Electrical thermometers for aircraft Page: 3 of 16
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ELECTRICAL THERMOMETERS FOR AIRCRAFT
Average temperature-e. m. f. relations for thermo-
couples of four combinations of materials are given in
table H. The data for the lower temperatures are
given at short intervals for convenience in applying
corrections for the reference junction temperature or in
calculations dealing with the range of compensators.FIOURE 3.-Thermcouples of the gasket and rivet types and a pair of leads.
A photograph of two types of thermocouple is shown
in figure 3. This particular equipment was constructed
according to specifications of the Bureau of Aeronautics,
Navy Department. The gasket-type thermocouple is
mounted in place of the regular spark-plug gasket. The
rivet-type thermocouple is inserted in a drilled hole,
3 inch in diameter and in depth, and a steel pin concen-
tric with the rivet is driven down to expand the copper
to hold the rivet securely in place. The thermocouple
wires are welded to the gasket or rivet head.
The United States Army has standardized on 2-ohm
iron-constantan thermocouples and the Navy, on 2-ohm
copper-constantan thermocouples; engine manufactur-
ers install 3-ohm iron-constantan thermocouples in
commercial airplanes. These resistance values include
both the resistances of the leads and thermocouples.
Thermocouple leads are made in lengths to fit any
installation requirement. In order to make leads of
different lengths interchangeable, the cross-sectionalarea of the stranded wire is varied directly as the
length so that all leads will have the same resistance.
The leads are composed of two insulated conductors,
of the same materials as the thermocouple, laid parallel,
covered over-all with braid and saturated with flame-
and moisture-resistant lacquer.
Selector switches.-When the temperatures of two or
more points are to be measured with the same indi-
cator, a selector switch of the required number of posi-
tions, a switch lead, and additional thermocouples are
required. The switch lead and switch contact resist-FIGURE 4.-Rotary selector switches for connecting an Indicator to any one of a
number of thermocouples.
ances are made low so as not appreciably to affect the
indicated temperature. (See fig. 2 for a diagram of a
4-position and fig. 4 for a photograph of a 4-position
and an 18-position rotary selector switch.) It is neces-
sary that these switches be of the 2-pole type to avoid635
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Peterson, John B. & Womack, S. H. J. Electrical thermometers for aircraft, report, December 15, 1936; (https://digital.library.unt.edu/ark:/67531/metadc66264/m1/3/: accessed May 9, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.