TPACC Fabric Evaporative Cooling Efficiency Test Method

TPACC can evaluate the evaporative cooling efficiency of clothing fabrics using standardized test method ASTM F3628. The cooling efficiency test method, developed by the TPACC labs, measures energy released from human skin by the evaporation of sweat, when influenced by the fabric’s intrinsic ability to absorb and transport liquid sweat. The protocol includes sweat and no sweat phases, simulating a use scenario that occurs when a wearer produces sweat during exercise followed by a period of rest without sweating. We calculate maximum heat loss, cooling energy in sweating and drying phases, drying time, and an overall cooling efficiency index for the test fabric.

What sets this method apart?

This new method removes all conduction contribution to heat loss to focus on heat released by the evaporation of liquid sweat, not vapor. It applies liquid moisture to the bottom of the fabric allowing capillary wicking to occur, unassisted by gravity. It provides a realistic drying time for a fabric. While some tests apply a consistent amount of moisture and measure the time it takes to dry, this method accounts for different amounts of moisture evaporation during active sweating. Rather than assuming a constant starting point, it allows the fabric to control how much moisture is present at the time of drying. The principle of measurement assumes that a high-performing cooling fabric should promote sweat evaporation during sweating phase to reduce body temperature, and dry quickly during a resting phase to avoid a chilling effect.

Figure 1. Dynamic hotplate system housed in an environmental chamber

The cooling method uses a dynamic sweating hot plate testing platform to measure the energy released from a human skin model by the evaporation of sweat (Figure 1). The sweating plate consisted of an 8 × 8 inches heated test plate surrounded by a 2-inch guard ring to prevent lateral heat loss (Figure 2). Unlike a conventional sweating guarded hotplate, the system uses a heat flux sensor embedded in the surface of the test plate to measure heat exchange from the plate and through the test material sample placed on the plate surface. Positive heat flux readings from the embedded thermal sensor indicate heat loss from the plate to the environment. A water circulation system located under the hotplate cools the sweating plate.

The hot plate sweats by delivering volumetrically controlled water to the surface of the heated plate. Heat exchange measurements occur with the plate configured to simulate sweating and evaporative heat exchange. The protocol incorporates sweating and drying phases.

To measure the endothermic thermal energy absorbed in evaporative cooling, the hot plate surface temperature is set at 35 °C, the same temperature as the ambient environment. The test sweating rate is selected to approximate the average sweating rate of a person engaged in vigorous athlete activities such as playing basketball, soccer or baseball.

graph heat loss over time — Figure 2. Idealized evaporative heat loss curve showing cooling energy in sweating and drying phases of test protocol. The areas under the heat loss curve (A1 and A2) are the cooling energies in the sweating and drying phases.

Figure 2 represents an idealized evaporative heat loss curve produced by the testing protocol. The curve shows an initial rapid rise in evaporative thermal energy absorption from the sweating plate that reaches a peak flux level. A rapid reduction in energy absorption follows the sweating phase as the fabric dries to initial state. The evaporative cooling properties represented by this heat absorption include the peak heat flux produced by evaporative heat absorption in the sweating phase of the test. This measure may predict the cooling effects of evaporation in the same way that peak transient heat flux values predict a fabric’s cool-touch. Another metric indicative of cooling performance is the sweating cooling energy, or the endothermic thermal energy absorbed from the sweating hotplate during the sweating phase of the test (A1 in Figure 2).

Figure 3 shows how the method compares two different fabrics with different evaporative characteristics.

Figure 3 shows how one fabric (B) can be expected to absorb heat from the and cool the skin more rapidly than the other fabric (A) that takes longer to dry.

*Partly excerpted from Gao, H., Deaton, A.S. and Roger L. Barker, A new test method for evaluating the evaporative cooling efficiency of fabrics using a dynamic hot plate, Journal of Measurement Science, Volume 33, Issue 12, December 2022.