5. Reference Junctions:
A thermoelectric temperature
measurement measures a (thermoelectric) voltage which is generated by the
thermocouple at the measuring point and whose size depends on the difference in
temperature between the measuring point and the reference junction. For
technical and economic reasons, the thermocouple is not directly connected to the reference junction in
most operating circumstances. Instead, the free ends of the thermocouple designated
as the connection points are usually connected to a ceramic base
(in the connection head) or to a suitable
plug, a compensating or extension cable being used to extend
the thermocouple from here to the reference junction. To be
able to determine the exact temperature at the measuring point,
the temperature at the reference junction must be constant and/or known.
A distinction is drawn here between external and internal
reference junctions. In the case of the so-called external reference junctions, appropriate
measures are used to keep the temperature at a constant and known
value (generally 0°C, 20°C or 50°C). A simple copper
cable is used to connect the external reference
junction to the downstream display or control instrument, which has been
adjusted, in turn, to the constant temperature of the reference
junction. However, measuring equipment with an internal (integrated)
reference junction is increasingly being used. In this case,
the compensating or extension cables are connected directly to the
clamps of the measuring equipment / temperature controller.
These feeder clamps constitute the internal reference junction. The current
clamp temperature is measured internally and converted to
an auxiliary voltage, which is used to correct the measured
thermoelectric voltage. We would like to point out here
that the transducers used for thermoelectric temperature measurement (output mostly 4 to
20 mA) also have an internal reference junction, which
means that they can be connected to the downstream device
by means of a simple copper cable. Depending on
design, these transducers are usually fitted in the connection head or in a
control cabinet and are either directly connected to the thermocouple
(connection head fitting) or by means of compensating/extension cables (control
cabinet or connection head fitting).
Go to Section 6 (Resistance Thermometers – Measuring Circuit) |
