INSULATION PERFORMANCE TEST
To test how well each flask retained heat, we firstly primed each flask for 5 minutes with boiling water. We then filled each flask with freshly boiled water. We allowed some initial cooling to take place for a minute or two until each flask contained water at 90°C. We then fitted the screw tops and cups, if provided, and re-opened the flasks after three-, six- and nine-hour intervals. During the test the ambient air temperature was 18°C and there was no wind. The flasks were rested on a table and not insulated by clothing in a rucksack, as this could create a wide variation in performance.