Int J Thermophys (2017) 38:70
Drift in Type K Bare-Wire Thermocouples from
E. S. Webster1
Received: 22 June 2016 / Accepted: 6 March 2017 / Published online: 14 March 2017
© Springer Science+Business Media New York 2017
Abstract Base-metal thermocouples play a signiﬁcant role in industrial measure-
ments, and among the many varieties and formats, bare-wire Type K is often preferred.
The reason for this preference is its low cost, durability and tolerance of high tem-
perature. Unfortunately, Type K, like all base-metal thermocouples, is made based
on a temperature-to-emf relationship and not on a speciﬁc metallurgical formulation.
The original Hoskins Chromel/Alumel couple was simple in composition, and it had
known thermal drift characteristics associated with reversible crystallographic changes
and irreversible oxidation, and these two drift mechanisms led to large instabilities
in use. To improve the stability of Type K, most modern manufacturers now adopt
compositions with alterations that can depart signiﬁcantly from those of the original
formulation. These alterations are usually made to improve the stability and/or man-
ufacturing processing of their wire. So, although the wire is made to meet the limits
of error and tables, this is only true at the time of manufacture. As soon as the wire
is exposed to temperatures above 150 ◦C, the supplier-dependent alloys can exhibit
a wide range of drift behaviors that depend on composition and even the batch of
the wire. This study investigates the change in Seebeck coefﬁcient as a function of
temperature for Type K bare-wires from different suppliers by using a linear-gradient
furnace and a high-resolution homogeneity scanner. Wires were exposed to tempera-
tures over the range ∼20 ◦C to 950 ◦C for time periods between 24 h and 500 h. The
results show that most wires have very different drift behaviors, which the end user
could not realistically predict or correct.
Selected papers of the 13th international symposium on temperature, humidity, moisture and thermal
measurements in industry and science.
BE. S. Webster
1Measurement Standards Laboratory, PO Box 31310, Lower Hutt 5040, New Zealand