

1200

2500

3250

3989

-B-

Ro@P

(kohms)

30.0

27.8

23.4

15.2

10.6

6.1

5.5

16.5

39.5

82.0

106.0

129.5

---A---

Ro

(kohms)

29.0

27

22.8

15.0

10.5

6.1

---c---

Ro

Vs(mV)

(kohms)

-40

-20

0

+25

+40

+60

1

1

1

1

1

1

13-9

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