Campbell 21X Operator's Manual page 150

SECTION
13.
21X MEASUREMENTS
built-in
thermistor.
Errors
in
the thermocouple
and reference temperature polynomials are
extremely small, and error
in the voltage
measurement is negligible.
To illustrate the relative magnitude
of
these
errors in the environmental range, we willtake
a
worst case situation where all errors are
maximum and
additive.
A temperature of
45oC
is measured with
a
type
T (copper-constantan)
thermocouple, using the *SmV range. The
nominal accuracy on this range
is
2.5pV (0.05%
of SmV), which at
45oC changes
the
temperature by
0.06"C. The
RTD
is 25oC but
is
indicating 25.3oC, and the terminal that the
thermocouple
is
connected to
is
O.3oC
cooler
than
the
RTD.
TABLE 13.4-4. Example of Errors in
Thermocouple Temperature
Source
Error
oC
o/"
of Total Error
loc
1ol"
Slope
Error
Error
13.4.2
USE OF
EXTERNAL REFERENCE
JUNCTION OR JUNCTION BOX
An externaljunction box is often used to
facilitate connections and to reduce the
expense of thermocouple wire when the
temperature measurements are to be made
at
distance from the
21X.
In most situations, it
is
preferable
to make the box the reference
junction,
in
which case
its
temperature
is
measured and used as the reference for the
thermocouples;
copper wires are run from the
box to the 21X (Section
7.4).
Alternatively, the
junction box can be used to couple extension
grade thermocouple wire to the thermocouples
being used for measurement, and the 21X
panel used as
the reference junction.
Extension grade thermocouple wire has
a
smaller temperature range than standard
thermocouple
wire, but meets the same limits
error within that
range.
The only situation
it
would be necessary
to use extension grade
wire instead of a external measuring junction
is
where the junction box
temperature is outside
the range of reference junction compensation
provided
by the
21X. This
is only a
factor when
Lsing type K thermocouples, where
the upper
limit
of
the reference compensation
li
is
100oC
and the upper limit
of
the extension
grade wire
is
200oC. With the other types of
thermocouples,
the reference compensation
range equals or
is
greater than the extension
wire range.
In
any case, errors can arise
if
temperature gradients exist within the junction
box.
Figure 13.4-3 illustrates
a typical
junction
box.
Terminal strips
will be
a
different metalthan the
thermocouple
wire. Thus,
if
a
temperature
gradient exists between A and
A'or
B
and
B',
the
junction box will act as another
thermocouple in series, creating an error
in
the
voltage measured
by the
21X.
This
thermoelectric offset
voltage
is a
factor
whether
or
not the
junction box is used for the reference.
It
can be minimized by making the thermal
conduction between the
two points large and
the distance
small. The
best solution in
the
case where extension grade wire is being
connected to thermocouple wire
would
be
to
use connectors which clamped the two wires
in
contact with each other.
Reference Temp.
TC Output
ANSI
0.01 x
20oC
Voltage
Measurement
Reference
Linearization
Output
Linearization
36.1
69.6
60.1
23.2
0.6
1.0
0.2
0.06
0.001
0.001
7.0
3.6
0.1
0.'l
0.1
0.1
Total Error
With
ANSI error
Assuming
17o
slope
error
100
100
1.662
0.862
13-14