Advantages of
Type N Thermocouples
Dr. Noel Burley of
the Australian defense command developed the Type N
Thermocouple in the 1970’s. It is the most recent
ISA Letter Designated thermocouple having obtained
letter designation in 1983. It was designed to
minimize some of the instabilities of the
conventional Chromel-Alumel Type K combination. It
differs from Type K in the addition of Silicon in
both the NP and NN wires and an increase in the
Chromium content of the NP wire. The NN wire also
contains a small amount of Magnesium.
This thermocouple
has an EMF output and Seebeck Coefficient similar to
that of a Type K and covers an equivalent
temperature range. It has lower drift rates at
temperatures up to 1200 C., it does not exhibit the
short range ordering problem of the Type K and it is
less susceptible to “Green Rot” compared to a Type
K. Type N Thermocouples can be used in the same
environments as Type K. Error tolerances on the Type
N are the same as the Type K and ASTM has published
both thermocouple tables and polynomials describing
the emf response of Type N just as it has for all
other Letter Designated Thermocouples. It is
somewhat surprising that the Type N has not seen
greater use.
Data from the
literature and experience in our laboratories
confirm that the Type N thermocouple is more stable
and drifts at a lower rate. The graph below from a
publication by Dr. Richard Anderson of Oak Ridge
National Laboratories helps illustrate this point.
Small diameter Type K and Type N thermocouples in
Inconel and Stainless Steel sheaths were tested for
drift at 1200 C. Note that while all of the sensors
began drifting almost immediately at these severe
conditions, the two Type N couples (triangles and
crosses) show lower initial drift rates and hence
greater stability than the two Type K’s. We see
similar results in our laboratories.
We believe that for
most applications Type N is superior to Type K. We
think that the reluctance to change to Type N is due
to the very large installed base and experience with
Type K. Many systems have been built around Type K
over the years and some of them are not easily
modified. With the advances in electronics most
signal conditioning units and smart transmitters now
come with multiple calibration options including the
Type N. Process instrumentation systems are slowly
being modernized and more capability is being
demanded of instrumentation and control systems.
This should lead to greater use of the Type N in the
future because of it’s obvious benefits.
Ref: Anderson, R.L.,
et.al., DECALIBRATION OF SHEATHED THERMOCOUPLES,
Temperature, (1982) pp 997-1007
You
may download a copy of this paper at
Type N Thermocouples.pdf
