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SECTION

13.

21X

MEASUREMENTS

lead

length.

lf

the

capacitive load

exceeds

0.1

pfd

and

the resistive

load

is

negligible,

V,

will

oscillate

about its

control

point.

lf

the

capacitive

load

is 0.1

pfd

or

less, V, will

settle

to within

O.1o/o ol its

correct

value

in

150ps.

A lead

length

of 2000 feet

is

permitted

for

the

Model

227

betore

approaching

the drive

limitation.

Sensor

Model#

Error

Table

13.3-6 summarizes

maximum lead

lengths

for

corresponding

error limits in six

Campbell

Scientific sensors.

Since the

first

three

sensors

are nonlinear,

the voltage error,

V",

is the most conservative

value

corresponding

to the error

over the

range

shown.

Maximum

Length(ft.)

TABLE

13.3-6.

Maximum

Lead

Length

vs.

Error

for Campbell Scientific

Resistive Sensors

MINIMIZING

SETTLING ERRORS

IN NON-

CAMPBELL

SCIENTIFIC

SENSORS

When

long

lead lengths are mandatory

in

sensors

configured

by

the user,

the following

general practices

can be used to minimize

or

measure

settling

errors:

1.

When measurement

speed is

not a

prime

consideration,

Instruction 4, Excite,

Delay,

and Measure,

can be used to insure

ample

settling

time for half bridge,

single-ended

sensors.

2.

An

additional low value bridge

resistor

can

be added to

decrease the source

resistance,

Ro. For example,

assume

a YSI

nonlinear

thermistor such

as the model

44032

is used with a 30 kohm

bridge

resistor,

Ri. A

typical configuration

is

shown

in Figure

13.3-74. The

disadvantage

with this

configuration is

the

high

source resistance

shown in

column

3

of Table 13.3-7.

Adding another 1K

resistor,

R1,

as shown in Figure

13.3-78,

lowers

the source

resistance of the 21X

input.

This

offers no improvement

over

configuration

A because Ri

still combines

with

the lead

capacitance to slow

the signal

response

at

point

P. The source

resistance

at

point

P

(column

5) is essentially

the

13-8

ve(pv)

5

1oOOl

5oo

18903

5

8652

1390

4gO2

-

2ooo3

5oo 18603

same as

the input

source resistance of

configuration

A. Moving

Ri out to the

thermistor

as shown in Figure 13.3-7C

optimizes

the signal

settling time because

it

becomes

a function

of Rr and

C*

only.

Columns

4

and

7

list the signal

voltages

as

a function of temperature

using

a 5V

excitation

for

configurations A and C,

respectively. Although

configuration A has

a higher output signal

(5V

input

range), it

does not

yield

any higher

resolution than

configuration C

which

uses

the

t500mV

input range.

Where

possible,

run

excitation

leads

and

signal leads

in

separate

shields

to minimize

transients.

AVOID PVC

INSULATED

CONDUCTORS

to minimize the

etfect of dielectric

absorption on input

settling time.

107

207(RH)

WVU-7

o24A

227

237

0.050c

1%RH

0.050c

30

10 kohm

Range

ooO to 4ooO

2Oo/"to

9Oo/"

ooc to 4oo-C

@ 3600

20k

to 300k

I

based on transient

settling

'

based on signal rise

time

'

limit

of excitation

drive

3.

4.

NOTE: Since Ri attenuates

the

signal in

configuration

B and

C, one might

consider

eliminating

it altogether.

However,

its

inclusion

"flattens"

the non-linearity of the

thermistor,

allowing

more

accurate

curve

fitting

over

a broader temperature range.

1 2 ... 139 140 141 142 143 144 145 146 147 148 149 ... 190 191

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