3He refrigerator using 3He as coolant is the most convenient, as well as one of the most popular methods to reach temperatures below 1.0 K. Similar to the 1.5 K system (pumped 4He) the basic working principle of 3He refrigerator is to reduce the vapor pressure of liquid 3He to reach lower temperatures.

Liquid 3He is not commercially available. The refrigerator needs to first generate liquid 3He by itself before the evaporative cooling. In other words, all 3He refrigerators include a two-step operations: step #1 is to condense (i.e., liquefy) 3He, and step #2 is to reduce the vapor pressure of liquid 3He and to cool it down to lower temperatures.

Selected properties of 3He are presented in this chapter. They provide a better understanding of the working principles of He-3 cryostat.

Two basic types of 3He refrigerators exist: know as the Single Shot system, and the other is the Continuous Flow (or referred to as Recirculating) system which can be operated with longer (or unlimited) holding time.

Great details of the Single Shot system such as its working principles, structures, fabrications, operation process, examples with typical performance, etc. are described.

Features unique to the Continuous Flow 3He cryostat such as the 3He condensing unit, hermetic pumps, etc. are briefly presented (more detailed description regarding the condensing unit is presented in Chapter 11: Cryogen-free dilution refrigerator).

Examples of Continuous Flow 3He cryostat with different 3He condensing unit are presented.