N.A.C.A. Technical Memorandum No. 905

By the requirement of sharply differentiated colors,
it was meant that colr.,r changes such as white to yellow,
yellow to brown, and light green to dark green were t o be
excluded as unsuitable, since in operation on the test
stand, slight color differences cannot be distinguised on
account of the soiling of the parts by the oil.   It was
therefore desired to obtain markedtcolor changes, as, for
example, red to blue, yellow to red, green to brown.
A property soon recognized as very important was the
sharp transition from one color to the other.   Intermedi-
ate shades will always occur to some extent, since the
color change is associated with chemical processes which,
although occurring at a definite temperature, are initiat-
ed within a small temperature interval.  This interval
within which the color change occurs is smaller the higher
the transition temperature.  It amounts to only a few de-
grees, so that it is always possible to recognize the
unique position of the isotherms.
The question was also considered whether, in contrast
to these sharp transitions, it would not be better to at-
tempt to find a coating with a uniform change of color
with temperature, which would thus be determined at each
point. Aside from the difficulty of producing a material
with this property, practical considerations soon showed
that difficulties are encountered in evaluating intermedi-
ate shades.  It becomes necessary to determine the temper-
ature with the aid of a calibrated color scale and then to
draw in the isotherms.
CHARACTERISTICS OF TEMPERATURE-INDICATITG PAINTS
Permanent paints for rendering temperature fields
visible, such as mercuric sulphate, mercuric iodide, lead
carbonate, cadmium carbonate, and others, have been ap-
plied in various ways. None of these materials, however,
could sufficiently satisfy the above-described require-
ments, because the sharpness of the transition, the colors,
and the unique correspondence of temperature to color
change reproducibilityy) were not satisfactory.
In order to attain any progress, it was necessary to
conduct comprehensive and laborious development tests at
the research laboratory of the Oppau Works; this work  led
to the use of entirely new materials. Those paints, which