"Ray B" <[EMAIL PROTECTED]> spaketh thusly:
>We have used the 60-hour packs from http://rldtheheatsource.com/ for several
>years, and are quite pleased with them, although trusting any heat pack for
>the full 60 hours is foolish. Personally, I try to ship stuff so it is
>delivered within 48 hours, just to be safe.
I, too, use the RLD heat packs. They work quite well.
Some years ago, I placed one in a box with plastic foam
peanuts, put in the temperature sensor to one of those $20
"indoor/outdoor" digital thermometers, and put the box in a
refrigerator at 40 F. The probe was a few inches above the heat pack,
along with a temperature datalogger. I could view the temperature
inside the box, and know when the experiment was over as the
temperature approached the ambient temperature of the refrigerator.
The datalogger showed that the temperature dropped slightly
over time, and plummeted at the 58 or 59 hour mark. I was surprised
at how well the temperature was maintained for so long.
>I don't understand how they could stop functioning below freezing, and
>suspect it is a misinterpretation of observation. These heat packs
>typically work by a simple, exothermic, chemical reaction - the oxidation of
>iron powder upon exposure to moisture (humidity) and oxygen - which begins
>once you open the air-tight bag. That process is not overly
>temperature-dependent, especially if you've started it at room temperature,
>so are already warm.
The reaction in these widgets is moderated by available
moisture (amongst other things); if it managed to get so cold as to
freeze the available moisture, the reaction would stop. But, of
course, if the temperature of the heat pack was below freezing (and
it would have to be quite far below that, as salt is also used in
these heat packs, further depressing the freezing point of the
moisture), the plants would be gone already anyway.
>Instead, I'd bet the chemical reaction is slowed by either a lack of
>moisture or a lack of oxygen. The simple fact is that folks typically seal
>their shipping boxes up tight to contain the heat, not realizing the heat
>pack will consume the oxygen faster than it can be replaced, so the process
>slows and little- to no heat is released. If the box is not totally sealed
>(I often poke holes in it), it usually woks fine. In the middle of the dead
>of the dry winter though, I'll sometimes stick a sopping paper towel inside
>of a perforated baggie in the box to ensure a source of humidity.
I usually tape the heat pack on the bottom seam of the box
to allow some fresh source of oxygen. Although I tape across the
bottom of the box, I do not tape all the way around the bottom so it
remains "drafty."
Only lost one winter shipment with these, and there was a
snap blizzard and an unheated post office involved.
-AJHicks
Chandler, AZ
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