1973 OPEL GT-R Service Manual

98-20 1973 OPEL SERVICE MANUAL
If we were to put a thermometer in the cold drain
water, we would see the temperature gradually creep
upwards. That is to be expected because heat is flow-
ing into the cold water making it warmer. Before
long the water would be as warm as the stored foods.
Then the water could no longer attract heat because
heat will not flow from one warm object to another
equally warm object. Since we no longer can draw
heat out of the foods we no longer are cooling them.
Now, let’s see what happens when we put ice instead
of cold water into the ice-box. This time, we’ll set the
thermometer on top of the ice (Fig. 9B-5). When wefirst look at the thermometer, it reads 32 degrees. A
couple of hours later, we open the ice compartment
door. The ice block is smaller because some of the ice
has already melted away
- but the thermometer still
reads 32 degrees. Again, still later, even more of the
ice has melted, yet the termometer continues to read
32 degrees. So long as any ice remains, no matter
how much of it has melted away, the temperature of
the ice stays right at 32 degrees.
All this time the ice has been soaking up heat, yet it
never gets any warmer no matter how much heat it
draws from the stored food. On the other hand, the
cold drain water got progressively warmer as it
soaked up heat. Why is it the addition of heat will
make water warmer yet won’t raise the temperature
of ice above the 32 degrees mark? If we till one
drinking glass with ice and another with cold water,
and put both glasses in the same room where they
could absorb equal amounts of heat from the room
air, we will find it takes much, much longer for the
ice to melt and reach room temperature than it did
for the water in the other glass to reach the same
temperature. Obviously, most of the heat was being
used to melt the ice. But it was the heat that appar-
ently disappeared or went into hiding because if
couldn’t be located with a thermometer. To best de-
scribe this disappearing heat, scientists turned to
Latin for the right word. They chose the word “la-
tent” which means hidden.
Latent Heat
So latent heat is nothing more nor less than hidden
heat which can’t be found with a thermometer.
What happens to the latent heat? Where does it
disappear to? At first it was thought it was in the
water that melted from the ice. But that wasn’t ex-
actly the right answer because, upon checking water
temperature as it melts from ice, it will be found that
it is only a shade warmer than the ice itself. It is not
nearly warm enough to account for all the heat the
ice had absorbed. The only possible answer is that
the latent heat had been used up to change the ice
from a solid into a liquid.
Many substances can be either a solid, or a liquid, ora gas. It just depends on the temperature whether
water for example was a liquid, or a solid (ice), or gas
(steam) (Fig.
9B-6).Figure 99-6 Temperature Determines State of Water
If we put some water in a tea-kettle, set it over a tire
and watch the thermometer as the water gets hotter
and hotter, the mercury will keep rising until the
water starts to boil. Then the mercury seems to stick
at the 212 degrees mark. If we put more wood on the
fire, despite all the increased heat, the mercury will
not budge above the 212 degree mark (Fig.
9B-7).Figure 98.7 Boiling Water Never Exceeds 2 12
DegreesEven though many housewives won’t believe it, no
matter how large or hot you make the flame, you
can’t make water hotter than 2 12 degrees. As a liquid
changes into a gas, it absorbs abnormally great
amounts of heat without getting any hotter. Here is
another instance where heat disappears.
Now we have two different kinds of latent heat,
which are quite alike. To keep their identities sepa-
rate, the first one is called latent heat of fusion. Since
fusion means the same as melting, it is a good de-
scriptive name. The other kind is called latent heat
of vaporization because‘ that means the same as
evaporation.
It may seem as though we have drifted into a story

REFRIGERANT COMPONENTS ALL MODELSSE- 21
about heat instead of refrigeration. But in doing so,
we have learned how a simple ice-box works. It’s
because the magic of latent heat of fusion gives ice
the ability to soak up quantities of heat without get-
ting any warmer.
Therefore, since it stays cold, it can continue to draw
heat away from stored foods and make them cooler.
The latent heat of vaporization can be an even better
“magnet” because it will soak up even more heat.
Whenever we think of anything boiling, we instinc-
tively think of it being very hot. However, that’s not
true in every case. Just because water
boi1.s at 212
degrees doesn’t mean that all other substances will
boil at the same temperature. Some would have to be
put into a blast furnace to make them bubble and
give off vapor. On the other hand, others will boil
violently while sitting on a block of ice.
And so each substance has its own particular boiling
point temperature. But regardless of whether it is
high or low, they all absorb unusually large quanti-
ties of heat without getting any warmer when they
change from a liquid into a vapor.
Consequently, any liquid that will boil at a tempera-
ture below the freezing point of water, will make ice
cubes and keep vegetables cool in a mechanical re-
frigerator.
Figure
9B-10 Simple R-12 Refrigerator
Refrigerant - 12Refrigerant-12 is used in the air conditioning system
and boils at 21.7 degrees below zero. Maybe that
doesn’t mean very much until we picture a flask of
R-12 sitting at the North Pole boiling away just like
a tea-kettle on a stove. No one would dare pick up
the flask with his bare hands because, even though
boiling, it would be so cold and it would be drawing
heat away from nearby objects so fast that human
flesh would freeze in a very short time. If we were toput a flask of R-12 inside a refrigerator cabinet, it
would boil and draw heat away from everything sur-
rounding it (Fig.
9B-10). So long as any refrigerant
remained in the flask, it would keep on soaking up
heat until the temperature got down to 21.7 degrees
below zero.
Now we can begin to see the similarity between a
boiling tea-kettle and a refrigerator. Ordinarily we
think of the flame pushing heat into the tea-kettle.
Yet, it is just as logical to turn our thinking around
and picture the tea-kettle pulling heat out of the
flame. Both the tea-kettle and the flask of refrigerant
do the same thing they draw in heat to boil
although they do so at different temperature levels.
There also is another similarity between the ice-box
and the mechanical refrigerator. In the ice-box, wa-
ter from melting ice literally carried heat out of the
cabinet. In our simple refrigerator, rising vapors do
the same job.Rdsing
Our R-l 2Water is so cheap that we could afford to throw it
away. But R-12, or any other refrigerant, is too ex-
pensive just to let float away into the atmosphere. If
there was some way to remove the heat from the
vapor and change it back into a liquid, it could be
returned to the flask and used over again (Fig. 9B-
11).There is a way, and that is where we find the biggest
difference between the old ice-box and the modern
refrigerator. We used to put in new ice to replace that
lost by melting. Now we use the same refrigerantover and over again.
Figure 9B-1 1 Re-Using Refrigerant

9B-22 1973 OPEL SERVICE MANUAL
We can change a vapor back into a liquid by chilling
it, or do the same thing with pressure. When we
condense a vapor we will find that the heat removed
just exactly equals the amount of heat that was neces-
sary to make the substance vaporize in the first place.
At last the lost is found! The latent heat of vaporiza-
tion the heat that apparently disappeared when
a liquid boiled into a vapor again reappears on
the scene when that same vapor reverts back into a
liquid. It is just like putting air into a balloon to
expand it and then letting the same amount of air out
again to return the balloon to its original condition.
We know that any substance will condense at the
same temperature at which it boiled. This tempera-
ture point is a clear-cut division like a fence. On one
side, a substance is a liquid. Immediately on the
other side it is a vapor. Whichever way a substance
would go, from hot to cold or cold to hot, it will
change its character the moment it crosses over thefence.But pressure moves the fence! Water will boil at 212
degrees under normal conditions. Naturally, we ex-
pect steam to condense at the same temperature. But
whenever we put pressure on steam, it doesn’t! It will
condense at some temperature higher than 212 de-
grees. The greater the pressure, the higher the boiling
point and the temperature at which a vapor will
condense. This is the reason why pressure cookers
cook food faster, since the pressure on the water
permits it to boil out at a higher temperature. We
know that R-12 boils at 21.7 degrees below zero. A
thermometer will show us that the rising vapors,
even though they have soaked up lots of heat, are
only slightly warmer. But the vapors must be made
warmer than the room air if we expect heat to flow
out of them. Also, the condensing point temperature
must be above that of room air or else the vapors
won’t condense.This is where pressure comes to the rescue. With
pressure, we can compress the vapor, thereby con-
centrating the heat it contains. When we concentrate
heat in a vapor that way, we increase the intensity of
the heat or, in other words, we increase the tempera-ture;because temperature is merely a measurement
of heat intensity. And the most amazing part of it all
is that we’ve made the vapor hotter without actually
adding any additional quantity of heat (Fig.
9B-12).
Use of Pressure in RefrigerationBecause we must live by press&s and gauges in air
conditioning work, the following points are men-
tioned so that we will all be talking about the same
thing when we speak of pressures.
All pressure, regardless of how it is produced, is
measured in pounds per square inch (psi).Figure 98.12 Compressing a Vapor Concentrates its
HeatAtmospheric Pressure is pressure exerted in every
direction by the weight of the atmosphere. At higher
altitudes air is raritied and has less weight. At sea
level atmospheric pressure is 14.7 psi.
Any pressure less than atmospheric is known as a
partial vacuum or commonly called a vacuum. A
perfect vacuum or region of no pressure has never
been mechanically produced. Gauge pressure is used
in refrigeration work. Gauges are calibrated in
pounds (psi) of pressure and inches of Mercury for
vacuum. At sea level
“0” lbs. gauge pressure is
equivalent to 14.7 lbs. atmospheric pressure. Pres-
sure greater than atmospheric is measured in pounds
(psi) and pressure below atmospheric is measured in
inches of vacuum. The “0” on the gauge will always
correspond to the surrounding atmospheric pressure,
regardless of the elevation where the gauge is being
used.
Basic Refrigerator OperationWe’ve now covered all the ground-rules that apply to
refrigeration. Most likely they still are a little hazy,
but it is easy enough to remember these main points.
All liquids soak up lots of heat without getting any
warmer when they boil into a vapor, and, we can use
pressure to make the vapor condense back into a
liquid so it can be used over again. With just that
amount of knowledge, here is how we can build a
refrigerator.
We can place a flask of refrigerant in an ice-box. We
know it will boil at a very cold temperature and will
draw heat away from everything inside the cabinet
(Fig. 9B-13).
We can pipe the rising vapors outside the cabinet and
thus provide a way for carrying the heat out. Once

REFRIGERANT COMPONENTS ALL MODELS96.23Figure 96-l 3 Basic Refrigerant Circuit
we get the heat-laden vapor outside, we can com-
press it with a pump. With enough pressure, we can
squeeze the heat out of “cold” vapor even in a warm
room. An ordinary.radiator will help us get rid of
heat.
By removing the heat, and making the refrigerant
into a liquid, it becomes the same as it was before, So,
we can run another pipe back into the cabinet and
return the refrigerant to the flask to be used over
again.
That is the way most mechanical refrigerators work
today. Now, let’s look at an air conditioning unit to
see how closely it resembles the refrigerator we have
just described.
Basic Air ConditionerWhen we look at an air conditioning unit, we will
always find a set of coils or a tinned radiator core
through which the air to be cooled passes. This is
known as the “evaporator” (Fig.
9B-14). It does the
same job as the flask of refrigerant we
spok.e about
earlier. The refrigerant boils in the evaporator. In
boiling, of course, the refrigerant absorbs heat and
changes into a vapor. By piping this vapor outside
the car we can bodily carry out the heat that caused
its creation.
Once we get vapor out of the evaporator, all we haveFigure 98.14 Evaporator Assembly
to do is remove the heat it contains. Since heat is the
only thing that expanded the refrigerant from a liq-
uid to a vapor in the first place, removal of that same
heat will let the vapor condense into a liquid again.
Then we can return the liquid refrigerant to the
evaporator to be used over again.
Actually, the vapor coming out of the evaporator is
very cold. We know the liquid refrigerant boils at
temperatures considerably below freezing and that
the vapors arising from it are only a shade warmer
even though they do contain quantities of heat.
Consequently, we can’t expect to remove heat from
sub- freezing vapors by “cooling” them in air tem-
peratures that usually range between 60 and 100
degrees heat refuses to
flow from a cold object
toward a warmer object.
But with a pump, we can squeeze the heat-laden
vapor into a smaller space. And, when we compress
the vapor, we also concentrate the heat it contains.
In this way, we can make the vapor hotter without
adding any heat. Then we can cool it in compara-
tively warm air.
That is the only responsibility of a compressor in an
air conditioning system (Fig.
9B-15). It is not in-
tended to be a pump just for circulating the refriger-
ant. Rather, its job is to exert pressure for two
reasons. Pressure makes the vapor hot enough to
cool off in warm air. At the same time, the compres-
sor raises the refrigerant’s pressure above the con-
densing point at the temperature of the surrounding
air so it will condense.
As the refrigerant leaves the compressor, it is still a
vapor although it is now quite hot and ready to give
up the heat that is absorbed in the evaporator. One
of the easiest ways to help refrigerant vapor dis-
charge its heat is to send it through a radiator- like
contrivance known as a condenser (Fig. 9B-16).
The condenser really is a very simple device having
no moving parts. It does exactly the same job as the
radiator in a typical steam-heating system. There,
the steam is nothing more than water vapor. In pass-
ing through the radiator, the steam gives up its heat
and condenses back into water.
The same action takes place in an air conditioning

9B-24 1973 OPEL SERVICE MANUAL
Figure 9B-15 Compressor Assembly - GT Shown
Figure 3B-16 Condenser Assembly
condenser. The refrigerant vapor gives up its heat,
which is quickly and easily radiated into the sur-
rounding air through the large finned surfaces of the
condenser. In giving up its heat, the refrigerant vapor
condenses back into liquid which collects in a pool
at the bottom of the condenser.
As we have said before, when the refrigerant con-
denses into a liquid, it again is ready for boiling in the
evaporator. So, we can run a pipe from the condenser
back to the evaporator.
Main Units of the SystemThese three units then; the evaporator, the compres-
sor, and the condenser are the main working
parts of any typical air conditioning system. We have
the evaporator where the refrigerant boils andchanges into a vapor, absorbing heat as it does so. We
have the pump or compressor to put pressure on the
refrigerant so it can get rid of its heat. And we have
a condenser outside the car body to help discharge
the heat into the surrounding air.
Pressure and FlowThere is one more unit that co-operates with thesethree. It doesn’t do any real work, but it does act as
sort of a traffic officer in controlling the flow of the
refrigerant through the system. To get a better idea
of what this does. let’s first do a li,ttle exoerimentine
with an ordinary’ tire pump.
When we use a
t,ire pump to Sate an automobile
tire, we are creating pressure only because we are
“pushing” against the air already entrapped inside
the tire. If you question this, just try pumping up a
tire that has a large puncture in it. You could pump
all day, and still not be able to build up any pressure.
As fast as you would pump the air in, it would leak
out through the puncture.
Abou~t all you would be
doing would be circulating nice fresh air through the
tire.
1Jnless you have something lo push against - to
block the tlow of air
- you can’t create more than a
mere semblance of pressure.
The same situation holds true in an air conditioning
system. The compressor can pump refrigerant vapor
through the system, but unless it has something to
push against, it cannot build up pressure. All the
compressor would be doing would be to circulate the
vapor without increasing its
pres,sure.Yet we can’t just block the flow through the system
entirely. All we want to do is put pressure on the
refrigerant vapor so it will condense at normal tem-
peratures. What’s more, this
musi: be done some time
after the vapor leaves the evaporator and before it
returns again as a liquid. We can’t have high pressure
in the evaporator because that would slow down the
boiling of the refrigerant and thus penalize the re-
frigerating effect.
Controlling Pressure and FlowPressure and flow can be controlled with a float
valve, or with a pressure-regulating valve. They do
the same job, but in a different way.
Since the float valve type will give us a better idea of
pressure and flow control, let’s look at it first (Fig.
9B-17).It consists simply of a float that rides on the surface
of the liquid refrigerant. As the refrigerant liquid
boils and passes off as a vapor, naturally the liquid
level drops lower and lower. Correspondingly, the
float, because it rides on the surface of the refriger-
ant, also drops lower and lower as the liquid goes
down.By means of a simple system of mechanical linkage,
the downward movement of the float opens a valve
to let refrigerant in. The incoming liquid raises the
fluid level and, of course, the float rides up with it.
When the surface level of the refrigerant liquid re-
aches a desired height, the float: will have risen far

REFRIGERANT COMPONENTS ALL MODELS9B- 2596.15
Figure 95.17 Float Type Flow Valve
enough to close the valve and stop the flow of refrig-
erant liquid.
For the sake of simplicity, we have described the
float and valve action as being in a sort of definite
wide open or tight shut condition. Actually, though,
the liquid level falls rather slowly as the refrigerant
boils away. Likewise, the float goes down gradually
and gradually opens the valve just a crack. New
refrigerant liquid barely seeps in through the
“cracked” valve. At such a slow rate of flow, it raises
the liquid level in the evaporator very slowly.
With that in mind, it is easy to see how it would be
possible for a stabilized condition to exist. By that,
we mean a condition wherein the valve would be/
DIAPHRAGMACTUATINGBACK.UP PLATE
PINS \
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>IAPHRAGM \
/
BoDyEQUALIZER\4]
PASSAGE
‘!!!ISEATSCkEEN:ARRIAGEORIFICE
AGE SPRINGIER ELEMENT:MOB”LBSPRING SEAT
OUTLET
W-16opened barely enough to allow just exactly the right
amount of refrigerant liquid to enter the freezer to
take the place of that leaving as a vapor.
Thermostatic Expansion ValveAutomotive air conditioning systems use a thermo-
static expansion valve in place of the float system.
Figure 9B-18 shows a cross-section of the valve
which consists primarily of the gas-filled power ele-
ment, body, actuating pins, seat and orifice. At the
high pressure liquid inlet is a tine mesh screen which
prevents dirt, tilings or other foreign matter from
entering the valve orifice.
When the valve is connected in the system, the high
pressure liquid refrigerant enters the valve through
the screen from the receiver-dehydrator (which acts
as a storage tank for the condensed refrigerant as it
leaves the condenser) and passes on to the seat and
orifice. Upon passing through the orifice the high
pressure liquid becomes low pressure liquid. The low
pressure liquid leaves the valve and flows into the
evaporator core where it absorbs heat from the
evaporator core and changes to a low pressure vapor,
and leaves the evaporator core as such. The power
element bulb is clamped to the low pressure vapor
line just beyond the outlet of the evaporator (Fig.
9B-20).The operation of the valve is quite simple. It is a
matter of controlling opposing forces produced by a
spring and the refrigerant pressures. For example:
The pressure in the power element is trying to push
the seat away from the orifice, while the spring is
trying to force the seat toward the orifice. These
opposing pressures are established in the design of
the valve so that during idle periods, i.e. when the
system is not operating, the spring force and the
refrigerant pressure in the cooling coil are always
Figure 9B-18 Thermostatic Expansion Valve
Figure
98.20 Expansion Valve Bulb Location

98-26 1973 OPEL SERVICE MANUAL
greater than the opposing pressure in the power ele-
ment. Therefore, the valve remains closed. When the
compressor is started, it will reduce the pressure and
temperature of the refrigerant in the cooling coil to
a point where the vapor pressure in the power ele-
ment becomes the stronger. The seat then moves off
the orifice and liquid starts to flow through the valve
orifice into the cooling coil.
The purpose of the power element is to help deter-
mine the quantity of liquid that is being metered into
the cooling coil. As the temperature of the low pres-
sure line changes at the bulb, the pressure of
the
vapor in the power element changes, resulting in a
change of the position of the seat. For example, if the
cooling coil gets more liquid than is required, the
temperature of the low pressure line is reduced and
the resultant lowering of the bulb temperature
reduces the pressure of the vapor in the power ele-
ment, allowing the seat to move closer to the orifice.
This immediately reduces the amount of liquid leav-
ing the valve. Under normal operation, the power
element provides accurate control of the quantity of
refrigerant to the cooling coil.
To employ our tire pump analogy once more for
clarity, it is the same situation that would exist if you were inflating a tire with a very slow leak. Providing
you pumped the air into the tire as fast as it leaked
out, you would be able to maintain pressure even
though the air would merely be circulating through the tire and leaking out through the puncture.
To Sum Up
So far, we’ve discussed only what each unit in an air
conditioning system does. We’ve learned that the
evaporator is the unit in which liquid refrigerant
soaks up heat from the air, the compressor is a pump
for squeezing this heat out of the vapor, the con-
denser is a radiator for getting rid of the heat, and the
thermostatic expansion valve is a device for regulat-
ing the pressure on the refrigerant. Now, let’s
find
out how the temperature of the cooled air is con-
trolled.
METHOD OF TEMPERATURE CONTROL
To achieve temperature control, the compressor is
run intermittently, automatically turning on and off
as necessary to maintain proper temperature.
Thermostatic Switch
The compressor can be started and stopped au-
tomatically through the use of an electro-magnetic
clutch and a thermostat affected by variations of temperature.
The job is usually done by a gas bulb thermostat (Fig.
9B-21).
Figure 9B-21 Thermostatic Switch Schematic
With the gas bulb type of thermostat, a highly expan-
sive gas is sealed into a metallic bulb which is located
in the air stream as it leaves the evaporator. A small
tube leads from the bulb to a bellows operated switch. As air temperature rises, the gas inside the
bulb expands, travels through the tube to the bellows
and closes the electrical switch that engages the com-
pressor clutch.
Of course, as soon as the compressor starts running,
the temperature begins to go down. As the air being
cooled gets colder, the gas in the thermostat bulb
begins to reduce the pressure on the switch bellows.
This
Ilips “off’ the switch and disengages the com-
pressor clutch.
REFRIGERANTS
No matter how scientifically refrigerating machinery
is built or how
efftciently it runs, it alone cannot
remove heat. The only thing that carries heat out of
a refrigerator cabinet or an automobile is the sub-
stance we call the refrigerant.
There are many refrigerants known to man. In fact,
any liquid that can boil at temperatures somewhere
near the freezing point of water can be used.
But a boiling point below the temperature at which
ice forms is not the only thing that makes a good
refrigerant. A refrigerant should also be non-
poiso-
nowand non-explosive to be safe. Besides that, we
want a refrigerant that is non-corrosive and one that
will mix with oil.
Since Nature did not provide an ideal refrigerant,
chemists went to work to see if they could do any
better. They did! But it wasn’t as simple as that.
At first, they tried to improve existing natural refrig-
erants. But after exploring innumerable trails along

REFRIGERANT COMPONENTS ALL MODELS99.27that line, they still hadn’t gotten anywhere. So, they
started from scratch and juggled molecules around
to make an entirely new refrigerant. Eventually they
succeeded by remodeling the molecules in carbon
tetrachloride. This is the same fluid that is used in
fire extinguishers and dry-cleaners’ solvents.
From this fluid, the chemists removed two chlorine
atoms and replaced them with two fluorine atoms.
This newly-formed fluid carried the technical chemi-
cal name of dichlorodifluoromethane. Today, we
know it as Refrigerant-12 or R-12.
Fluorine is an extremely temperamental substance.
Under most conditions it is toxic and highly corro-
sive, and after is is manufactured, it has to be stored
in special containers because it will eat through glass
and will dissolve most metals in short order.
Despite its rambunctious character though, fluorine
is completely tamed when it is combined with the
other substances that go to make up the refrigerant.
Each is non-toxic, non-inflammable, non-explosive,
and non- poisonous; however, breathing large quan-
tities of R-12 should be avoided.
Pressure. Temperature Relationship of R-12A definite pressure and temperature relationship ex-
ists in the case of liquid refrigerants and their satu-
rated vapors. Increasing the temperature of a
substance causes it to expand. When the substance is
confined in a closed container, the increase in tem-
perature will be accompanied by an increase in pres-
sure, even though no mechanical device was used.
For every temperature, there will be a corresponding
pressure within the container of refrigerant. A table
of the temperature-pressure relationship of R-12 is
presented below. Pressures are indicated in gauge
pressure, either positive pressure (above atmos-
pheric) m pounds or negative pressure (below atmos-
pheric) in inches of vacuum.
“F-40
-35
i#Pressure
11.0*
8.3*
“F
50
50#Pressure
46.1
52.0
-30~
5.5*6057.7
-252.3*6s67 7__.
-200.6
io70.1
-152.4
76.9
-104.584.1
1;6.8 9.2tz99.6 91.71;
11.8 14.712116.9 108.1
1517.7105126.2
2021.1110136.0
2524.6115146.5
3028.5120157.1
;:
30.1
125167.5
32.6
131)179n
4037.0
4541.7*Inches of Vacuum.-. _.-
1402045
150232.0Thus if a gauge is attached to a container of R- 12 and
the room temperature is 70 degrees, the gauge will
register 70 psi pressure; in a 100 degrees room the
pressure will be 117
ps~
AIR CONDITIONINGBecause air conditioning has always been very
closely allied with mechanical refrigeration, most of
us are apt to think of it only as a process for cooling
room air.
But true air conditioning goes beyond the mere cool-
ing of the air. It controls the humidity, cleanliness,
and circulation of the air as well.
Whenever it gets warm and muggy in the summer-
time, someone is almost sure to say, “It’s not the heat
it’s the humidity.” But that is only partly right.
Actually it is a combination of the two that makes us
feel so warm temperature alone is not the only
thing that makes us uncomfortable.
Humidity is nothing more nor less that the moisture
content of the air. To a certain extent, it is tied in
with the temperature of the air. Warm air will hold
more moisture than will cold air. When air contains
all the moisture it can hold, we say it is saturated,
and the relative humidity is 100 percent. If the air
contains only half as much water as it could possibly
hold at any given temperature, we say that the rela-
tive humidity is 50 percent. If it contains only a fifth
of its maximum capacity, we say that the relative
humidity is 20 percent and so on. This amount
of water vapor, or relative humidity, affects the way
we perspire on hot days.
Nature has equipped our bodies with a network of
sweat glands that carry perspiration to the skin
sur-faces. Normally, this perspiration evaporates and, in
doing so, absorbs heat just like a refrigerant absorbs
heat when it is vaporized in a freezer. Most of the
heat thus absorbed is drawn from our bodies, giving
us a sensation of coolness. A drop of alcohol on the
back of your hand will demonstrate this principle
very convincingly. Because it is highly volatile, al-
cohol will evaporate very rapidly and absorb quite a
bit of heat in doing so, thereby making the spot on
your hand feel unusually cool.
The ease and rapidity with which evaporation takes
place, whether it be alcohol or perspiration, governs
our sensation of coolness and to a certain extent,
independently of the temperature. Of even more im-
portance, the ease and rapidity of the evaporation are
directly affected by the relative humidity or com-
parative dampness of the air. When the air is dry,
perspiration will evaporate quite readily. But when
the air contains a lot of moisture, perspiration will
evaporate more slowly; consequently less heat is car-
ried away from our body.