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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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