Page 184 - Mechanical Engineers' Handbook (Volume 2)

8 Temperature-Sensitive Coatings 173

Hewlett-Packard quartz crystal thermometer uses shear-wave propagation in the resonance
mode to achieve its very high precision (on the order of 0.0001 K). 64
Tuning-fork resonance probes of pure sapphire have been run at 2000 C in a laboratory
environment with an apparent stability of 1% or better by Bell et al. 65
While capable of high precision under favorable conditions, the resonance mode is very
sensitive to deposits or accidental contact between the resonator and its support. It is also
inherently a line-averaging technique since it is the temperature of the oscillating stem, not
that of the mass at the end, that determines the resonant frequency. For those reasons, most
acoustic velocity systems proposed for high-temperature work use the time-of-flight tech-
nique known as the pulse-echo technique.
63
Tasman and Richter offer the following description of the pulse-echo system:
The pulse-echo technique measures the transit time of a single sound pulse through a thin wire.
A magnetostrictive transducer transforms an electrical pulse into a sound pulse, which is injected
into a transmission line. Discontinuities in the line, and the end of the line, reflect (part of) the
sound pulse, which is then converted back into electrical signals by the transmitting transducer.
The actual sensor is the wire joining the discontinuities. Almost any kind of discontinuity will
do to produce an echo, even a kink in the wire. One sensor may contain several discontinuities,
apart from its end, and thus measure the longitudinal temperature profile over several consecutive
sections.
For good resolution, the sensor material should have a large change in propagation
velocity over the temperature range to be encountered. Thoriated tungsten is one candidate
material. Tasman and Richter quote the round-trip time difference between echos as increas-
ing from about 4.5 to about 5.5 s/cm when the temperature changes from 20 to 2700 C.
Time differences can be measured to about 1 ns, which means that a 50-mm-long sensor
could resolve temperature to 0.2 K at 2000 C (in theory). Practical considerations seem
to limit the resolution to about 3 K for the 50-mm sensor. Tungsten is very sensitive to
oxidation at high temperatures, and pure tungsten cannot be used as an acoustic velocity
sensor because it continues to recrystallize at high temperatures, causing a continuous shift
in calibration. Thoria blocks the grain growth, however, and the calibration of thoriated
tungsten is stable (in the absence of oxygen).

8 TEMPERATURE-SENSITIVE COATINGS
66
Paints and crayons are available that are designed as temperature indicators, up to 2500 F
(1371 C) with a quoted accuracy of 1%. Two types are available: phase change and color
change. When the phase change materials melt, they yield easily discernible evidence that
their event temperature has been exceeded. Color change materials are subtler and less easy
to interpret. There have been some complaints of calibration shifts of these paints when used
on heavily oxidized materials which are believed due to alloying of the oxide with the paint.
These paints are nonmetallic and, therefore, have different radiation properties than
metals. They should be used only over small areas of metallic surfaces (small compared with
the metal thickness), or else their different emissivities will lead to a shift in the operating
temperature of the parts. Since nonmetals tend to have higher emissivities than metals, the
painted regions may have different radiation properties than the substrate material. Trial
specimens should be checked if precise data are needed. If the specimens are to be heated
by radiation from a high-temperature source, the different radiation properties can have a
significant effect (2–3% in temperature).
Paints are useful in examining the distribution of temperature over a surface. Discrete
spots should be used, rather than lines, since contact with molten material facilitates the
melting of neighboring material even if it has not yet reached its own melting point.