Bimetal thermometers are called so because there are two pieces of metal inside the shaft. As the temperature rises, the difference of space between the two pieces is measured and this translates into a temperature reading. These thermometers come in different face and stem sizes. The one that is best for you will depend on the visual and space requirements you have for your particular project. For instance, Dura Choice, which is one of the brands we carry, offers 3" and 5" face sizes. They offer stem sizes that range from 2 1/2" to 9" - which should give you enough variety to choose from for your plans.