Testing the limit of heat packs in freezing temps

I conducted a heat pack test trying to push the limits to the extremes to see what the temp in the box would be.

I should have took photos (sorry) but I will try to explain the best I can what I did:

I had a 8x8x6 shipping box insulated on all sides with 3/4 inch Styrofoam . The box had one very small air hole punched in only one side.

Inside the box I have a plastic pre-punched deli cup. I put a remote themometer inside the deli cup and snapped the lid on.

I unwrapped and started TWO 40 hour heat packs and let them warm up in a towel for about 45 min. I then took the warming packes and wrapped them in newspaper and placed them on eaither side of the deli cup...as if I where packing a real chameleon.

Newspaper was packed in all the loose areas securing the deli cup in the middle.

I sealed up the box and let it sit inside for a two hours to simulate it sitting inside at the UPS center. Them moved outside where it was slightly cooler.

I also had a thermometer to measure outside temp.

Heat packs where started about 10:00 and put in the box at about 10:55


11:00pm / 76 F / 78 F

3:00AM / 70 F / 88.5 F (4 hours in the box)

Moved box at 3am (yes I got up at 3 am in the name of science) to the refrigerator set on the highest settings (45 F) After 4 hours at 45 degrees I turned the temp setting of the fridge down to 32 degrees

7:00 AM / 45 F / 68 F (8 hours in the box)

8:00 AM / 41 F / 67 F (9hrs)

9:00 AM / 39 F / 64 F (10hrs)

Now I decide to push the results more and put the box in the freezer The freezer is on its highest setting which is 8 degrees. My outside thermometer started to giveme error readings that looked like "L L L" ( I will just assume that means 'freakin cold')

10:00 AM / ERROR / 51 F (11hr)

10:30 AM / ERROR / 47.5 F (11.5hr)

11:00 AM / 14 F / 42 F (12hr)

12:00 PM / 19 F / 43 F (13hr)

1:00 PM / 15 F / 40 F (14hr)

1:30 PM / 15 F / 38.7 F (14.5 hr)

I ended my experiment there but placed the box back in the fridge just to see how much longer the heat packs would heat the box. I went out to run errands and returned at 5:00 pm

5:00 PM / 39 F / 50.3 F (18 hrs)

I unpacked the box at 5:30pm after 18 hours inside the box. The heatpacks seemed warm to the touch but not hot. the newspaper not in direct contact with the packs where cold.

This was the only heat pack experiment I have done and I used Two 40 hour heat packs. I found the results very interesting. It seemed to do very well when the outside temp is 40 degrees.

Also I was surprised that the temp inside the box never dipped to freezing (32 degrees) even when the outside temp was 14 degrees.

I also now know that my settings on my fridge and freezer are not accurate. Setting the temp on the freezer to 8 degrees yeiled a inside temp of 14 degrees and also the fridge set on 32 never got lower than 39.._​


Have you considered testing the heat packs less the hole on the side of the box? I understand it's intent and purpose but I would side against do so when shipping animals in extremely cold weather.

The hole was very tiny and was made with a bamboo skewer (like what you use to make kabobs)

I put the hole in because I have read debates that heat packs need air flow to properly heat up. I suppose I could test to see if a tiny hole really makes a difference or not .

I would also like to test the differences between a 40 hr heat pack and a 72 hour heat pack. also one heat pack vs 2 as I used here.
another question and/or point–

I dunno when exactly you changed the fridge and freezer temps during the course of the experiment, but most fridges will tell you that it takes about 24 hours to properly stabilize to a new temperature setting. I'm not sure if you pre-chilled them, or turned the temps down as you put in the box.

if you do repeat any variations of the experiment, I would definitely suggest the pre-chill if it wasn't done this time, and also have an accurate thermometer in the compartments so you really know what temps the box is being subjected to.

cool test, though!
Well that explains why the freezer settings did not match the actual temps I was reading inside.

But it really did not matter since I did have a thermometer on the outside of the box (in the firgde and freezer) as well as inside the box and did not rely on the settings

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