A Comparison of TPS Methods for Measuring Thermal Effusivity

The Transient Plane Source – Effusivity Meter is a portable thermal effusivity meter for textiles with a thermal effusivity measurement range of 35 to 1700 (W√s/m2K) and a temperature range of -10 ºC to 50 ºC. A single measurement can be set to 2 or 10 seconds in duration. The TPS-EFF operates in accordance with the ASTM D7984-16 standard of testing. The TPS-EFF has a 5% accuracy and a 2% reproducibility, making it a highly accurate and precise instrument for measuring thermal effusivity of textiles and other materials.

tps effusivity

Figure 1: Thermtest TPS-EFF Portable Thermal Effusivity Meter.

One application of thermal effusivity is the measurement of coolness or warmth feel of materials on textiles and fabrics. When a textile or fabric is measured from the surface with short test times by any transient method or instrument, the measured effusivity includes various heat transfer mechanisms, including conductivity, convection and radiation, as well as contact resistance between the sensor and sample. As the dominant property in the thermal effusivity expression is thermal conductivity, which is valid for solids only, the term used to properly describe the measured value is thermal conductance. When considering this point in terms of thermal effusivity, the more appropriate term becomes thermal effusance. With this understanding, thermal conductivity should not be determined from the thermal effusivity expression. This is an important fact when considering qualitative use of thermal effusivity for testing non-solids. 

In this experiment, the thermal effusance of various materials was measured with: a TPS-EFF, a TPS instrument, and a MTPS instrument. Test times of 2 and 10 seconds were used to simulate both short and long touch times. The TPS instrument features data at the longer duration only, as the short test time resulted in a poor fitting for the model. The MTPS instrument features data at the shorter duration only, as it is not calibrated for longer test times. Results can be seen in the table below.

Table 1. Thermal Effusance results for multiple materials measured with multiple methods.

Sample ID Test Interval
(s)
Thermal Effusance (W√s/m2K)
TPS TPS-EFF MTPS
PVC Foam 2 39.5 40.5
10 47.7 47.2
85% Polyester, 15% Cotton 2 259.5 251.3
10 224.6 257.1
60% Cotton, 40% Kapok 2 207.5 204.4
10 214.6 222.3
PVC 2 559.5 545.2
10 530.0 561.0
Delrin 2 836.2 864.6
10 830.1 853.0
Pyrex 2 1323 1532
10 1449 1357

Note: The TPS instrument features data at the longer duration only, as the short test time resulted in a poor fitting for the model. The MTPS instrument features data at the shorter duration only, as it is not calibrated for longer test times.

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