When you find delicious warmed sake is gentle to the body and relaxes your mind, then you will certainly be particular about the utensils to warm sake. A sake warmer is used in a broad sense to include a tokkuri and a chirori (sake tanpo), but in general, it means a machine or a device to warm a tokkuri and a chirori.
The traditional sake warmers used to be mainly metallic as to be immersed in hot water. Now there are a variety of sake warmers, such as the electric one to keep the water warm for a long period of time, a desktop type which integrates with a tokkuri or a katakuchi server to raise the temperature automatically to the designated one, a compact type about the same size as a tokkuri, and the one using a microwave.
The temperature of warmed sake depends on the type of sake and your taste, but first you should not boil the water. If you are not used to warming sake, keep an eye on the temperature not allowing to exceed 60 degrees Celsius. Prepare a thermometer if you want to make slight adjustments. However, the best thing is to have a taste.