In order to determine long-term changes in lake temperatures as well as the timing of stratification and mixing events, thermistors have been deployed at UK UWMN lake sites since 1999. Thermistor pairs are used, one being positioned 1.5m from the lake bottom and the other 1m from the water surface. Both are attached to one of the UK UWMN sediment trap arrays at each site, and are downloaded annually in August. Historically Vemco Minilog12-T devices were used, recording every two hours, but as these units have a reliable non-replaceable battery life of four years the Network has gradually replaced them with Tinytag Aquatic 2 units from Gemini Data Loggers recording at a half-hourly frequency.
In April 2010 a thermistor chain was installed at Llyn Llagi and in summer 2013 thermistor chains were installed at all other lake sites.