|The Sodium Acetate Heat Packs contain a
saturated solution of Sodium Acetate as well as one or more small stainless
steel concave discs.|
Almost any liquid is reluctant to freeze when it reaches
its freezing point. It is only after the first crystal forms, in the freezing
process, that the balance follows suit quite rapidly. During this freezing
process the liquid will stay at the temperature that it freezes at, until the
very last crystal has solidified, and only then will the frozen substance
conform to the ambient temperature of its surroundings. For example, while water
is freezing it will remain at 32 degrees Fahrenheit (0 degrees Celsius) until
completely frozen and will only then cool to the temperature of the surrounding
atmosphere. How rapidly it conforms to the surrounding, ambient, temperature
depends on how well it is insulated. If insulated well it will be slow but if
not insulated at all the process will happen more rapidly.
It is possible
to "super cool" water and keep it in its liquid form well below its
freezing temperature, but this is difficult as the water must be totally pure
(distilled) and the container must be completely smooth to ensure that there are
no blemishes to give the first ice crystal an opportunity to form. Even then it
is very hard to achieve as even the slightest tremor will set the freezing
process in motion (like the vibration of the compressor in a domestic
Regardless of how cold we managed to get our
container of water when it freezes it will return to 32 degrees Fahrenheit until
completely frozen. This sudden gain in temperature results in it heating its
immediate environment by shedding the unwanted energy in the form of heat.
acetate freezes at 130 degrees Fahrenheit (54 degrees Celsius).
acetate forms a very stable liquid making it easy to cool it way below its
freezing point while remaining in its liquid form. In its liquid form it can
tolerate very low relative temperatures before it freezes spontaneously and is
also reasonably immune to movement, which is why it is the ideal substance for
use in reuseable heat packs. Sodium acetate reusable heat packs can safely be
stored in liquid form at room temperature for prolonged periods.
the small stainless steel disc in the heat pack causes one or more crystals to
form in its immediate proximity, which is then followed rapidly by all of the
contents of the heat pack crystallising. The heat pack's temperature will
immediately rise to 130 degrees F (approximately the temperature produced by
domestic hot water systems) and will remain at 130 degrees F until the entire
contents of the sodium acetate heat pack has crystallised. Depending on the size
of the heat pack and the insulation of the reusable heat pack it will then
gradually lose warmth until it reaches the ambient temperature (between half an
hour to two hours).
Raising the temperature of the rechargeable heat pack
to above 130 degrees Fahrenheit will "melt" the sodium acetate and
return it to liquid (every single crystal must be dissolved otherwise it will
freeze upon cooling) which can then be allowed to cool without solidifying
making it available for use later.