One factor that has made the Hot Disk Transient Plane Source a top of the line thermal conductivity measurement system is the continued emphasis placed on research by the Hot Disk company. Its founder, Dr. Silas Gustafsson, and CEO, Dr. Mattias Gustavsson, have spent the last twenty years perfecting the system and extending its measurement capabilities. In this applications page, we will be discussing research performed by Gustavsson and Gustafsson (2006) to quantify the ability of the Hot Disk TPS to determine the quantity of fat present in milk based on thermal conductivity measurements. This is an excellent example of how the system can be used to detect the smallest of changes in a material, and is applicable to the field of quality control in liquids.
Figure 1. Milk undergoes testing and processing prior to being available for commercial purchase. Part of this process includes skimming the fat from the milk to produce milk products with a variety of fat contents.1
Thermal conductivity measurement in liquids with low viscosities is often difficult, as convection resulting from the increase in temperature from the test quickly takes over as the dominant method of heat transfer. Fortunately, the Hot Disk TPS has enviably short test times which can measure thermal conductivity prior to the influences of convection beginning. The two-sided Hot Disk sensor is composed of a nickel spiral surrounded by a Kapton insulator, and can simply be placed into a container holding the liquid to be tested. A liquid sample cell is also available for use with the system. This set-up seals the Hot Disk sensor in the middle of a small volume (2-3 ml) of the liquid to tested in order to obtain maximum probing depth for the sample size. This is an invaluable apparatus for clients looking to test fluids that are expensive to produce.
Figure 2. Diagram of the liquid sample cell available for use with the Hot Disk TPS system. The two pieces seal around the Hot Disk sensor (right) to obtain maximum probing depth during the test.
For this research, Gustavsson and Gustafsson (2006) measured the thermal conductivity of milk samples with fat contents of >0.1, 0.5, 1.5 and 3.0%. A Hot Disk sensor (#7577) with a 4 mm diameter was dipped into each sample. Care was taken to ensure that all tests occurred at the same temperature. Power output was set at 0.15 W, and test times were limited to 0.96 seconds to ensure convection did not affect the results. Two measurements were performed on each sample. The authors determined that the total temperature rise in the milk during the test was 1.7 K, and the probing depth (the distance the heat from the sensor travels in the sample) was 0.8 mm.
Results indicated that thermal conductivity and the fat content had a linear correlation at fat contents over 0.5% (Figure 3). The ability of the Hot Disk TPS to detect minute changes in thermal conductivity enables it to differentiate between the fat content present in the milk. Gustavsson and Gustafsson (2006) determined that their system was capable of measuring the milk fat content with a sensitivity of 0.09%.
Figure 3. Thermal conductivity results of milk with various fat contents obtained by Gustavsson and Gustafsson (2006) using the Hot Disk TPS.
This research illustrates how the Hot Disk TPS excels in the thermal conductivity measurement of liquids. This ability sets it apart in a field where many methods struggle to produce a measurement before convection begins to affect results. The sensitivity with which the TPS system performs tests makes it a powerful tool for those looking to detect small changes for quality control purposes.