Convective Cooling, Conductive Cooling, and Pressure Distribution Testing of Life-Saving Equipment

Introduction

When analyzing choices for lifesaving equipment such as plate carriers and duty belts, along with accompanying accessories such as insoles and shoulder pads, it can be difficult to quantifiably measure what would be the best option for the consumer. Many characteristics and their combinations can heavily influence how effective a certain piece of equipment can be for an individuals needs, for example, certain load bearing capabilities, cummerbund features, MOLLE rows in particular locations, concealability, integration with aftermarket accessories, or overall comfort. However, pressure distribution, convective, and conductive cooling are characteristics of body worn items that are often neglected, a design afterthought, and have not had a metric of comparison that has been effectively tested. This report features an overview of an extensive series of tests conducted with regards to pressure distribution of belts and shoulder pads, convective cooling of plate carriers, shoulder pads, insoles, and belts, as well as conductive cooling of plate carriers and shoulder pads.

Pressure Distribution

 

Purpose: 

Pressure distribution testing aids in comparing plate carriers, belts, insoles, and shoulder pads across the market to identify pressure points and improve design to increase user comfort. Higher user comfort results in a decrease in fatigue and strain on the end user and can therefore allocate that lost energy to perform higher for a longer duration. The effects of fatigue drastically increase with higher weights over longer periods of time, stressing the importance of comfort through design and material selection.

 

Methodology:

To test the various products impacted by pressure distribution, FujiFilm Prescale is used to highlight pressure points visually. To ensure the accuracy and realism of the tests, 2 mm of ballistic gel is placed in between the pressure film and the testing product to better represent how human pressure receptors are stimulated as these pressure receptors typically sit ~2 mm below the skin. 

 

Results: 

The results for all of the pressure distribution tests will yield the same metric to compare results: pressure film images. For all shoulder pad/plate carrier tests, 20lbs in bumper plates was hung from the webbing strap. To test varying/higher pressures on the specified shoulder pads, the 4ULW film and up must be used.

Shoulder Pads/Plate Carriers:

Figure #1: Simulated Plate Carrier Pressure Film

Figure #1 shows the tests conducted on 6 shoulder pads available to the market, as well as 3 prototype options. The top row features, from left to right, ICEVENTS® Classic, ICEVENTS® Aero, T.Rex Arms AC1, SHAW Concepts ARC V2, Crye Precision JPC 2.0, and Spiritus Systems LV-119, all with 20lbs applied. The following row features the ICEVENTS® Aero and T.REX AC1 pads with only 10 lbs. The bottom row features prototype options, from left to right, 2x ICEVENTS® fully enclosed, and ICEVENTS® fully enclosed with a layer of tegris placed on top of the enclosed ICEVENTS®.In interpreting the results of the tests, the more saturated the color of the magenta, the greater the pressure, therefore, the more dispersed/lower the saturation, the more effective the shoulder pad is at distributing pressure. 

 

Based on the shoulder pads tested, the Shaw Concepts ARC V2 appeared to have the most even pressure distribution with the least amount of peaked pressure points. ICEVENTS® appear to have even pressure distribution although ICEVENTS® Aero has a smaller contact area equating to greater stress. Prototype options show promising results with a more even pressure distribution in utilizing the enclosed stimulite. All of the other options appear to decently distribute the pressure although there are more pressure points to be seen as is expected from very thin foam or none at all.

 

Belts

Figure #2: Inner Belt Pressure Distribution

Figure #2 shows the pressure distribution results from testing the AWS/Ronin, BFG CHLK, and ICEVENTS® INNER BELT. The top of each of the strips is the front left groin portion of the mannequin. Of note are both front groin areas, shown on both ends of the strips, as well as the middle of each strip which is where there was limited contact with the mannequin's lower back. The areas in between are where the areas of the belt contacted the pressure film. Will’s Ronin belt setup was used across all 3 tests.

Based on the test conducted, the results are somewhat inconclusive as the areas of error are very large as well as the sensitivity of the film being too high for the given pressure for the pressure applied by the pants and belt. A more conclusive test would be to lay the belts flat, similar to the shoulder pad pressure distribution test.

Insoles

Figure #3: Insoles Pressure Distribution

Figure #3 shows pressure distribution tests conducted on both ICEVENTS® Insoles and SuperFeet insoles. While the specific pressure film used for this test was too sensitive for the applied pressure (human body weight), it does serve to visually represent the sensations created by the different material choice between stimulite and a traditional foam sole + plastic heel cup. The untouched slashes seen throughout the various insoles are from the film folding/collapsing under the foot.

 

Conductive Cooling

 

Purpose: 

Wearing plate carriers inherently insulates a large area of the torso. Material selection, surface area, and thickness are all important components of the equation that models heat transfer through thermal conductivity. To reduce thermal conductivity, the insulating plate carrier would ideally have minimal surface area, thickness, and be constructed of a material with a higher K value. However, all plate carriers hold armor plates, and whether they are constructed of ceramic composites or steel, make up a large surface area with a relatively thick material with a low K value. Additionally, other areas of the plate carrier such as the shoulder pads and cummerbund must strike a balance between pressure distribution, where pressure is alleviated through a greater surface area, functionality, and lastly thermal properties/heat transfer. 

 

Methodology: 

To test and highlight thermal conductivity of plate carriers, a heated mannequin torso with a combat shirt and plate carrier worn will be photographed/videographed with a FLIR camera. Once the carrier has been worn for a sufficient period for the simulated body temperature of the mannequin to be heated, the plate carrier will be removed and the thermal camera will highlight hot spots on both the torso as well as the plate carrier.

 

Material Specific Test: Additionally, a more specific material test can be conducted to identify the K value of select materials. A non absorbent heating pad heated to a constant 100 degrees fahrenheit will have the test material lying on top. On top of the test material, a layer of foil with ice formed in the shape of the test material will be placed in contact with the material. This sets up a test to identify the specific K value of the testing material with all other components of Fourier's law being known.

Figure #4: Fourier's Law

With the thickness of the test material, temperatures on both sides of the material, area of the material, and heat transfer rate with the specific heat of fusion of ice, mass of ice, all being known, the only variable needed to be recorded is the time it takes for the ice to melt. With all variables either known or recorded, simple algebra solves for K, the last remaining variable.This is also the basis for the cooling vest test video where a heated mannequin with cooling vests placed on top and time taken to melt the ice is used for attaining heat transfer values. For more information on this experiment reference the second cited source at the bottom of this paper.

 

Results: 

For the practical plate carrier test, thermal imaging will provide a visual element for the viewers and product designers/engineers to identify hot spots on carriers. All carriers will likely perform similarly with minor differences so it would be a good opportunity to market ICEVENTS® and ICEPLATE® Curve. ICEVENTS® offsets carriers from the body, effectively reducing the surface area of the hot insulating carrier/plates on the body and enabling high rates of evaporative cooling with increased airflow while also being made of a material with a higher K value. ICEPLATE® Curve adds a conductive cooling element over a large surface area, etc. 

The thermal images indicate that carriers such as the Crye Precision AVS that exhibit a greater contact area on the body insulate more heat as compared to ICEPLATE® Exo or Crye Precision JPC 2.0. The Shaw Concepts design shows that even with material such as spacer mesh being used, an air gap/spacing greatly reduces the contact area which insulates less and allows for convective cooling to take place much faster than when the body is in contact with the carrier and is an important note especially considering that it is a full duty plate carrier designed for a role similar to the Crye Precision AVS.

For the material specific test, K values are a scientifically acquired value that states a material's ability to conduct heat. This can be inputted back into Fourier's law with different parameters to illustrate the K values' impact on total heat transfer.

 

	Shoulder Pad	Thermal Conductivity Coefficient "K"	ICEVENTS ® percent more efficient
1	ICEVENTS® Aero	0.000440
2	Crye Precision	0.000188	134.15
3	T.REX Arms	0.000072	507.09
4	Spiritus Systems	0.000041	980.04

Figure #5: Shoulder Pad Conductive Cooling Thermal Conductivity Values

Figure #5 indicates both the raw figures attained for the thermal conductivity coefficients of each shoulder pad along with the %greater efficiency that ICEVENTS® provides. Note that while the actual percent efficiency may be great, the thermal conductivity coefficient plays a somewhat small role in Fourier’s Law.

 

Metric of Comparison:

The first metric of comparison (shoulder pads) is the raw K value attained by the testing which differentiates the specific material property of each test sample that impacts the test samples ability to transfer heat.

 

The second metric of comparison (plate carriers) are FLIR images which can be seen here.

 

Convective Cooling

 

Purpose:

A vital component of the body's natural cooling process is the evaporative cooling of sweat which is best facilitated by convective cooling or airflow through the heated/sweat-through region. To test material selection, design, and construction, the following tests are performed to determine design flaws and improvements to be made.

 

Methodology:

Two different test methods are used to test evaporative cooling. Both involve soaking a shammee material to a consistent weight to mimic heavy sweating experienced during strenuous activity (0.5 kg/m^2/hr). The first method is to enclose a mannequin heated to a constant human body temperature in an enclosed chamber and monitor the humidity increase over time from the shammee evaporating. The second method involves weighing the soaked shammee over time as water evaporates.

 

Plate Carriers: plate carriers will be tested in the acrylic chamber while monitoring humidity increase. With the mannequin set to body temperature (100F), the soaked shammee (weighed to 3.78oz soaked) is placed on the mannequin with the tested carrier loaded with plates covering all of the shammee. An arduino humidity sensor hangs inside the chamber.

 

Belts, Insoles, Shoulder Pads: these three sets of wearables are all tested on the acrylic encased heating bed. With the desired test sample, a soaked shammee is placed between the heating pad (set to 100F to simulate body temperatures outdoors during strenuous activity) and the test material. Both the test sample and shammee are weighed every 10 minutes to measure evaporation over time. Measurements are taken every 10 minutes until all water has evaporated.

 

Results:

These tests measure the convectivity and evaporative cooling elements of the given test samples. With the measured variable for both the plate carrier and belt/insole/shoulder pad tests being rate of evaporation through humidity increase and water loss respectively, these rates indicate which test samples allow more surface area of the shammee to be exposed to airflow and thus promote evaporation, accounting for both convective and evaporative cooling  properties. To contextualize the data against ICEPLATE® Curve, the wattage/J/s can be used as a comparison to the evaporative cooling power. This energy transfer rate is found by multiplying the heat of vaporization of water (2.25x10^6 J/kg) by the rate of evaporation of water. Based on the data obtained by these tests, the following figures can be obtained.

 

Please reference the Convective Cooling Data Collection Spreadsheet here for all raw data points.

Shoulder Pads

	Results: Average Sweat Loss (oz/min)	Energy Transfer (W orJ/s)	ICEVENTS® % more efficient	Average Sweat Loss (oz/min)
1	ICEVENTS® Aero	2.14		0.00202
2	Spiritus Systems LV-119	1.93	11.35	0.00181
3	SHAW Concepts ARC V2	1.26	70.42	0.00118
4	T.REX Arms AC1	0.85	152.58	0.00080
5	Crye Precision JPC 2.0	0.84	154.32	0.00079

Figure #6: Shoulder Pad Evaporative Cooling Energy Transfer and Sweat Loss Figures

After 3 trials were conducted for each test material, the change in weights were calculated for each time difference (every 10 mins), then averaged to attain an average weight loss/min figure, then averaged again across the three trials to attain the figures seen in the column furthest to the right. To find the constant energy transfer rate (Watts or J/s), the average sweat loss found is converted from oz/min to kg/s then multiplied by the heat of vaporization of water to attain a heat transfer rate “Q” as seen in the left-most data column in the figure above. Finally, the percentage increase in efficiency of ICEVENTS® over competing test samples is given in the middle data column above.

 

Belts

	Results: Average Sweat Loss (oz/min)	Energy Transfer (W orJ/s)	ICEVENTS ® % more efficient	Average Sweat Loss (oz/min)
1	ICEVENTS® Aero	2.14		0.00201
2	ICEVENTS® w/ AWS/Ronin	1.68		0.00158
3	BFG CHLK	1.49	44.15	0.00140
4	AWS/Ronin	1.39	21.11	0.00131

Figure #7: Duty Belt Evaporative Cooling Energy Transfer and Sweat Loss Figures

Similarly to the shoulder pads testing, 3 trials were conducted for the belts listed in Figure #6. The change in weights were calculated for each time difference (every 10 mins), then averaged to attain an average weight loss/min figure, then averaged again across the three trials to attain the figures seen in the column furthest to the right. To find the constant energy transfer rate (Watts or J/s), the average sweat loss found is converted from oz/min to kg/s then multiplied by the heat of vaporization of water to attain a heat transfer rate “Q” as seen in the left-most data column in the figure above. Finally, the percentage increase in efficiency of ICEVENTS® over competing test samples is given in the middle data column above. The results of the test yielded differences between inner belt design and material choice.

Insoles

	Results: Average Sweat Loss (oz/min)	Energy Transfer (W orJ/s)	ICEVENTS ® % more efficient	Average Sweat Loss (oz/min)
1	ICEVENTS® Insoles	1.16		0.00109
2	No-Name Foam Insoles	0.87	32.76	0.00082

Figure #8: ICEVENTS® Insoles Evaporative Cooling Energy Transfer and Sweat Loss Figures

Similarly to the shoulder pad and belt convective cooling tests, 3 trials were conducted for the ICEVENTS® insoles. The change in weights were calculated for each time difference (every 10 mins), then averaged to attain an average weight loss/min figure, then averaged again across the three trials. To find the constant energy transfer rate (Watts or J/s), the average sweat loss found is converted from oz/min to kg/s then multiplied by the heat of vaporization of water to attain a heat transfer rate “Q,” resulting in a 1.16 J/s rate of energy transfer and a .00109 oz/min average evaporation rate. Another market option whose construction featured foam as the reinforcement material and a felt liner as the contact material featured a lower sweat loss rate as the water soaked through the material which prolonged the evaporative process.

 

Plate Carriers

Figure #9: Plate Carriers Normalized Humidity Increase over Time

After 3 trials of data collection per plate carrier configuration, an average increase in humidity was calculated per trial, then the humidity increase/min was calculated from this to normalize the data from zero to properly compare humidity increases across carriers as the initial humidity values vary from trial to trial and carrier to carrier. The initial increase in humidity before the curve begins to flatten out/reach equilibrium is most significant. The steeper the initial slope, the greater the rate of evaporation per minute of the given carrier. The overall increase in humidity over the entire 60 mins is unable to show the total amount of evaporation since the chamber has reached its equilibrium humidity to which the time it takes for more water to evaporate and saturate the air increases greatly or doesn’t occur at all. To normalize the starting point of each humidity increase, the average delta between each average minute is calculated and added to zero.

 

There are a few ways of interpreting the data collected from this set of tests. Firstly, is graphical interpretation. In Figure #9, the first 25 minutes is of most importance as it highlights the initial increase in humidity before all of the rigs reach an equilibrium humidity. A higher resolution of Figure #9 can be seen on the spreadsheet as well as other comparisons and raw data graphs.

 

	Total Evaporative Cooling, Q	J
1	ICEPLATE® EXO w/ ICEVENTS®	53.84
2	SHAW Concepts ARC V2 w/ ICEVENTS®	38.53
3	Crye Precision AVS w/ ICEVENTS®	36.56
4	ICEPLATE EXO®	32.91
5	Crye Precision JPC 2.0	30.87
6	Crye Precision AVS	28.69
7	Spiritus Systems LV-119	27.88
8	SHAW Concepts ARC V2	26.32
9	T.REX Arms AC1	13.82
10	Crye Precision AVS w/ Crye Pads	10.42

Figure #10: Plate Carrier Evaporative Cooling Raw Energy Transfer Figures

Figure #10 indicates the placement of plate carriers and configurations and their respective evaporative cooling values. Of note is the increase in cooling power provided from ICEVENTS® and the decrease in cooling power provided by the Crye Precision AVS pads which reduced the AVS’s cooling power from its base configuration. While the Crye Pads did offset the carrier from the body slightly, the material that the pads are made from are very effective at absorbing water and thus extremely ineffective at hosting evaporation. 

Figure #11: Humidity vs. Time ICEVENTS® Comparison

Figure #12 : ICEVENTS® Comparison % Difference in Evaporative Cooling

Figure #11 and #12 indicate the percent difference that ICEVENTS® make to increase evaporative cooling as well as the average percent increase in evaporative cooling provided by ICEVENTS®. ICEVENTS® also provides 250.68% greater convective cooling power than using the Crye AVS Pads, which, in context of the total Joules is 36.56 J vs. 10.42 J.