Omnigrad M TR13, TC13
2
Function and system design
Measuring principle
Resistance thermometer (RTD)
These resistance thermometers use a Pt100 temperature sensor according to IEC 60751. The
temperature sensor is a temperature-sensitive platinum resistor with a resistance of 100 Ω at
0 °C (32 °F) and a temperature coefficient α = 0.003851 °C
-1
.
There are generally two different kinds of platinum resistance thermometers:
•
Wire wound (WW):
Here, a double coil of fine, high-purity platinum wire is located in a ceramic
support. This is then sealed top and bottom with a ceramic protective layer. Such resistance
thermometers not only facilitate very reproducible measurements but also offer good long-term
stability of the resistance/temperature characteristic within temperature ranges up to
600 °C (1
112 °F). This type of sensor is relatively large in size and it is comparatively sensitive to
vibrations.
•
Thin film platinum resistance thermometers (TF):
A very thin, ultrapure platinum layer,
approx. 1 μm thick, is vaporized in a vacuum on a ceramic substrate and then structured
photolithographically. The platinum conductor paths formed in this way create the measuring
resistance. Additional covering and passivation layers are applied and reliably protect the thin
platinum layer from contamination and oxidation, even at high temperatures.
The primary advantages of thin film temperature sensors over wire wound versions are their smaller
sizes and better vibration resistance. A relatively low principle-based deviation of the resistance/
temperature characteristic from the standard characteristic of IEC 60751 can frequently be observed
among TF sensors at high temperatures. As a result, the tight limit values of tolerance category A as
per IEC 60751 can only be observed with TF sensors at temperatures up to approx. 300 °C (572 °F).
For this reason, thin-film sensors are generally only used for temperature measurements in ranges
below 400 °C (932 °F).
Thermocouples (TC)
Thermocouples are comparatively simple, robust temperature sensors which use the Seebeck effect
for temperature measurement: if two electrical conductors made of different materials are connected
at a point, a weak electrical voltage can be measured between the two open conductor ends if the
conductors are subjected to a thermal gradient. This voltage is called thermoelectric voltage or
electromotive force (emf.). Its magnitude depends on the type of conducting materials and the
temperature difference between the "measuring point" (the junction of the two conductors) and the
"cold junction" (the open conductor ends). Accordingly, thermocouples primarily only measure
differences in temperature. The absolute temperature at the measuring point can be determined
from these if the associated temperature at the cold junction is known or is measured separately and
compensated for. The material combinations and associated thermoelectric voltage/temperature
characteristics of the most common types of thermocouple are standardized in the IEC 60584 and
ASTM E230/ANSI MC96.1 standards.
