I'm still a bit confused. Suppose you have another table, call it temperatures with columns id and temperature, where every row contains only a valid temperatur

I'm still a bit confused. Suppose you have another table, call it temperatures with columns id and temperature, where every row contains only a valid temperature (no NULL records), and you have a main logging table with date and temperature_id so that you can join on temperature_id = temperatures.id. This seems to be what you mean, with a canonical measurement record table related via the temperature_id foreign key. But then if your sensor fails to record a measurement don't you end up with NULL for that row's temperature_id?