4 years ago

Total Heat Loss (THL) vs. Evaporative Resistance (Ret)

Which is the best measure of comfort?

A new debate recently emerged within several NFPA committees involved in standards development for different types of protective clothing, including firefighter turnout gear, chemical protective clothing and emergency medical clothing. The debate is about determining the best test method for the evaluation of comfort.

Heat stress is potentially dangerous and probably the biggest concern for users today that wear different types of protective clothing. Heat stress is a difficult thing to measure and the test methods being used have not been correlated to actual user stress conditions.

The body regulates heat through the evaporation of sweat from the surface of the skin. Heat is also transferred away from the skin through airflow across the surface of the skin. Protective clothing which is considered impermeable does not allow for the evaporation of sweat and the core temperature of the body increases.

So the addition of comfort requirements to protective clothing standards can be considered a good thing. However, caution must be exercised to prevent creating requirements for protection that might ultimately cause potential harm to the user. 

There are three basic test methods that are currently in use to evaluate comfort. 

Water (Moisture) Vapor Transmission Test – ASTM E 96 

This test method is simple and easy to perform. A cup or cell is filled with water and sealed with the test fabric. The cup is weighed and left for a period of time. After the period of time, typically 24 hours, the cup is weighed again. The difference in weight represents the weight of the water that has evaporated. The test can be performed at different temperature and humidity conditions as well as different cup orientations. The results of this test are expressed as the weight of water evaporated per surface area per time.

Total Heat Loss (THL) Thermal andEvaporative Resistance – ASTM F 1868

This test method is complicated and requires special equipment to perform. Only one lab in the US can currently conduct this test method. 

A heated plate is placed in a chamber with special conditions of temperature and humidity.

The test fabric is placed over the plate. The test is run first in the dry condition.

This measures only the heat transfer from the plate to the atmosphere. The machine is designed to keep the plate at a constant temperature. If heat transfer occurs, the machine will provide electrical current to keep the plate at the temperature. This measured current is used to calculate the resistance of the fabric. The test is then run again with the water on the plate between the plate and the test fabric. The calculation is performed again for this test. The results of the two tests are combined to create a number. This test method can be run at different conditions of temperature, humidity and airflow across the fabric surface. The test is typically run at a plate temperature of 35 degrees C and an air temperature of 25 degrees C with a humidity of 65%. The results of the test are expressed as watts per meter2.

Evaporative Resistance – ISO 11092 

This test method is similar to portions of the ASTM F 1868 but is conducted in isothermal conditions, which means the plate and the air are at the same temperature.

This basically means that all heat loss is based on the evaporation of the moisture only. This test is conducted at a humidity of 40%, which is not typically representative of what would be expected in most of the United States. The results are expressed as pressure per surface area times energy, which is very confusing to interpret.

Those involved in the development of the standards and incorporation of breathability requirements agreed that some additional requirements for evaporation are needed. Too many examples exist of fabrics with no evaporative characteristic that meet THL requirements, particularly in NFPA 1994 Class 3. One of the problems that still exists is that the committees are trying to correlate requirements between the different standards. This is not easily done, because the fabric structure designs vary significantly. Fabrics for turnout gear are multi layer composites that are thick and bulky.

The fabrics for 1994 or 1999 are single layer thin lightweight membranes. So the debate still carries on as to the best method for measuring breathability. Until proper human subject testing can be performed and attempts made to correlate the various breathability testing methods, the debate will continue with just opinions provided to justify each position.

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