The use of this phase change material cool vest provided relief from thermal stress in spite of the addition of 3.7 kg weight. Skin temperature and sweat loss were both significantly reduced when the vest was employed. Rectal temperature rate of increase, average heart rate, and respiratory volume were reduced when the vest was used, but not significantly. It should be noted that a larger (than six) sample size may have strengthened this measurement effect.
Use of the vest caused the suit to be somewhat tight on those subjects that were custom fitted to their own suits. There was also some noted loss in mobility, specifically in bending in the waist.
An important advantage of this 65º F (18.3ºC) phase change material was its comfort next to the skin compared to products at normal ice (32º F or 0ºC), temperatures. In this latter case, vasoconstriction of blood vessels near the skin is observed and it is expected that the ability of the body to radiate heat would be decreased.
At the conclusion of each test, the sixteen envelopes of phase change material were withdrawn from the vest pockets to assess the amount of use or phase conversion. In no case was more than half the phase change material converted to the liquid state. For this particular application, it would be possible to reduce the amount of material carried by about half, hence a weight savings of 1.8 kg (4lb.) It would be important to maintain the same amount of skin exposure area during this reduction.
A major advantage of this technology is the easy maintenance. The phase change material enveloped can be converted to the solid (frozen) state in ice water in about 15 minutes. There are no pumps, batteries, tubes or valves to fail.
User acceptance was excellent, an important feature of any personal protective equipment. The psychological relief from heat stress was subjectively very good with all subjects positively responding.
Subsequent to this series of tests, the vest was tested under the Category I Propellant Handler’s Ensemble. It was comfortably worn under the environmental control unit and provided significant (subjective) heat stress relief. The vest is now worn under Category I and Category IV PHE at the propellant farm at the Cape Canaveral Air Station.