Page 257 - Instrumentation Reference Book 3E

Temperature scales 241

Table 14.1 Coefficients of linear expansion of solids heat through matter. Convection is the indirect
Extracted from Tables of Plg~sicrrl arid Clzeniical transfer of heat by the thermally induced circulation
Consranis by Kaye and Laby (Longmans). The values of a liquid or gas; in “forced convection,“ the
given are per kelvin and, except where some circulation is increased by a fan or pump. Radi-
temperature is specified. for a range about 20 degrees.
ation is the direct transfer of heat (or other form of
energy) across space. Thermal radiation is electro-
magnetic radiation and comes within the infi-ared,
Aluminum 25.5 visible, and ultraviolet regions of the
Copper 16.7 electromagnetic spectrum. The demarcation
Gold 13.9 between these three classes of radiation is rather
Iron (cast) 10.2 indefinite but as a guide the wavelength bands are
Lead 29.1 shown in Table 14.2.
Nickel 12.8
Platinum 8.9
Silver 18.8 Table 14.2 Wavelengths of thermal radiation
Tin 32.4
Brass (typical) 18.9
Constantan (Eureka) 17.0
60 Cu, 40 Ni Infrared 100-0.8
Duralumin 22.6 Visible light 0.8-0.4
Nickel steel, Ultraviolet 0.4-0.01
10% Ni 13.0
30% Ni 11.0
36% Ni (Invar) -0.3 to t2.5 So far as the effective transfer of heat is con-
40% Ni 6.0
Steel 10.5 to 11.6 cerned the wavelength band is limited to about
Phosphor bronze. 16.8 10 pm in the infrared and to 0.1 pm in the ultra-
97.6 cu. 2Sn, 0.1 P violet. All the radiation in this band behaves in
Solder, 2 Pb, 1 Sn 25 the same way as light. The radiation travels in
Cement and concrete 10 straight lines, may be reflected or refracted, and
Glass (soda) 8.5 the amount of radiant energy falling on a unit
Glass (Pyrex) 3 area of a detector is inversely proportional to the
Silica (fused) -80” to O’C 0.23 square of the distance between the detector and
Silica (fused) 0’ to 100 “C 0.50
the radiating source.
coefficient of expansion, which is the increase in 14.2 Temperature scales
volume per degree rise in temperature. divided by
the volume at 0 “C, so that volume Lrt at tempera- To measure and compare temperatures it is neces-
ture f is given by: sary to have agreed scales of temperature. These
v-v 0(1 +Pf) (14.2) temperature scales are defined in terms of physical
r
-
phenomena which occur at constant temperatures.
where Yo is the volume at 0°C and n is the The temperatures of these phenomena are known
coefficient of cubical expansion. as “fixed points.”
There is also the mean coefficient of expansion
between two temperatures. This is the ratio of the
increase in volume per degree rise of temperature, 14.2.1 Celsius temperature scale
to the original volume. That is. The Celsius temperature scale is defined by inter-
national agreement in terms of two fixed points,
(14.3) the ice point and the steam point. The tempera-
ture of the ice point is defined as aero degrees
where V,, 3s the volume at temperature 11, and Vt2 Celsius and the steam point as one hundred
is the volume at temperature tl. degrees Celsius.
This definition is useful in the case of liquids The ice point is the temperature at which ice and
that do not expand uniformly, e.g., water. water exist together at a pressure of 1.0132 x
10’ N . m-? (originally one standard atmosphere
= 760mm of mercury). The ice should be pre-
14.1.3 Radiation pared from distilled water in the form of fine
There are three ways in which heat may be trans- shavings and mixed with ice-cold distilled water.
ferred: conduction, convection, and radiation. Con- The steam point is the temperature of distilled
duction is, as already covered, the direct transfer of water boiling at a pressure of 1.0131 x IO’N . rn-?.