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SECTION
13.
21X MEASUREMENTS
=
r\ | -
Jvt\
vtilvl
A)
Ro
=
RsRf/(Rs+Rf),
Vs
=
VxRf/(Rs+Rf)
B)
Ro@P
=
ns(Rf+Rf)/(Rs+Rf+Rf)
=
JOK
OHM
:
| \l
C)
Ro
=
Rf, Vs
=
VxRf/(Rs+Rf+Rf)
FIGURE
13.3-7. Half Bridge Configuration
for
YSf #44032
Thermistor Connected to
21X
Showing: A) Large source resistance,
B) large
source resistance at point
P,
and
C) configuration optimized for input settling
21X
SHIELD
FIGURE
13.3-8. Measuring Input Settling
Error with the
21X
13-10
FIGURE
13.3-9. Incorrect Lead Wire
Extension on Model
107
Temperature
13.4
THERMOCOUPLE
MEASUREMENTS
A thermocouple consists of two
wires, each of a
different metal or alloy, which
are
joined
together at each
end.
lf
the two
junctions are at
different temperatures,
a
voltage proportional to
the
difference
in
temperatures is induced
in
the
wires.
When
a
thermocouple is used for
temperature measurement, the wires are
soldered or welded
together
at
the measuring
junction. The
second
junction, which becomes
the reference junction,
is
formed where
the
other ends
of
the wires are connected to
the
measuring
device.
(With the
connectors
at
the
same temperature,
the chemical dissimilarity
between the thermocouple wire and
the
connector
does not induce any
voltage.)
When
the temperature
of
the reference junction
is
known, the temperature of
the measuring
junction can be determined by measuring the
thermocouple voltage and adding
the
corresponding
temperature difference to the
reference temperature.
The 21X determines thermocouple
temperatures using the following sequence.
First, the
temperature
of
the reference junction
is
measured.
lf
the reference junction
is
the
21X panel, the
temperature is measured with
the built-in thermistor (lnstruction
17).
The
reference junction
temperature
in
oC
is stored
in
an input location which
is
accessed by the
thermocouple measurement instruction
(lnstruction
13
or
14). The
21X calculates the
voltage that
a
thermocouple
of
the type
specified would output
at
the reference junction
temperature if its reference junction were at
OoC,
and adds this voltage to
the measured
thermocouple
voltage.
The temperature of the
measuring
junction
is
then calculated
from
a
SHITLD
HI
OR
LO
VX
V5
1.,
l-q
R
Cry
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