Operator`s manual

SECTION 13. 21X MEASUREMENTS
13-14
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 will take a
worst case situation where all errors are
maximum and additive. A temperature of 45
o
C
is measured with a type T (copper-constantan)
thermocouple, using the ±5mV range. The
nominal accuracy on this range is 2.5µV (0.05%
of 5mV), which at 45
o
C changes the
temperature by 0.06
o
C. The RTD is 25
o
C but is
indicating 25.3
o
C, and the terminal that the
thermocouple is connected to is 0.3
o
C cooler
than the RTD.
TABLE 13.4-4. Example of Errors in
Thermocouple Temperature
Source Error
o
C % of Total Error
1
o
C 1% Slope
Error Error
Reference Temp. 0.6 36.1 69.6
TC Output
ANSI 1.0 60.1
0.01 x 20
o
C 0.2 23.2
Voltage
Measurement 0.06 3.6 7.0
Reference
Linearization 0.001 0.1 0.1
Output
Linearization 0.001 0.1 0.1
Total Error
With ANSI error 1.662 100
Assuming 1% 0.862 100
slope error
13.4.2 USE OF EXTERNAL REFERENCE
JUNCTION OR JUNCTION BOX
An external junction box is often used to
facilitate connections and to reduce the
expense of thermocouple wire when the
temperature measurements are to be made at a
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 of
error within that range. The only situation where
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
using type K thermocouples, where the upper
limit of the reference compensation linearization
is 100
o
C and the upper limit of the extension
grade wire is 200
o
C. 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 metal than 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.