Red spirit-filled thermometers provide increased safety and economic pricing. They contain a kerosene-based column with red dye, which is significantly safer than mercury. These thermometers all have 6 to 7mm outer diameters, and the bulbs have equal or smaller diameters than the stems. Partial immersion thermometers are useful if you have a shallow container which holds the substance to be measured. These thermometers have a small indicator ring that denotes the depth to which the thermometer should be submerged to obtain an accurate temperature reading. Full immersion thermometers need to be submerged deep enough so the desired temperature is at the meniscus. For example, if you wish to measure the temperature of a hot water bath to make sure it is 100°C, then you need to submerge the thermometer up to the 100°C mark.