You can change the global unit of measurement in your DIgiQuatics account on the Settings page, under General Settings. Here you can change the Unit of Measurement to either Imperial or Metric. This will change the units displayed in the Chemical Records module where water and air temperature are recorded. Changing this setting will also change the measurements displayed on all historical Chemical Records as well. 

Important: This will change the default unit of measurement for all users on your DigiQuatics account.