There was this cool thermometer in my house growing up. It was a bunch of coloured glass bulbs that would float in water with a liquid in them. You would read the temperature by looking between them to see where the biggest gap was. There may not be a free icon or graphic like that below, but I'm sure you'll find one that is free and available for your project or design.
All the icons and illustrations above can be used for both Personal & Commercial purposes.