HOTMATES are a re-chargeable instant source of heat that may be carried and used just about anywhere you can imagine. They consist of a liquid encapsulated in a plastic pad that undergoes a user initiated exothermic state change that provides heat for a period of time until the reaction is complete. The HOTMATE may be then re-used by reversing the state change by applying heat to the pad, this may be done by boiling for instance.
HOTMATES originally appeared a couple of years ago but due to a design problem were withdrawn after a short while. They have now been re-designed and are on the market again. They were primarily designed for use by medical organisations as flexible Physio Heat Pads and several hospitals approved their use; recently they have also been finding a market in outdoor pursuits activities and related rescue organisations.
The HOTMATES come in six different sizes ranging from a small hand pad (3" x 4") to a large pad measuring 16" x 6". Most are rectangular although one is foot shaped for slipping into the base of a sock. The pad is made from durable plastic and contains an aqueous solution of Sodium Acetate Trihydrate. This substance if left in an open container at room temperature would solidify from liquid to solid (freeze) at around 54C. However, if this solution is encapsulated it remains a stable clear liquid down to as low as -20C and so the pads may be frozen and used as cold packs if so required. The solution is encased in an envelope at temperature with a measure of pure water. At 85C the Tri-Hydrate crystal takes additional molecules of water into its structure and it prevents precipitation within the solution until user activation.
Each pad contains a trigger mechanism that consists of a small plastic covered stainless steel disc made to a calculated thickness. By pressing the convex side of the trigger to concave, a kinetic shock wave is generated on a frequency level which interferes with the water molecules and the Trihydrate molecules crystallise in the area around the trigger. A visible chain reaction then ensues and only stops when all the liquid has solidified. As the liquid reverts to its solid form by a crystallisation process the phase change releases heat and the pad instantly rises in temperature to around +54C. The maximum temperature the substance can achieve is a few degrees above this and so can be considered a safe heat. The heat is dissipated over a period of time depending on the pad's size, shape and protection from external ambient temperatures.
The pad may be re-used by reversing the state change of the substance from crystal to liquid by raising its temperature above 85C such as placing it in boiling/simmering water for 5 to 10 minutes. The manufacturer gives figures of about 100-200 cycles before the pad would need to be replaced due to the trigger failing to generate the correct frequency wave.
I was supplied with eight pads as a free sample and I took them to the Matienzo Expedition (Northern Spain) last summer. I was hoping that they could have been used as part of an underground bivy, but as this didn't materialise I generally used/abused them above and below surface. In the past I have used another sort of personal heat source (WARMPAC) underground whilst camping in Darren and I feel a comparison between the two is therefore worth while.
WARMPAC's resemble catering size tea-bags and contain a solid substance that can undergo a non-reversable exothermic oxidising reaction. Once the WARMPAC's reaction has been started, by exposing it to the air, it can last for about 14 hours giving out +40C before gradually cooling to external ambient temperatures 10 hours later. These were found to be ideal for using around the camp and a heat source to sleep with. The WARMPAC could be made to last several days by reducing the air flow to it by re-sealing it in a plastic bag.
Size for size the HOTEMATE's are about three times more expensive than WARMPAC's although WARMPAC's are NOT reusable. My next main concern, as with most caving kit, is how survivable would the HOTMATE's be? The plastic coating of the pad seemed to be tough enough and carrying them next to my thrutching body (ugh!) or in a mucky rigging bag seemed to do them no harm. I suspect that as long as you didn't try using them as knee pads they were well up to the job. Because of the WARMPAC's breathable paper outer skin they are unable to put up with any contact with mud or water before they became unuseable. HOTMATE's plastic skin seemed therefore to make them entirely suitable for use in "hostile" environments. The trigger mechanism on only one occasion took more then a couple of clicks to get the crystallisation started and the benefits of being able to produce an instant heat source waiting underground during intermidable bolting sessions was obvious. Back at camp, re-charging them by boiling took no more than a few minutes and it was found that even leaving them out in the midday sun could achieve a semi-state change. The pads were also found to be useful for taking the chill off the night air whilst waiting for the meal to be cooked and also as cold paks for cooling the San Miguel in the cool-box, its a hard expedition in Matienzo!
My two main criticisms of the pads are as follows. Although the initial heat radiated by the pad is very good, it was found that because of the low ambient temperature of the cave or at night time the HOTMATE's seemed to last only for a couple of hours at the most before they became cold; WARMPAC's give out a better heat for a longer time without human interference. The manufacturer claims that the pads last longer if encased in an insulated pouch, this hasn't been tried however. By massaging the pad also, it may be made to generate yet more heat but this is obviously only useful if you have both hands available at the time. The second criticism is more minor; when in crystal form the pads become very hard and set in the shape that they have been last left in, this is not a problem with the small pads but the large pads become somewhat unwieldly.
In summary the HOTMATE pads seem ideal for hostile environments where an instant heat source is required (ie. emergencies) but until they can be made to last for longer WARMPAC's are still a good choice for bivying even allowing for the greatly increased cost. If you require anymore information then please contact X-Pak Survivor Ltd., Postscript, Bleadon Mill, Bridgewater Road, Bleadon, Weston-Super-Mare, Avon, BS24 0BD.