Campbell Scientific CR10 CR10 Measurement and Control - Page 90

SECTION

7.

MEASUREMENT

PROGRAMMING

EXAMPLES

R

AI

N

GAGE

1000

3OO FOOT

LEAD

LENGTH

FIGURE

7.8-1. Wiring

Diagram

for

Rain

Gage

with

Long

Leads

7.8

TIPPING BUCKET RAIN GAGE

WITH

LONG LEADS

A tipping bucket rain gage is measured with

the

Pulse

Count

Instruction configured for Switch

Closure.

Counts

from

long intervals will be

used, as the

final

output desired

is

total

rainlall

(obtained

with InstructionT2,

Totalize).

lf

counts

from

long intervals

were

discarded, less

rainfallwould be

recorded than was actually

measured

by

the

gage (assuming there were

counts

in

the

long

intervals). Output

is

desired

in millimeters

of precipitation. The

gage

is

calibrated for

a

0.01

inch tip, therefore, a

multiplier of

O.254

is used.

In

a long cable

there

is

appreciable capacitance

between

the

lines.

The capacitance

is

discharged across the switch

when

it

closes.

In

addition to shortening switch life,

a

transient

may be induced in other wires packaged with

the

rain gage leads each time the switch closes.

The

100

ohm

resistor protects

the

switch

from

arcing and the associated transient from

occurring, and should be included any time

leads longer

than

100

feet

are

used with a

switch closure.

PROGRAM

Pg

Pulse

1

Rep

1

Fulse

Input Chan

2

Switch closure

11

Loc

[:RAIN

mm

]

0.254

Mult

0

Offset

7.9

1OO

OHM

PRT

IN

4

WIRE HALF

BRIDGE

lnstruction

9

is

the

best choice for accuracy

where

the

Platinum Resistance

Thermometer

(PRT) is separated

from other

bridge

completion resistors by a lead length having

more

than

a

few thousandths of an ohm

7-6

resistance.

In

this example, it

is

desired to

measure

a

temperature

in

the

range of -10 to

40'C"

The

length

of

the cable from

the

CR10

to

the

PRT is 500 teet.

Figure 7.9-1 shows the

circuit

used

to

measure

the

PRT.

The

10

kohm resistor allows

the

use

of

a

high excitation voltage and

low

voltage

ranges on

the measurements. This

insures

that

noise

in

the

excitation

does

not have an effect

on signal

noise.

Because the

fixed

resistor

(R)

and

the

PRT

(Rs)

have approximately the same

resistance,

the differential

measurement

of

the

voltage drop across

the

PRT

can

be made

on

the

same

range as

the ditferential

measu

of

the voltage drop

across

R1.

lf

the voltage drop across

the

PRT

(Vr)

is kept

under 50mV, self heating

of

the

PRT should

be

less than

0.001'C

in

still

air.

The best

resolution is obtained when

the

excitation

voltage

is large enough to cause

the

signal

voltage to

fillthe

measurement

voltage

range.

The

resolution of

this

measurement on

the

25mV

range

is

+0.04oC.

The voltage drop

across

the

PRT

is

equalto V,

multiplied by the

ratio

of R,

to the

total

resistance, and

is

greatest

when

R,

is greatest

(Rs=1

15.54 ohms

at

40'C).

To

find the

maximum excitation

voltage that

can

be used, we assume

V2

equal

to

25

mV

and

use Ohm's Law to solve for the

resulting

current,

l.

|

=

25mV/Rs

=

25mV/115.54 ohms

=

0.216

mA

Next solve for V*:

Vx

=l(Rt+R.+Rl)

=

2'21V

lf the

actual

resistances were

the

nominal

values, the CR10 would not over range with

V"

=

2.2V.

To allow for the

tolerances

in

the

actual resistances, it

is

decided to set

V"

equal

to 2.1

volts

(e.9.,

if

the

10 kohms resistor is

57o

low,

then

Rr/(R1+Rr+Rr)=1

15.54/971

5.54, and

V"

must be 2.102V

to

keep

V.less

than

25mV).

01:

01:

o2:

03:

o4:

05:

06:

Campbell Scientific CR10 CR10 Measurement and Control - Page 90

Oo Foot

Page 90 highlights