Operation
2.5
be selected in UNIT.
Temperature Sensors
The TT301, as previously explained, accepts several types of sensors. The TT301 is specially designed
for temperature measurement using thermocouples or thermoresistances (RTDs).
Some basic concepts about these sensors are presented below.
Thermocouples
Thermocouples are the mot widely used sensors in industrial temperature measurements.
Thermocouples consist of two wires made from different metals or alloys joined at one end, called
measuring junction. The measuring junction should be placed at the point of measurement. The other
end of the thermocouple is open and connected to the temperature transmitter. This point is called
reference junction or cold junction.
For most applications, the Seebeck effect is sufficient to explain thermocouple behavior:
How the Thermocouple Works
When there is a temperature difference along a metal wire, a small electric potential, unique to every
alloy, will occur. This phenomenon is called Seebeck effect.
When two wires of dissimilar metals are joined in one end, and left open in the other, a temperature
difference between the two ends will result in a voltage since the potentials generated by the dissimilar
materials are different and does not cancel each other out. Now, two important things must be noted.
First: the voltage generated by the thermocouple is proportional to the difference between the
measuring-junction and the cold junction temperatures. Therefore the temperature at the reference
junction must be added to the temperature derived from the thermocouple output, in order to find the
temperature measured. This is called cold junction compensation, and is done automatically by the
TT301, which has a temperature sensor at the sensor terminals for this purpose. Secondly, if the
thermocouple wires are not used all the way to the terminals of the transmitter (e.g. copper wire is used
from sensor-head or marshalling box) new junctions with additional Seebeck effects will be created and
ruin the measurement in most cases, since the cold-junction compensation will be done in the wrong
point.
The relation between the measuring junction temperature and the generated millivoltage is tabulated in
thermocouple calibration tables for standardized thermocouple types, the reference temperature being
0
o
C.
Standardized thermocouples which are commercially used, whose tables are stored in the memory of
the TT301, are the following:
NBS (B, E, J, K, N, R, S, T)
DIN (L, U)
GOST (L)
ASTM-E (W5Re/W26Re)
Thermoresistances (RTDs)
Resistance Temperature Detectors, most commonly known as RTD's, are based on the principle that
the resistance of a metal increases as its temperature increases.
Standardized RTDs, whose tables are stored in the memory of the TT301, are the following:
JIS [1604-81] (Pt50 & Pt100)
IEC, DIN, JIS [1604-89] (Pt50, Pt100, Pt500, Pt1000)
GE (Cu 10)
Edison Curve (Ni 120)
GOST (Pt50, Pt100, Cu50, Cu100)
IEC 751-95 (Pt100)
MILT (Ni120, Pt100)
For a correct measurement of RTD temperature, it is necessary to eliminate the effect of the resistance
of the wires connecting the sensor to the measuring circuit. In some industrial applications, these wires
may be hundreds of meters long. This is particularly important at locations where the ambient
temperature changes a lot.
The TT301 permits a 2-wire connection which may cause measuring errors, depending on the length of
connection wires and on the temperature to which they are exposed (see Figure 2.3).
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