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Fundamental Concepts of Environmental Testing Techniques in Electricity and Electronics
Part 1: Fundamental concepts of physics and chemistry regarding heat and humidity |
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Toshio Yamamoto*
This article is the first in a four-part series on basic issues concerning reliability quality control in electricity and electronics. The issues to be covered deal with "temperature and humidity testing", and are primarily directed toward beginning technicians who are participating in environmental testing.
The series is organized as follows.
Part 1: Fundamental concepts of physics and chemistry regarding heat and humidity
Part 2: Concepts concerning the behavior of standard environments and moisture required for designing temperature and humidity tests
Part 3: Concepts concerning the establishment of public test standards
Part 4: Concepts serving as reference for effectively applying environmental testing The aim of this series is to present the fundamental
concepts required for performing
temperature and humidity testing, the most basic testing in the various kinds of test methods. In addition, we sincerely hope that these fundamental concepts will lay the foundation for understanding current environmental tests, designing the next generation of environmental tests, and developing new test methods. |
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Fifteen
years ago in Japan, the electrical and electronics
industry used "environment" to refer to natural
environments. Nowadays, though, the term is
used to include other meanings.
The rapid development of industrial
technology has widely popularized a great
variety of industrial products which have
provided enormous convenience in our lives.
However, this situation is also leading to
immense changes in the natural environment
on a global scale. It is well-known that these
changes are giving rise to a variety of current
social problems.
Modern industrial products have
increasingly numerous opportunities to be
directly and strongly influenced by environmental
changes, including naturally occurring periodic
weather changes such as the changing seasons,
and the environment of the geographical location,
but even more by artificial environmental
changes brought about by environmental changes
in the rooms in which the products are placed,
as well as by nonperiodic operation of other
products located close by. In other words,
the influence of the artificial interior environment
has become greater than the influence from
the global natural environment. (Our own bodies
are also receiving the effects of this same
environmental influence on a daily basis.)
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| 2. Physical properties of temperature
and humidity |
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Temperature
refers to thermal energy, and we can only
grasp the level of thermal energy through
some sort of matter. In other words, thermal
energy is not matter. Because of this, we
use appropriate instruments (thermometers
and calorimeters) to index the order of coolness
and heat and its increase and decrease-in
other words the amount of thermal energy and
the level of change in that energy.
On the other hand, humidity is
a combination of water and dry air, and the
water can easily change into a variety of
forms within our life temperature zone. Occasional
high and low temperatures cause the humidity
values to fluctuate widely.
We know from experience that when
a hot substance comes into direct contact
with a cold substance, the hot one cools and
the cold one warms. This change does not continue
in perpetuity, but is rather one that reaches
equilibrium after time, and the change stops.
At this time the two substances involved are
said to have reached thermal equilibrium.
We should also note that as the temperature
of a substance increases, molecular motion
correspondingly increases in the molecules
making up the substance. However, as the temperature
drops, molecular motion also slows. It is
well known that at extremely low temperatures,
in the vicinity of -273°C ,
molecular motion in most molecules stops.
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2-1 Molecular motion of
gases
Fig. 1 illustrates the velocity
of one oxygen molecule in a vacuum. As the
temperature rises, the velocity increases.
In other words, the higher the temperature
of a gas, the greater the kinetic energy that
it has (the thermal motion of the molecule
increases).
Next, let's observe the molecular
motion of groups of molecules. |
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| Fig. 1 Velocity of
one molecule of oxygen |
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2-1-1 With groups of identical
molecules
Even when a number of identical
molecules form a gas, the velocity of all
the molecules making up the gas is not equal.
Each of the molecules conforms to the uniform
velocity distribution (1) determined by the
temperature. As Fig. 2 clearly shows, the
higher the temperature, the more the distribution
is inclined toward an area of greater velocity.
For example, the velocity distribution of
N2 molecules in nitrogen
gas is as shown in Fig. 2. When most of the
molecules are temporarily at the average velocity
v for that temperature, the value of v for
the N2 molecules changes
to 425 m/s at 0°C ,
910 m/s at 1,000°C ,
and 1,220 m/s at 2,000°C .
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| Fig. 2 Temperature
and the distribution of molecular velocity |
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2-1-2 With groups of different
types of molecules
Even at the same temperature,
the velocity distribution of groups of molecules
with lower molecular weight have a distribution
more inclined toward an area of greater velocity
than groups of molecules with greater molecular
weight. For example, when comparing hydrogen
molecules (molecular weight, 2) with nitrogen
molecules (molecular weight, 28) as in Fig.
2, at the same temperature of 0°C ,
the velocity v of the H2
molecules is 1,680 m/s, but the velocity v
of the N2 molecules
is 425 m/s.
The higher the temperature, the
greater the average molecular velocity v.
This value of v is known to be in proportion
to the square root of absolute temperature
T. The average velocity can be found with
the following formula, in which M is the molecular
weight.
 (Where
R is the gas constant.)
This type of temperature change
creates thermal stress by changing the velocity
distribution of the molecules composing the
substance that is in thermal equilibrium,
i.e., stable. Therefore, changing the temperature
of a substance causes repeated changes in
velocity at the molecular level, so the substance
is not static even though its macro stability
can be observed. From the micro standpoint,
a substance is normally in an unstable condition.
The greater the change in temperature, the
greater the loss of stability. To put it the
other way around, the smaller the change in
temperature, the greater the relative stability.
Next, let's consider humidity.
When dealing with the subject of moisture
content, the prerequisite for humidity, we
must bear in mind the characteristics of water.
Water is at its greatest density at 4°C
(1 atm, 3.98°C), and undergoes phase changes
according to the temperature. In other words,
depending on the temperature, water will exist
as water vapor (gaseous phase), water (liquid
phase), or ice (solid phase). Within our life
temperature zone we don't often see liquids
freeze (with the exception perhaps of only
glacial acetic acid (2) and benzol (3) ).
By the way, when discussing phase change,
we must not forget the existence of intermolecular
force.
First of all, when a gas is cooled or compressed,
it becomes a liquid (in rare instances changing
directly to a solid). For example, when air
is compressed and cooled, it becomes liquid
air, and when carbon dioxide is cooled, it
becomes a snow-like solid (dry ice). These
phenomena indicate that the molecules attract
each other. This force is called intermolecular
force, or Van der Waals force (Van der Waals,
1837 to 1923). This intermolecular force is
created only when molecules are contiguous,
and is rarely created when the intermolecular
distance is great. Therefore, in solids and
liquids, intermolecular force results in molecules
being held almost immobile or merely able
to flow. When the substance becomes a gas,
the molecule can fly about almost freely (although
within the range of mean free path (4) ).
For example, butane (C4H10)
boils at 10°C ,
and at room temperature is a gas, but is a
liquid in a gas lighter. This is because the
butane is compressed and put into the lighter,
forcibly reducing the intermolecular distance
between the butane molecules. This compression
creates intermolecular force, forming a liquid
and making it possible to insert 200 times
as much butane into the lighter.
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| Fig. 3 1 mol
of butane (liquid and gas) |
Fig. 4 Intermolecular
distance |
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Intermolecular
force is strongly affected by intermolecular
distance, and is inversely proportional to
the intermolecular distance raised to the
power of 7. For example, if the intermolecular
distance is doubled, the intermolecular force
becomes 1/27
, i.e., 1/128. Intermolecular distance is
a weaker force than chemical bonds (ionic
bonds (5) , covalent bonds (6) , and metallic
bonds (7) ), but it is incomparably stronger
than universal gravity. However, coming too
close together creates repulsion force, so
molecules can't approach closer than a certain
uniform distance. The atomic radius determined
from this distance is called the Van der Waals
radius,which is considerably larger than the
covalent bond radius.
We don't have room to go more
into detail on this subject here, so if you
would like to go into more detail on any of
these items, please refer to other sources.
By the way, water undergoes a
change in volume when changing from a liquid
to a gaseous phase or to a solid phase, and
because of that exerts an extremely strong
mechanical force on its surroundings. Also,
existing at the boundaries of the various
changes are special characteristics such as
the appropriate gain and loss of kinetic energy
as latent heat (8) in cases in which there
is no temperature change. Another current
problem is that even when the amount of moisture
is insufficient to cause problems as independent
water, when combined with other major factors
it can form a complex environment with properties
that severely affect electrical and electronic
products. |
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