Operator's Manual
Issue 14
/
Mar 2016 / UMC0071
Mercury
iPS
©2016 Oxford Instruments NanoScience. All rights reserved.
Page
71
Newer files, with a higher density of data points, are available for some sensors. These have
the prefix Merc, such as MercTG57-2.
6.6 More about thermocouples
6.6.1 Thermocouple principles
A thermocouple consists of two parallel conductors of dissimilar materials, usually alloys, joined
at one end, or possibly both ends but with a break-out for voltage measurement. One junction is
placed on the sample at unknown temperature, the other at some reference temperature, so
that there is a temperature gradient,
T
, along both conductors.
The inhomogeneous distribution of electrons along the conductor gives rise to a thermoelectric
emf
(T) which is a function of the temperature gradient
T.
The precise form of
(T) is
determined by scattering of the conduction electrons by the lattice which is, in turn, energy
dependent. Therefore, the shape of the Fermi surface of the metal or alloy and the conduction
electron mean-free path are significant factors in determining the detail of
(T).
In practice, however, thermocouples suffer from poor low temperature sensitivity and are
subject to errors arising from inhomogeneities in the metals used. In addition,
(T) is usually
magnetic field dependent. For these reasons, a resistance thermometer normally provides
better performance, especially at low temperature.
6.6.2 Configuring for thermocouples
The
Home
page below shows a
Mercury
iPS configured for a Au-Fe/chromel thermocouple with
a liquid nitrogen reference. The widgets have been configured as follows:
