ASTM C108-46(2004)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation:C 108–46(Reapproved2004)
Standard Symbols for
Heat Transmission
This standard is issued under the fixed designation C 108; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
SPECIALPRACTICES RELATING TO HEAT FLOW
q is in general the time derivative of total quantity of heat
transferred. For steady-state heat flow, q is total quantity of heat
TERMS
divided by time.
Terms Ending in “ivity”—Terms ending in “ivity” designate
k = thermal conductivity; heat flow rate, per unit of area, per “degree
characteristics of materials, normally independent of size or
per unit of length”
shape, sometimes called “specific properties.”
dq/d[mathit]A = −k (dt/dL) general expression giving value at each
Examples: Conductivity and resistivity.
point
Terms Ending in “ance”—Terms ending in “ance” designate
k=(q/A)/(Dt/L) for substantially uniform and steady-state heat flow
properties of a particular object, depending not only on the through a homogeneous medium of thickness L, with plane parallel
faces of area A, and a constant temperature difference Dt applied
material, but also upon size and shape, sometimes called
to the faces
“total quantities.”
1/k = thermal resistivity; reciprocal of conductivity
Examples: Conductance and transmittance.
R = thermal resistance; degrees, per unit of heat flow rate, for a
Terms Ending in “ion”—Terms ending in “ion” designate
particular body or setup (where the area may not be known)
time rate of the process of transfer; flux, flow rate.
R= Dt/q for substantially uniform and steady-state heat flow,
Examples: Conduction and transmission.
generally
Transmission—“Transmission,” “transmissivity,” “transmit-
R = L/kA for substantially uniform and steady-state heat flow
tance” usually refer to transfer by one or more of the
through a homogeneous medium of thickness L with plane parallel
faces of area A
processes of conduction, convection, and radiation.
Conduction—“Conduction,” “conductivity,” “conductance”
1/R = thermal conductance; reciprocal of thermal resistance (C is also
usually refer to transfer within a medium, and without bodily
used)
1/RA = thermal conductance per unit of area; heat flow, rate, per unit of
displacementasoccurswithconvection,andwithouttransfer
area, per degree
at a distance as occurs with radiation.
RA = thermal resistance of unit area; degrees, per “unit of heat flow rate
per unit of area.” R is used for resistance for a setup with a
HEAT FLOW SYMBOLS
particular area (which may not be known) and RA for resistance of
A = area
unit area. In some British texts R is used for thermal resistance of
p = density; pounds, kilograms, etc., per unit of volume
unit area, here called RA
L = length of path of heat flow
h = surface coefficient of heat transfer; heat flow rate, per unit of area,
Q = total quantity of heat transferred (with subscripts for particular cases
per degree, across a boundary surface
and to distinguish for Q for volume rate)
Q = volume rate; discharge by volume; fluid rate of flow by volume.
dq/dA=hDt general expression giving value at each point
(There also is used q with a subscript to distinguish from heat flow
h = (q/A)/Dt for substantially uniform and steady-state
m m
rate.)
heat flow, where h and Dt are constant over the area A or where
m m
c = specific heat
one (but not both) of them varies but may substantially be
T = temperature on absolute scale
represented by a mean value. h is an average property of a
t = temperature, degrees Celsius or Fahrenheit
particular boundary condition or film and is not neces
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