Thermal conductivity of water

 According to literature1 the thermal conductivity of water is 0.598 W/m·K at 20 °C. Here the limits and accuracy of the Thermtest THW-L2 thermal conductivity meter for fluids is explored by testing a selection of water samples.

For calibrating the THW-L2, deionized ultrafiltered (DIUF) water is used. With a calibrated instrument the thermal conductivity of a set of water samples was tested: DIUF water, distilled water, tap water, a 5 mass% sugar solution, and a 5 mass% salt solution.

Retesting DUIF water certifies that the calibration is good and provides a baseline for the precision of the method. Distilled water is readily available in most hardware stores and can potentially be used as a low-cost calibration material. Tap water varies from location to location and the here used water is from Veddige, Sweden. It is excellent drinking water, not chlorinated and has a hardness of 4 °dH and pH 8.3. It was tested twice, first freshly poured and then again after settling overnight in an open container. Sugar and salt will impact water in different ways and solutions of such are interesting examples to study.

All testing was performed with a Thermtest THW-L2 water thermal conductivity, which is an advanced, yet affordable instrument for testing the thermal conductivity of liquids, pastes and gels with a thermal conductivity range of 0.01 to 2 W/m·K in accordance with ASTM standard D7896-19. For full details, please see the THW-L2 product page.

Sample temperature was controlled with a dry bath, which is operated through the THW software.

The THW-L2 sensor used for the experiments consist of a bare 60 mm alumel wire and a pt-100 temperature sensor. During a test, the probe is fully immersed in the fluid sample and the thermal conductivity is derived from the temperature increase of the wire during the measurement time, which is 1 second long. The heating power is automatically detected and optimized by the instrument software.

All tests were performed at 20 °C and the sample volume used was ca 20 ml for each sample. For each specimen, the testing was repeated 6 times with 10 minutes between tests, to demonstrate the stability in the results.

The Thermal conductivity of water of different quality. Presented data are average values from six repeated tests on each sample. Standard deviation is close to 0 %. Full data sets showing the excellent precision of the THW-L2 are found in the appendix below.

As can be seen, DIUF, distilled and tap water show very similar results. Freshly poured tap water shows a slightly lower value, likely due to air trapped in the fluid. The value then goes up after settling overnight, allowing the gas to leave the sample.

Adding sugar to water clearly reduces the conductivity, while salt has a more limited impact. It should here be noted that care must be taken when a salt solution is tested. With the addition of salt, the electrical conductivity of the water is greatly increased. This can interfere with the result if exposed areas of the probe, i.e. temperature sensor and solder joints, are not electrically insulated from the sample.

When testing thermal conductivity with the THW-L2, results are presented directly in the software as seen in the picture below. In the software, the testing parameters are presented along with the results and the quality of the data is automatically controlled. To allow the user to confirm good data or to detect anomalies, all data is graphically depicted with both transient charts and a residual plot. The residual plot shows if the experimental is correctly fitted to a straight line, which is a requirement for acceptable results.

The combination of a far superior thermal conductivity, the ability to fit more copper coil into a cylinder than a stainless steel coil and the ability to incorporate a finned coil to further increase the surface area, means that copper is a dramatically more efficient material to be used for hot water cylinders.

So when you are examining how you can enhance the environmental aspect of your next project and also give your client a low running cost solution, remember to consider the hot water system and ensure that you are using the best and most efficient material for the job.