Heat Pack Study

[BeakerBob]

As the temperature outside gets colder, there are a number of threads being posted on heat packs with questions. I did this study several years ago but can't find it in the R2R search function, so I may not have posted it on this site, only locally.

Since this information is still relevant, it may be helpful to review and open it to discussion.

HEAT PACK STUDY: How efficient are heat packs when used to ship insulated boxes to maintain a warm temperature during the cold winter months in Michigan? Which heat packs should be used? What is the temperature of the water when received?

These are questions that have been on my mind when I ship coral frags during the winter months, so I designed a study to see how heat packs impact the shipped water temperature and the effectiveness of the insulated box. To design the study, many variables had to be controlled to make the proper comparisons meaningful, i.e. apples to apples. I used the same size and density Styrofoam containers, a uniform volume of water at a specific temperature and stored them in a controlled cold room at 4C +/- 1C (40F +/- 1F) degree. NIST calibrated temperature data loggers were sealed in plastic bags to keep them dry, then placed into the sealed water bag and recorded the water temperatures every 5 minutes for 48 hours.

Thank You
A special acknowledgement goes to Debbie Berlin (diyreef), Toddhttp://www.diyreef.com)/Todd Cherry (Cherry Corals) and Reefone for donating some different heat packs for testing and also for Jimsflies for helping me upload some of the data.

The Heat Pack Study follows below and in subsequent posts, however, I was having difficulty with the pdf files, so I have to convert the Excel charts to jpg format:

TEST CONTROLS:
First, every test must have some controls designed into the study for confidence in the results. Precision, accuracy, and blank samples are often used. For the purpose of precision, I started the study testing heat packs in triplicate to see if they were uniform in heating the water. After several tests on the Little Hotties, I concluded that testing in triplicate was no longer necessary as they were reproducible (precision). Accuracy is a little more difficult to show in this type of study, so the NIST calibration will have to suffice. In order to compare the results in a meaningful fashion, I prepared two blank controls, , blank meaning no heat packs. The first blank control was 27C bagged water with no Styrofoam box. The second blank was 27C bagged water packaged the same as the other tested boxes, but without any heat packs. These two blanks serve as control comparisons.

PACKAGING:
All packaging was uniform for every test (except the blank as described). A liter of 27C water was sealed with an activated temperature data logger in the bag, and then placed in the bottom of the Styrofoam cooler. A 2-ply newspaper was folded 4x and placed on the water bag as a barrier between the water and the heat packs. The heat pack(s) were taped to the inside of the lid and activated prior to use. Once assembled, a strip of tape was placed over the lid and sides of the box to make it secure, then placed in a temperature controlled environment. Shortly after 48 hours, the package was disassembled and the data logger downloaded to provide the readings and charts.

Next: Heat Packs Used
​

 

[BeakerBob]

HEAT PACKS USED: Note the red arrows I put in to point out the pack operating temperatures!

Little Hotties 18 hour Hand & Body Warmer Heat Pack

DIYREEF.com, 20 hour Heat Pack

Ultra Warmer, 24+ hour Heat Pack

UniHeat, 30+ hour Heat Pack

Next: Controlled Testing Conditions

 

[BeakerBob]

CONTROLLED TESTING CONDITIONS:

STORAGE TEMPERATURE:
• Ambient temperature maintained at 4C

STYROFOAM CONTAINER:
• Thermalok TL-645F with lid
o Outside Dimension: 9-1/2 x 7-7/8 x 8-1/16,
o Inside Dimension: 6-1/4 x 4-5/8 x 5,
o Volume: 144.53 cubic inches, 2.4 liters, 0.634 gallons, or 81.15 ounces
o Wall thickness: 1.5”

WATER CONDITIONS:
• Volume: 1 liter, 0.26 gallons, or 33.8 ounces
• Temperature 27C (80.6F)
• Double bagged, no air bubble

TEMPERATURE DATA LOGGER:
• Recording interval: Set to record temperature at 5 minute intervals
• Duration: 48 hours
• Placement: Heat-sealed in plastic bag, double bagged and secured inside 1 liter water bag.

HEAT PACKS:
1. Little Hotties 18 hour Hand & Body Warmer, states 37C (135F) average temperature
2. DIYREEF.com, 20 hour Heat Pack, no other information available.
3. Grabber Mycoal 24+ hour Ultra Warmer, states 51C (124F) average temperature, 65C (149F) maximum temperature.
4. American Pioneer International, 30+ hour UniHeat, states 38C (100F) average temperature, 49C (120F) temperature

• Heat packs activated and warm prior to insertion into water bag
• Heat packs taped to lid along edges to maintain clear air holes

ASSUMPTIONS:

1) There is an adequate oxygen supply for the heat packs in the container.
2) Shipped packages are stored at an average of 40F during the cold winter months.
3) All packaging is prepared in a similar manner.

REAL LIFE VARIABLES:

1. The actual storage temperature (ambient temperature) is dependent on shipping environment.
2. Packaging varies in box size, density, and thickness which impacts heating efficiency.
3. Water volume is extremely variable and has significant impact on temperature.

Next: Packaging Pictures

 

[BeakerBob]

PACKAGING PICTURES:

Water volume and temperature:

Temperature Data Logger heat-sealed in plastic bag:

Water and Data Logger Sealed in plastic bag:

Heat Packs taped to lid and activated:

Packaging Components ready for assembly:

Water bag with data logger in first:

Newspaper barrier in next:

Lid with activated heat packs placed on box and secured with tape:

Next: Charted Results

 

[BeakerBob]

CHARTED RESULTS:

Chart of Little Hottie 18 hour testing in triplicate:

Chart of DYREEF.com 20 hour heat packs, single and double:

Chart of Ultra Warmer, 24+ hour Heat Pack, single and double:

Chart of UniHeat, 30+ hour Heat Pack, single and double:

Chart showing the results of all the testing:

Next: Study Observations

 

[BeakerBob]

STUDY OBSERVATIONS:

Temperature increase: The DIYREEF 20 hour and the UltraWarmer 24+ hour single and double heat packs actually raised the water temperature for a period of 3 to 5 hours to highs between 87F and 89F before decreasing.

At the 18 hour mark, typical of shipping out at 5PM one day and receiving at 11AM the next day, the water temperatures ranged from 61F to 65F.

At the 24 hour mark, the temperature ranged from 56F to 60F.

Lets open this to discussion for questions, comments, suggestions and what else might be explored in the future for this study. Don't forget to rate this thread to show if this type of information helps the reefing community or not. Rating can be done by clicking on the upper right portion of the first thread.

Next: Some Q & A Received

 

[BeakerBob]

Previous Questions (black) and Answers (blue)

Q-I think I read a 40 degree average temp outside the box to simulate shipping temp? I think this is way low, as many locations such as inside the 'store' or 'house' before being picked up by the shipper will be more like 70, and then inside at least some of the shipping facilities this will be temp controlled as well. I'm guessing you tossed the box in the fridge at 40 degrees? I'd be curious to see what the curves were like at an average of 50.

A - I selected 40 degrees for a number of reasons...I had a controlled 40 +/-1 degree room available and considered that the "average temperature" from shipping. Yes, you are right that most boxes sit at room temp in stores waiting to be shipped, but then they sit at the shippers warehouse at the airport for a length of time. I don't think they are temperature controlled during the actual shipping in the aircraft and definitely not during the ride in the delivery van.


Q - I see the tape is only across the top, and not 'sealing' the seam. I'd be curious to see if doing a lap around the lid to seal this seam would have any impact. A couple of our shippers do this, and claim it helps to keep the heat in better.

A -I taped across the top only and did not seal the box as I didn't want to limit the amount of air needed to keep the heat pack activated. I have received several boxes that were sealed across the top too and wondered if they were more efficient.

Q - Finally, was the stryo wrapped in a cardboard box and then taped shut also? I can't figure the r-value of cardboard is all that good, but bums use it so maybe...

A - The only reason I used the cardboard box was to preserve the styro container for repeated shipping. Without the cardboard shell, they get banged up and dirty through the shipping process.

Q - As far as heat pack placement, some of our shippers will tape them to the top using minimal tape so to not impact the surface area exposed on the packs. Others simply wrap the heat pack in newspaper and toss it in the box.

A - I taped the heat pack to the top being careful not to impede the air flow to the pack. I've shipped both ways, but for the sake of consistency in the study, I wanted to control the placement.

Q - I'd also be curious to see about bigger boxes with more water. We receive enough shipments, and for the most part unless they are very delayed, the temps are pretty solid, upper 60's sometimes, but usually more like low 70's. Most of these are bagged up in the afternoon one day and unpacked between 9am and noon the following day, so 16-20 hours or so, although for the last 30 minutes or so they are sitting in the warm airport lobby, or in the warm car/truck.

A - The bigger boxes contain larger volumes of water that act as potential heat sinks to "buffer" the temperature in the box. I am not surprised that they maintain a higher temperature, but I think it is due to their volume, not to the heat packs. The heat packs would only be able to maintain the air temperature and slow down the loss of the water's thermal capacity.

 

[BeakerBob]

Addendum

After several years of shipping and tweaking the packaging, I now drill a 1/2" hole through the lid of the styrofoam and tape the heat pack over the hole. If I use two heat packs, I drill two holes and place each heat pack over each hole. The packs are taped along the sides only so that maximum air exposure is possible inside the container. The holes through the top provide oxygen that is needed for the heat reaction in the packs and insure that the oxygen levels are adequate. It has been my suspicion that using small sealed styrofoam containers limit the available air for the heat to be produced.

 

[wysiwyg]

awesome right up beaker very informative

 

[robert]

Where to start -

First the thermal overrun evident in your charts suggest that the 20H DIY and Ultra warmers exhaust available O2 in a very short time (30 minutes to an hour from the data presented). Subtract this initial increase in heat and you see that the heat packs rapidly exhaust available O2 do little to sustain the temperature of the packaging after the initial overheat. In effect - you would be as well off to take you shipping water, put it in the microwave to heat it up before shipment. A crazy idea - but that's all the heat-packs accomplish, In actuality its worse as this spike occurs in your car or the shipping facility where the ambient temps are likely to be 70 degrees or better - not in the 40 degree ambient assumed by the study - resulting in even a higher temp spike. While its not evident in the graph because the ambient temp was reduced to 40 degrees immediately - the Ultra-heat slow release heat packs are also likely to cause over-temps both in transport to and at the shipping drop off facility (especially when dealing with smaller water volumes.)

As you suggest, you didn't wrap tape around the styofoam seam to allow air flow into the container so the heat packs could work but you don't really answer the question as to whether you also placed the syrofoam container into in a snug fitting cardboard box which in turn is taped. I suspect you did not, but that's usually how these are shipped.

In your follow up you recommend drilling half inch holes to allow air to keep the heat pack functioning but do not test this scenario. Basically you cut a hole in the insulation allowing convection to enter into the equation, assuming that heat generated will offset the increased heat loss. This did not work out in my tests - the additional holes resulted in a net heat loss in all cases. Also you cannot guarantee that holes are not obstructed in stacking or orientation.

So in summary - I applaud you taking a experimental approach and sharing your results. I think your findings do demonstrate that heat packs - even multi-hour heat packs consume enough available O2 to go cold in a very short time - even when the shipping container is not well sealed. That heat packs can themselves pose a danger of overheating - especially when shipping from a warm environment to a cold environment as their maximal activity is immediate - they may pose an unexpected but potentially serious threat to corals held initially in a warm holding environment.

Bob the real issue is shipping into a cold environment where the box sits out on an unheated truck in the cold for an extended period of time. The other way around - shipping from a cold location into a warm one is usually much less problematic. The first 12 hours of the test should maintain an ambient temp representative of that found at the drop off facility. The last 12 should be the ambient in the delivery truck. The package should be as it would be shipped - no lose tape so the heat pack can work. Exactly as you would ship it - sealed so no water can possibly leak even if a bag breaks. If your going to recommend holes - test that too - I think you'll be surprised.

I think if you do that you'll conclude as I have that heat-packs are ineffective at best and dangerous at worst. The best course is to ship with as much quality insulation as possible with early delivery or better yet - pick up at the facility.

 

[robert]

Bob - one final question.

How do you explain that the ultrawarmer single pack test out performed the ultrawarmer double pack test while the Uniheat double pack test did better than the Uniheat single pack test? Seems a bit odd doesn't it? Only the 2 vs 1 DIY test has the inflection point that would suggest O2 starvation.

 

[BeakerBob]

Robert, thank you for the comments and critique of the study. Note that this was an initial study with some more planned after the initial observations and tweaking of packaging. The main goal of this study was to see how hot the water would get from the heat packs, how long the heat would be retained in the water, and how to better package the corals in the future. My initial suspicion of oxygen starvation in this size container was confirmed. I have not designed a new study yet, but if/when I do, it will be with a styro and air holes to see if the pack will sustain its heat with a continued supply of oxygen and that there is still heat benefit to the coral water after the heat loss from the holed insulation box.

In answer to some of your questions:

1) You are correct in observing that the temperature was raised and that oxygen depletion occurred within the 1st hour after packaging.

2) The styros were not enclosed in an outside cardboard box when placed in the temperature controlled room at 40F. The cardboard provides minimal insulation and additional factors that could vary based on the package taping.

3) Since I am shipping from Michigan to warmer states during the winter, I selected 40F as a starting point in the study. My boxes are dropped off at the FedEx facility 15 minutes before being picked up. Since my drop-off facility is maintained at ~70F and the boxes are stored inside until FedEx pickup, this is not an issue. The unknown factor is how long they remain in the truck until off-loaded into the FedEx facility for sorting/shipping, what temperature is encountered in the actual flight/truck, and the temperature at the destination. Personally, I don't ship in the winter if the temperature is very cold (<20F) and will hold off until it gets warmer. I look at the weather on both ends, originating and final destination when I ship and use only Overnight Priority Shipping to arrive no later than 10:30AM. It's those shipments that are delayed that cause me the greatest worry.

4) I am not ready to state conclusively that heat packs are worthless or dangerous. Additional studies are needed to reach that conclusion.

5) I am unable to explain why 1 pack would do better than 2 packs, unless there is a formulation change in the manufacturer heat pack between the two brands. The Ultra warmer is a 24 hour pack and 2 packs may have starved out faster than the one pack. The Uniheat is a 30 hour plus pack and it is possible that they are formulated to use oxygen at a slower rate, thus allowing two packs to last longer and provide additional heat than one pack. Uniheats also sells 40, 60 & 72 hour packs, so I think there is a formulation change in the manufacturing process that allows for slower oxygen uptake to regulate longer heating times.

Robert, based on the results of the initial study and questions that have followed, a new study is needed. I will apply for a grant to some organizations to see if they will fund a study. If you are interested in helping design the study, let me know via PM. I would like to look at a similar study with holed styro boxes to compare to the old study as well as a real life situation where several boxes are shipped out in the winter and shipped back with similar packaging to show shipping conditions, cold to warm destinations and warm to cold destinations.

 

[robert]

Hi Bob,

Thanks for taking the time to respond.

When I ran these tests myself, I didn't get the same results. I found heat packs to be totally ineffective. My experiment was essentially the same as yours - I had a slightly smaller box volume. I had different insulation - My ambient test temperature was below freezing - I used different heat packs - and I also experiment with oxygen generation inside the shipping container.

When all tests yielded disappointing results I stepped back and looked into the chemistry of the reaction.

Lets do the same using your configuration - we''ll ignore heat loss and just look into the heat generation side.

The chemical reaction of the heat packs is the exothermic reaction creating rust.
This reaction produces 410kjoules per molde of rust porduced.

The internal volume of your container is 2.4 liters.
Of the 2.4 liters space - 1 liter is occupied by water leaving 1.4 liters of air inside the box.
Now 1 mole of any gas at standard temperature and pressure occupies 22.4 liters of space.
So we have 0.0625 moles of gas inside the box of which 21% is O2.
0.0625 x 0.21 = 0.013125 moles of O2 inside the shipping container.
It takes 1.5 moles of O2 to produce 1 mole of rust resulting in 410kj of heat.
0.013125 / 1.5 = 0.00875
so with the O2 in the box we can make 0.00875 moles of rust.
0.00875 x 410kj = means the reaction inside the box liberates 3.5875 kj of heat.
It takes 419kj of heat to raise a kilgram (1 liter) of water 100 degrees C.
so 3.5875 / 419 = .00856 = this is enough heat to raise our water temp .856 degrees C.
Converting to Fahrenheit = 1.54 degrees F.

So the air in the shipping container could at the very best, with 100% heat transfer and no loss, raise the temperature of a liter of water 1.54 degrees F.
Now as the O2 is consumed it changes state from gas to solid resulting in an air intake. So it pulls in 21% fresh air of which 21% is O2 which can further feed the reaction resulting in an additional .323 degree rise. Running this replenish cycle 4 times - we get a maximal rise in temperature of 1.54 + 0.32 +.07 +.01 = 1.94 degrees F - lets call it 2 degrees.

At this point the box is exhausted of all O2 - the heat packs go cold and without an influx of fresh air - no more heat can be generated. In essence the box is vapor locked - there is nothing to bring fresh air into the system. Even with--out sealing the box - the air exchange is insufficient. That's why the external cardboard is so important - not because of its insulating value.

After running my experiments - and these back of the envelope calculations - I experimented with O2 generation inside the container. The math simply didn't work.

These calculations support my experimental results and my conclusions. Heat packs do nothing in reality. They heat up after the shipment arrives and the customer thinks you did everything you could. That's their value and nothing more.

Please check my calculations - I make all sorts of mistakes.
But with all this out of the way - there are lessons to be learned - and more to try.

 

[BeakerBob]

Robert,
To help others understand your explanation, heat packs are made up of a combination of iron powder, water, activated carbon, vermiculite, wood powder, and salt wrapped in a paper/cloth material that encloses the powders. This heat pack is sealed in a plastic package to prevent air (oxygen) from activating it. Once the plastic seal is opened, the air allows oxygen into the bag to react with the iron. The reaction of iron and oxygen cause rust to form while releasing heat.

The basis for the exothermic reaction in the heat pack is 4Fe(s) + 3O2(g) -> 2Fe2O3(s) ΔH= -1652KJ

A quick review of the math in your post appears to be correct, but you are assuming a limiting factor on oxygen availability based solely on a sealed styrofoam unit with zero air infiltration. While I agree that the limiting factor in the production of heat is the amount of oxygen available to the heat pack, I am not in agreement that the only air available to the heat pack is what is trapped in the styrofoam container.

Your calculations are based on the assumption of a sealed unit (thus limiting the oxygen availability) concludes that a heat pack could only raise the temperature of 1 liter of water 1.54F. The study shows a temperature rise from about 78F to as high as 89F in the case of the DIY 20 Hour Heat Pack, yielding a temperature rise of 11F! The results were all similar in showing a temperature increase and some testing was done in duplicate and even triplicate to ensure data quality.

It would appear that there is some air infiltration feeding the heat pack, but it eventually starves itself and quits working. More stuff to examine and study! I have a suspicion that using a holed styro will also result in a starved heat pack condition as the rust forms immediately over the air intake area and will form a plug that would reduce the air infiltration.

 

[robert]

I would concede that there is some infiltration - particularly when the cardboard shipping box is omitted from the test. But its not much.
I did a quick overlay using your DIY graph:

Note how after the initial heat is subtracted, the rate of decay is identical between the heat pack and the syrofoam only test. I made the point in my first post that the initial heat could have been accomplished by heating the water the corals were shipped in. (Not desirable at all.)

In fairness, other graphs show more evidence of air infiltration - but again since the styrofoam was not boxed as it would be in shipping - I have to rely on my own experimental results where the box was included.

 

[robert]

I realized after posing the above that the proper adjustment should have been to shift the graph to the left to eliminate the initial temperature spike - not move it down.

Doing so results in the following.

This transposition does suggest that the heat pack is indeed slowing the rate of cooling. While this totally conflicts with my own observations - it is the proper way to eliminate the initial heat spike.

 

[robert]

Robert,
...

Your calculations are based on the assumption of a sealed unit (thus limiting the oxygen availability) concludes that a heat pack could only raise the temperature of 1 liter of water 1.54F. The study shows a temperature rise from about 78F to as high as 89F in the case of the DIY 20 Hour Heat Pack, yielding a temperature rise of 11F! The results were all similar in showing a temperature increase and some testing was done in duplicate and even triplicate to ensure data quality.

It would appear that there is some air infiltration feeding the heat pack, but it eventually starves itself and quits working. More stuff to examine and study! I have a suspicion that using a holed styro will also result in a starved heat pack condition as the rust forms immediately over the air intake area and will form a plug that would reduce the air infiltration.

Actually my calculation allowing for a 1.54 degree rise is based solely on the air in the container, but I also allowed for infiltration of new air caused by the consumption of the O2 in the box and found the maximal rise of ~2 degrees. I did not assume a sealed container.

I agree that all heat packs in the study suffered O2 starvation - that is the only explanation for the lack of substantial difference between tests with two heat packs and one.

These were only rough calculations - and I assumed perfect transfer into the water - in real life I would be surprised if you get 30% transfer as a lot of the heat would go into the insulation and never reach the water. But going back to the graphs and still assuming perfect transfer - the 10 degree delta (using my adjusted version of your graph) would imply an absolute minimum of 5 full air exchanges over 36 hours. As there is no pressure differential - I can only see the osmotic diffusion of the increased Nitrogen inside of the packaging as reliably bringing in any new O2. I don't have a sense of how robust that effect might be -

While a minimum of 5 full air exchanges might explain your findings - I think the required number is conservatively closer to 15-20 to see the recorded heat effect.

Since in your experiment the seam was not sealed - I see a potential that air movement - a breeze if you will - was present in your cold chamber - and that absent the enclosing box - allowed more air exchange than would be seen in packaging and actual shipment.

I think we can agree that simply adding any number of heat packs without adequate air exchange can't work.

Your tests suggest that heat packs could work with sufficient air exchange but falls short in my opinion, of replicating a suitable shipping configuration.

 

[BeakerBob]

"....................................

I think we can agree that simply adding any number of heat packs without adequate air exchange can't work.

Your tests suggest that heat packs could work with sufficient air exchange but falls short in my opinion, of replicating a suitable shipping configuration.

Yes Robert, we agree that adding additional heat packs without adequate air exchange can't work.

However, the point of the study was to provide information that: 1) packaging with no insulation or heat pack will not sustain the coral in colder temperature climates; 2) using insulation as the only packaging material is better than no insulation at all; and finally, 3) that heat packs as used with the packaging in this study, would provide an amount of heat over time.

To understand the problem and reach a scientific, reproducible conclusion, the study was completed under a set of controlled conditions to remove as many unknown variables as possible. To properly perform this study, a number of conditions were required: a temperature controlled environment, calibrated temperature autologgers set to 1 hour recording, same packaging materials, and same packaging technique. The only variable was the heat pack manufacturer. The study was not designed for "replicating a suitable shipping configuration".

I posted the study to provide the information, have discussion on the observations, and perhaps to come up with better packaging. You have stated that you have performed similar testing with different results. It would be helpful to see your study design and data and open it up for discussion.

Now that the study is complete and the issues identified, another study should be done to see what packaging changes can be made to provide sufficient oxygen to the heat packs. Once the oxygen issue is resolved, then another study can be completed to simulate actual shipping conditions, with temperature autologgers documenting the outside box air temperature, the inside box air temperature and the water temperature.

 

[BeakerBob]

As an aside, the last 3 coral shipments I have received have been in boxes with six individually cut sides, held in place by the outside cardboard box. This loosely put-together box would allow as much oxygen in as possible for a heat pack, but the heat loss from such a poorly insulated and constructed box would reduce any benefit of a heat pack.

 

[robert]

Hi Bob -

Although it may feel like it - its not my intention to attack you or your study. I applaud your effort - I'm just confounded because I got a different answer.
While I harp on the shipping aspect - I think I took you at face value in that assumption. Your intro to your study states in part:

"How efficient are heat packs when used to ship insulated boxes to maintain a warm temperature during the cold winter months in Michigan? Which heat packs should be used? What is the temperature of the water when received?

These are questions that have been on my mind when I ship coral frags during the winter months, so I designed a study to see how heat packs impact the shipped water temperature and the effectiveness of the insulated box." (emphasis mine)

It was not evident to me from that statement, that the study was a comparison of heat pack manufacturers only. I may have read more into your intro than I should have.

As for my experiments - I didn't carry these out to publish a study - only to improve my own packaging. I recorded the data - but did not preserve it unfortunately.
I can describe my methodology if your interested. In a nutshell, I compared how long it took for water in my package to drop from eighty to sixty degrees for various shipping configurations. I used a much steeper temperature gradient and I ended my tests when the internal temp reached 60 degrees. This allowed me to iterate my package design several times in the course of an evening. The presence of heat packs didn't make much difference in cooling rates in my tests. At the end of the tests I would open the box and in every case found the heat pack was dead cold. I never found a boxed configuration which I felt was suitable to ship which allowed the heat pack to breath sufficiently to significantly impede cooling. I even tested the packaging of several shipments I received - including several of the six panel design. Despite the "looser" design. the heat packs in those go cold as well.

I think we may find that the outer cardboard box when taped shut is not permeable enough for the heat packs - that's my suspicion and why I dwell on this aspect of your tests so much.

After many shipments into cold climates - I have developed the opinion that its not just the temperature the corals arrive at that's important, but just as important (or maybe more so) is how long they endure the suppressed temperatures. A deltaT x time effect.

I now only ship into the cold with guaranteed am delivery or pick-up if the temps are near freezing. In one case with a rural delivery in your area - I triple boxed - with 1.5" corning extruded foam as my outer container. I put in a couple of heat packs - they did nothing - but they didn't hurt either - all was well with afternoon delivery.

My experience tells me not to rely on heat packs - they're one of those things that you should not count into your shipping strategy. If a miracle happens and they keep things a smidge warmer - great - but don't bet on it.