This is the first post in a series of blog posts where we investigate the thermal conductivity of liquids found in many households. Part two and part three of the series are coming soon. For these tests the Thermtest THW-L3 sensor for the portable Measurement Platform Two, MP-2, is used. With these experiments we show how easy and fun testing thermal conductivity of various fluids with the Transient Hot Wire method can be. The MP-2 is affordable and portable, yet powerful and versatile. We also hope this can be an inspiration for research and teaching in the field of thermal conductivity alike!
In this first instalment we look at two liquids found in the Thermtest Europe kitchen: Vegetable milk made from rye Totally Nordic Rye by Alpro and Italian olive oil Farchioni.
Both samples were tested at a room temperature of approximately 22 °C and the tested volumes were roughly 15 ml. The instrument configuration was MP-2 and THW-L3 sensor, adhering to ASTM standard D7896-19. Test time was 1 s and the heating power as suggested by the instrument software’s automatic power detection function.
The MP-2 instrument with the THW-L3 thermal conductivity sensors is easy to work with. The sample is poured into the sample holder and the probe is dipped in the fluid. It’s important to make sure that the entire heating wire is below the sample surface top not partly test air. The on-board software then assists the user to select the right heating power to increase the temperature of the heating element just the right amount for any given liquid. The actual test time of a Hot Wire experiment with the MP-2 device is only 1 second, but before the transient test if performed temperature stability is monitored to confirm good data quality. In total, you will have a result in less than one minute from starting the test. The instrument can also be programmed to automatically perform several tests in succession, something that is suggested as it allows the calculation of standard deviation and detection of trends in the resulting thermal conductivity data. In these experiments each test was repeated three times and we are presenting the average value and the standard deviation in percent.
Table 1. Thermal conductivity of Olive Oil and Rye Milk. Data as measured with the MP-2 handheld and THW-L3 sensor. Presented thermal conductivity data are average values of three repeated tests and the related standard deviation.
Distilled Water (Control)
In addition to the rye milk and olive oil, distilled water was tested to confirm the calibration of the sensor and the quality of the data. The measured thermal conductivity of the water is within 1 % of the theoretical value.
As seen, they rye milk, which is of course water based, has a thermal conductivity not far from that of water. The oil on the other hand has a much lower thermal conductivity, which is common for most oils. The standard deviation within each set of tests is close 0.5 % which confirms the excellent precision of the MP-2 instrument.
The MP-2 is as the name indicates a platform for several transient methods of testing thermal conductivity. The methods currently featured include Transient Plane Source (TPS), Transient Line Source (TLS) and Transient Hot Wire (THW).
The TPS method is represented by two sensor types, TPS-4 for testing thermal conductivity and TPS-EFF for testing thermal effusivity. The former operates as described in ISO 22007-2 with some modifications to fit the portable format and the latter operated in accordance with ASTM D7984.
The TLS method is represented by no less than four needle type sensors. TLS-100, TLS-50, TLS-150 and TLS-100 vCp. TLS-100 is the classic 100 mm long ASTM D5334 probe for testing soil. TLS-50 allows convenient testing of hard samples through its 50 mm length and increased 4 mm diameter. TLS-150 is 150 mm long and adds the IEEE 442-2017 standard for soil testing while also following ASTM D5334. Finally, the unique TLS-100 vCp measures thermal diffusivity in addition to thermal conductivity, which in turn allows the calculation of the specific heat per unit volume – all this by still following ASTM D5334.
The THW method is represented by two probes, the THW-L3 for liquids, pastes, and powders and the THW-S for testing insulation, soft materials, and powders.