When to add milk? Using a coffee cooling experiment to teach measurement device selection

A key challenge in physics education is enabling students to select appropriate measurement devices and critically evaluate measurement data while accounting for underlying measurement principles. This is becoming increasingly important, as modern measurement devices often operate as ‘black boxes’ that can obscure limitations and sources of uncertainty. Despite the importance of measurement to physics education, there exist few practical activities that support students to work through the process of how to measure real-world phenomena. In this paper, we present an experiment for students to plan and carry out that investigates the cooling of coffee when milk is added at different times. Two conditions (milk addition after 30 s and 300 s) are compared over a longer period. The temperature is recorded simultaneously using four different measurement devices (a conventional liquid thermometer, two digital probes–one with a digital display and one app-based–and a thermal imaging camera), which differ in their measurement method, among others. The resulting measurement data indicate that early milk addition leads to a slightly higher final temperature but also differ in the shape and fluctuations across the measurement devices. From a pedagogical perspective, this experiment can be used in physics classrooms to discuss how measurement devices influence both the presentation and interpretation of data. By comparing multiple datasets of the same phenomenon, students can reflect on how the choice of measurement method can affect the conclusions they draw from analysing measurement data. In this way, the setup also promotes critical thinking and experimental competence. We further outline ways of implementing the setting in physics classrooms and report insights from classroom practice, which indicate that students often find it challenging to base the selection and evaluation of measurement devices on the measurement system itself.