© 2013 SATRA Technology Centre. Reproduction of SATRA Spotlight in part or in its entirety is not permitted in any shape or form, whether in print or digital media, without prior written permission from SATRA.
Testing of extreme
temperature clothing
by Tom Bayes
cold, its internal mechanisms may be unable
to replenish the heat that is being lost to the
surroundings. The result is not just feeling
cold – a few degrees drop will severely affect
brain function and impair judgement.
Hypothermia
Hypothermia occurs as the body’s defences
attempt to limit heat loss. If the body did
not do this, death would result very quickly.
Hypothermia occurs in three stages:
Stage 1
The core body temperature drops by 1-2°C
below the normal temperature of 37°C.
Mild to strong shivering occurs and the
person becomes unable to perform
complex tasks with their hands, as they
become numb. Blood vessels in the outer
extremities contract and breathing becomes
quick and shallow in order to minimise
heat loss. Goose bumps form, raising body
hair on end in an attempt to create an
insulating layer of air around the body,
which has a limited advantage in humans.
Frequently a person will experience a warm
sensation, as if they are recovering, but
they are in fact heading into Stage 2.
Clothing designed to provide protection against sub-zero
temperatures needs to offer a very high level of insulation.
Although the body is normally able to
regulate both its skin and core temperature
through reactions such as sweating, it soon
loses the ability to keep up with the heat loss
in extremely cold conditions. Heat flows from
the warm surface of the body through the
clothing and out into the cold atmosphere.
Cold weather protection is largely a question
of reducing this outflow of heat to retain
body warmth.
Normal core body temperature for a
healthy human is 37ºC, with skin
temperature around 34ºC. Under low
ambient temperatures, we soon begin to feel
uncomfortably cold although, in fact, core
body temperature will initially remain static.
Dangers of heat loss
Any deviation in core body temperature, even
a degree, is serious and causes significant
problems for the human body. Hypothermia
refers to any condition in which the
temperature of the body drops below the
level required for normal metabolism and/or
bodily function to take place. In humans,
core body temperature is maintained at or
near a constant level through biologic
homeostasis. When the body is exposed to
Stage 2
As body temperature drops by 2-4°C,
shivering becomes more violent and
muscles struggle to coordinate. Movements
become slow and laboured, accompanied
by a stumbling pace and mild confusion,
although the sufferer may appear alert.
Surface blood vessels continue to contract
further as the body focuses its remaining
resources on keeping the vital organs
warm, especially the brain. The person
becomes pale and his/her lips, ears, fingers
and toes may become blue as blood flow
ceases to these parts.
Stage 3
The body temperature drops below
approximately 32°C and this is when
shivering usually stops. Speech becomes
difficult, thinking sluggish with amnesia,
and the individual is unable to use his/her
hands. Cellular metabolic processes shut
down below 30°C and the victim exhibits
© 2013 SATRA Technology Centre. Reproduction of SATRA Spotlight in part or in its entirety is not permitted in any shape or form, whether in print or digital media, without prior written permission from SATRA.
1
2
3
4
Figures 1-4 show how reduced insulation (clothing layers) increases heat loss (heat loss shown in red)
incoherent and irrational behaviour such as
terminal burrowing. This is a bizarre
survival behaviour pattern occurring in the
last stages of hypothermia, during which
the afflicted seeks to enter small, enclosed
spaces such as wardrobes, cupboards, and
closets. Pulse and respiration rates
decrease significantly but fast heart rates
can occur. Major organs fail and clinical
death occurs. However, because of
decreased cellular activity due to the cold,
the body will actually take longer to
undergo brain death.
Heat loss in extreme cold conditions
Air is a very effective insulator and many
garments rely on trapping a layer of still air
to prevent heat loss. Because water is such
a good conductor of heat, it is poor at
insulating. Moisture will build up inside the
garment due to sweating, especially if the
wearer is engaged in physical exertion. This
can double heat loss through clothing
because of the reduced insulation (see
figures 1-4).
Moisture trapped in materials can
double the thermal conductivity of the
material. The moisture may be disposed of
using breathable materials or membranes.
However, this also adds to heat loss. The
moisture will evap orate and in belowfreezing conditions, where the ambient
humidity is close to zero, this evaporation
will be at a very high rate. Whilst good for
keeping dry, large amounts of heat will be
lost due to the evaporation process. Heat
loss through moisture is compounded by a
property commonly known as ‘wind chill’.
Moisture loss from the body in still air tends
to slow down as the air surrounding the
body becomes saturated and forms a
boundary layer. The body relies heavily on
this boundary layer of still air to insulate
against heat and cold. In fact, many
animals have hair or fur to help preserve
this layer. Air movement removes this
saturated layer, replacing it with dry air,
SATRA’s sweating guarded hotplate apparatus can determine insulation and water vapour
properties of a material
© 2013 SATRA Technology Centre. Reproduction of SATRA Spotlight in part or in its entirety is not permitted in any shape or form, whether in print or digital media, without prior written permission from SATRA.
SATRA’s environmental chamber can be altered from -40°C to +40°C
thus increasing evaporation rates. The
higher the wind speed, the greater the
evaporation rate and heat loss become. At
an air temperature of 0°C, a wind speed of
50km/h will make it feel colder than
standing in stationary air at -20°C.
Clothing design
No matter what high performance materials
are used, openings must be provided for
the limbs, neck and head. The garment,
apart from needing good insulation
properties, must trap a layer of air between
itself and the body. This layer must be
maintained for any garment to function
effectively. If this trapped air is allowed to
exchange with the air in the environment,
no amount of insulation will prevent heat
loss. Openings must be carefully designed
with effective closures to preserve the
trapped air and prevent draughts.
When testing garments designed to
protect against cold, it is important to
understand the capabilities of the materials
involved, as well as the whole garment.
Durability is important and tests such as flex
cracking at low temperatures will show the
suitability of the materials. Sophisticated
tests such as the sweating guarded hotplate
should be used to determine both
insulation values and water vapour
permeability of the materials. Air
permeability is also an important property if
the garment is designed to be windproof.
Whole garment testing
SATRA has an environmental chamber in
which temperature can be altered from
-40°C to +50°C and the humidity from 5
per cent to 95 per cent RH (relative
humidity). The room is large enough for
several people to carry out tasks at different
levels of activity. Whole garment testing can
be done in two ways. The first method uses
a manikin pre-heated to a known
temperature, usually 20°C. The manikin is
dressed in the test clothing and
instrumented with thermo-couples so that
the ‘skin’ temperature can be monitored.
The clothed manikin is then placed in the
cold environment and the time taken for
the ‘skin’ to cool to a set temperature is
measured.
The second method involves using
human subjects, monitored both through
the measurement of skin temperature (for
safety) and questionnaires relating to
subject comfort and warmth. This is a very
effective way of evaluating the performance
of cold weather clothing. Subjects can also
carry out a number of different tasks to
simulate physical activity. Strenuous activity
followed by periods of rest can create
conditions that chill the body. The clothing
performance in these conditions can be
measured only through the use of subjects.
Although the manikin doesn’t perspire, it is
a useful technique where test conditions
raise ethical issues regarding the use of
human subjects. We carry out subjectbased
trials on a regular basis down to -30°C.
Colder temperatures require special
monitoring and are considered dangerous.
The subject must have their core
temperature measured and remain under a
close watch. The manikin is especially useful
where testing involves prolonged periods of
extreme cold or where it is simply
impossible to use human subjects, for
example when testing children’s clothing.
How can SATRA help?
Please email [email protected] for
more information on SATRA’s climatic
chamber or the development and/or testing
of cold weather products.