Campbell 21X Manuale dell'Operatore Scaricare il pdf (Pagina 143)

Equation

13,13-7 can

be solved

for

the

maximum

lead

length,

l,

permitted

to

maintain a

specified

error

limit.

Qombining

Equations

13.3-7 and

13.3-4 and splving for

L

gives:

1-

=

-(RoC1+ (Vln(V"A/"o)))/RoC*

[13.3-15]

where V" is

the

measurement error

limit.

EXAMPLE

I-.]EAD

LENGTH

CALCU

LATION

FOR CAMPtsELL

SCIENTIFIC

107

TEMPERATURE

SENSOR

Assume a

lirhit of O.05oC

over

a OoC

to

+40oC

range is est4blished

for

the

transient

settling

error.

This limit is

a

reasonable

choice

since

it

approximatep

the

polynomial

error

over

that

range.

The

putput

signalfrom

the thermistor

bridge

varie$ nonlinearly

with temperature

(refer

to 107 Operdtor's

Manual),

ranging from about

2oopvfc

atlo"c to l

oopvfc

at

4ooc.

Taking

the

most corfservative

figure

yields

an

error

limit

of

V"

=

5pV. The other values

needed

to

calculate

th( maximum lead

length are

summarized

in Table

13.3-5

and

listed

below:

SECTION

13.

21X

MEASUREMENTS

1)

Veo

=

100mV,

peak

transient

at

4V

excitation

2) V"

=

SltV, allowable

measurement

error

3)

t

=

4501rs, 21X

input

settling time

4) Ro

=

1 kohm,

107

probe

source

resistance

5) Cr

=

3.3nfd,

21X input capacitance

6)

C*=

42pld/ft.,

lead

wire capacitance

Solving Equation

13.3-15

gives

a maximum

lead

length of:

L

=

1003 ft., error

=

0.O5oC

Setting

the allowable

error at 0.1oC or

approximately

1OpV, the maximum

lead length

increases

to:

L

=

1085

ft.. error

=

0.1oC

Input

Range(mV)

15

150

15

500

50

500

TABLE

Sensor

Model

#

Ro Cw

(kohms) (pfd/ft.)

1

1

1

0.1-1

1

0-5

Vx(mV)

V"o(mV)**

4000

100

3000

130

4000

0

500

0

5000

125

1000 0-90

Summary

of Input

Settling

Data For

CampbellScientific

Resistive Sensors

T,*

(us)

107

207(RH)

WVU-7

227

237

o24A

8641

8771

8723

8641

8641

8771

42

41

62

42

42

41

45

44

65

5-45

45

't-222

*

Estimated time constants

are

for 1000

foot

lead lengths

and

include 3.3nfd 21X input capacitance.

**

Measured

peak

transients

for 1000 foot lead lengths at corresponding

excitation, V*.

13.3.4

SUMMAFY

OF

SETTLING

ERRORS FOR

SCIENTIFIC RESISTIVE

SENSORS

Table

13.3-$ summarizes the

data

required

to

estimate

thQ effect

of

lead length

on settling

errors

for

sensors.

the

input

Scientif

ic's resistive

the transient

level, V"o, to

,

one suspects

that transient

errors

are the

most

likely

limitation

for

the 107

sensor.

Thd sensors in the WVU-7

are

the

same as

in

lhe

Model

107

(the

lead

wire

is

different), bqt

the

signal

leads

for the WVU-7

wet and

dry'bulbs are not subject

to excitation

transients

because

they

are

shielded

independently

f rom the excitation.

The

comparatively

smalltransient

yet

large

source resistance of the 0244 sensor

indicates

that

signal

rise time may

be

the rnost

important

limitation. The analysis in Section

13.3.2

confirms

this.

The

Model 227 Soil

Moisture Block has a

relatively

short

time constant and essentially no

transient. Lead

lengths

in excess

of 2000 feet

produce

less than a 0.1 bar

(0-10

bar

range)

input

settling error.

With

this sensor,

the drive

capability of the excitation channel

limits the

13-7

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