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FIGURE
tg.4-2.
Diagram
of Junction Box
An
external reference junction box must
be
constructed so that
the entire terminal area
is
very close
tO
the same temperature. This
is
necessary
sO
that
a
valid reference temperature
can be measured. and to avoid
a
thermoelectric
otfset voltagB which
will be induced
if
the
terminals at
Which
the
thermocouple leads are
connected (points A and B in Figure 13.4-3) are
at different
tpmperatures. The box should
contain elernents
of high thermal conductivity,
which will
adt
to rapidly remove any thermal
gradients to which
the box is
subjected.
lt is not
necessary
tQ
design a constant
temperature
box; it is de$irable that
the box respond slowly
to external temperatu
re
f
luctuations. Radiation
shielding
m{st
be provided when
a
junction box
is installed ifi the
field.
Care must also
be
taken
that
a
thermpl gradient is not induced by
conduction
frrough the
incoming
wires. The
21X
can be
Used
to measure the temperature
gradients wifhin the
junction box.
13.5
BRIDGE RESISTANCE
MEASUREMENTS
There
are 6 bridge measurement instructions
included
in
the standard 21X
software.
Figure
13.5-1
shovr4s
the circuits that would typically be
measured with
these instructions.
In
the
diagrams,
t(e
resistors labeled
R"
would
normally
be
the sensors and those labeled
R1
would normhlly
be
fixed resistors. Circuits other
than
those {iagrammed could be measured,
provided
th{
excitation and type of
measu
remefrts
were appropriate.
With
the
exOeption
of Instructions 4 and
8,
which apply an excitation voltage then wait
a
specified
tirne before making a measurement,
SECTION
13.
21X
MEASUREIT,IENTS
allof
the bridge measurements make one set of
measurements with
the excitation as
programmed
and another sel of measurements
with
the excitation polarity reversed. The error
in
the
two measurements due to thermal emfs
is
then accounted for
in
the processing of the
measurement
instruction. The excitation
is
switched on 450ps before
the integration portion
of
the measuremenl stads and is grounded as
soon as
the integration
is
completed.
Figure
13.5-2 shows the excitation
and measurement
sequence
for Instruction 6, a
4
wire
full bridge.
When more than one measurement per sensor
is necessary (lnstructions 7 and 9),
excitation
is
applied separately
for each measurement. (For
example, in Instruction 9
(6
wire
full
bridge),lhe
differential measurement of the voltage drop
across
the sensor is made with the excitation at
both polarities;
excitation
is
again applied and
reversed for
the measurement of the outpul
voltage.)
Instruction
8
applies an excitation voltage,
delays a specified
time, and makes a differential
voltage measurement, lf
a
delay of
0
is
specified,
the inputs for the differential
measurement
are not switched for a second
integration as is normally the
case (Section
13.2). The
result stored
is
the
voltage
measured.
Instruction 8
does not have as good
resolution or
common mode rejection as the
ratiometric bridge
measurement instructions" lt
does provide
a
very rapid means of making
bridge measurements as well as supplying
excitation to
circuitry requiring differential
measurements. This instruction does not
reverse
excitation. A
1
before
the excitation
channel number (1X) causes the channel to
be
incremented
with each repetition. The output of
lnstruction
8
is
simply the voltage
measurement. When 8 is used to measure
a
full bridge (same connections as Instruction 6
in
Figure 13.5-1),
the result
is
Vl
which equals
V, (Rs/(Rs+Ra)
R2l(R1+R2)).
(ln other words, to make the output the same as
Instruction 6, use
a
factor of 10004/,
in
the
multiplier.)
13-15
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