Campbell 21X Manuale dell'Operatore Scaricare il pdf (Pagina 65)

SECTION

7. MEASUREMENT

PROGRAMMING

EXAMPLES

Figure 7.2-1.

Since

a single- ended

measurement

is referenced

to

the 21X

ground,

any voltage

difference

between the

sensor

ground

and

21X

ground

becomes

a

measurement

error.

A

differential measurement

avoids this error

by measuring

the

signal

between the two

leads

without

reference

to

ground.

This example

analyzes

the

ground potential

error

of

a single-ended

measurement

of

pH

using a Martek

Mark V

water

quality

analyzer.

01:

01:

02:

03:

o4:

05:

06:

corrected

in

programming.

However, it is

better

to use a differentialvoltage

measurement

does not rely on the

current drain remaining

constant. The

program

that follows illustrates

the use

of

Instruction

2 to make the

measurement.

A multiplier

of 0.014

is

used to

convert

the

millivolt

output into

pH

units.

PROGRAM

Volt

(DIFF)

Rep

5000 mV slow

Range

lN Chan

Loc

[:pH I

.014

Mult

Offset

7.3 THERMOCOUPLE

TEMPERATURES

USING 21X REFERENCE

The

use of

the built in

21X thermocouple

reference

thermistor

is described in the

introductory

programming

example

(Section

ov4).

7.4 THERMOCOUPLE

TEMPERATURES

USING AN

EXTERNAL

REFERENCE

JUNCTION

When

a number

of

thermocouple

are made

at

some

distance from the 21X, it is

often

better

to use

a

reference

junction

box

located

at the

site rather

than using the 21X

panel

for

the

reference

junction.

This

reduces

the required length of

expensive thermocouple

wire

as regular copper wire

can

be used

between

the

junction

box

(J-box)

and 21X. In

addition,

if the temperature

gradient

between

J-box

and

the

thermocouple

measurement

junction

is smaller than the

gradient

between

21X

and the

measurement

junction,

thermocouple accuracy is

improved.

In

the

following

example, an external

reference

junction

is used on 5 thermocouple

measurements. A

Campbell

Scientific

107

Temperature Probe

is used

to measure the

reference

temperature. The connection

is

shown in

Figure

7.4-1.

P2

1

5

1

1

EXTERNAL

12

VOLT

BATTERY

FIGURE 7.2-1.

Typical Connection

for

Active

Sensor with

External

Battery

The wire

used to

supply

power

from

the external

battery is 18

AWG with

an average resistance

of

6.5 ohms/1000

feet. The

power

runs

to the 21X

and

pH

meter

are 2 feet and 10 feet,

respectively. Typical

current

drain

for

the

pH

meteris

300mA. When

making measurements,

the

21X

draws

about

60mA.

Since

voltage

is

equal to current

times

resistance

(V=lR), ground

voltages

at the

pH

meter

and

the 21X relative

to

battery

ground

are:

PH

meter

ground

=

0.3A x 10/1000

x

6.5ohms

=

+0.01

95V

21X

ground

=

0.0604

x2l10OO

x 6.5ohms

=

+0.0008V

Ground at the

pH

meter is 0.0187V

higher

than

ground

at the 21X.

The meter output

is

0-1 volt

referenced

to meter

ground,

for the full

range

of

14

pH

units,

or O.O714V/pH. Thus,

if the

output

is

measured with

a single-ended voltage

measurement,

it is

0.0187V or

0.26

pH

units too

high. lf this

offset remained

constant,

it

could be

POWER IOFT, 18

AWG

7-2

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