Thermometers are small, but important items in a darkroom. You can buy old style glass and mercury thermometers or newer digital models. Whatever you buy, make sure it is accurate and reliable as you need to know the temperature of the developing chemicals you are using, along with the water. Some thermometers read the temperature in Celsius and other models read the temperature in Fahrenheit. You can also get thermometers that will read the temperature in both units of measurement. Thermometers are also available in various sizes, shapes, materials and styles. A good thermometer should be fog and leak proof and anticorrosive. You should be able to buy a darkroom thermometer from as low as bout $10.