If you just want on off control you could rig up some type of magnetic float in your sight glass that would pick up a read switch mounted on the outside of the sight glass, just slide the switch up and down the sight glass to set your level. You could have multiple switches for different levels...
You are looking for a temperature data logger. The make them that are battery powered and plug into your USB port to download the data. Prices start at around $50 for the simple ones that record abient temps with an internal probe. Here is one that has a thermcouple probe, for $90...