This is the third and final part in this set of blog posts where the thermal conductivity of liquids found in many households and workplaces is investigated. Part one is found here and part two is found here. For all experiments, the Thermtest THW-L3 sensor in combination with the portable Measurement Platform Two, MP-2, is used. In doing these experiments, we hope to convey the fleeing that it can be both straight forward and rewarding to test thermal conductivity using the Transient Hot Wire method. The cost-effective and portable MP-2 device is capable, flexible, and well adapted for research, teaching, and product development in the field of thermal conductivity alike!
In this third episode, we analyse two liquids found in the Thermtest Europe workshop: 30 % Ethylene glycol mixture DOWTHERM™ SR-1 which is a heat transfer fluid, and white spirit/mineral spirits Landora which is used when cleaning paint brushes or to dilute oil-based paints.
All measurements were carried out at room temperature, approximately 22 °C, and the tested sample volumes were close to 15 ml. The MP-2 handheld was fitted with the THW-L3 Hot Wire sensor to operate in accordance with ASTM D7896-19. The onboard software automatically detected optimum power setting for the test and all other setting were default.
From starting a test, results are made available on-screen within one minute. During this time, the sample temperature stability is measured, transient test performed, and results calculated. It’s also easy to program the instrument to perform several tests with a waiting time between each test. This is a great way to determine if a fluid is stable or to generate statistics.
The MP-2 can also be connected to, and operated from, a PC. The MP-2 package includes a companion software, which makes it easy to export data to excel and control the device.
Table 1. Thermal conductivity of 30 % ethylene glycol mixture (SR-1) and white spirit. 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.
30 % Ethylene Glycol
Distilled Water (Control)
In addition to testing the ethylene glycol mixture and white spirit, distilled water was analysed to confirm sensor calibration and the data quality. The thermal conductivity measured in the distilled water was within 1 % of the expected value.
The thermal conductivity of pure ethylene glycol is 0.251 W/m/K at room temperature, the measured value of the SR-1 mixture is placed in between this value and that of water, with a shift towards the water. This is the expected behaviour.
The mineral spirits, being a hydrocarbon, is expected to have a low thermal conductivity which is here confirmed. Many hydrocarbons such a fossil fuels, oils, etc. are found in this thermal conductivity range.
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 here discussed THW-L3 for liquids, pastes, and powders and the THW-S for testing insulation, soft materials, and powders.