Conventional single compressor, mechanical refrigeration system condensing units are capable of achieving temperatures of about –40° Celsius. If lower temperatures are required then cascade refrigeration systems must be used. A two-stage cascade system uses two refrigeration systems connected in series to achieve temperatures of around –85° Celsius. There are single compressor systems that can achieve temperatures colder than -100° Celsius but they are not widely used. These systems are sometimes referred to as "auto cascading" systems. The main disadvantage of the auto cascading system is that it requires the use of a proprietary blend of refrigerant. This characteristic generates three service-related problems. First, since the refrigerant blend is made up of different types of refrigerant with different boiling points, a leak in the system can cause the loss of only some of the refrigerant making up the blend, resulting in an imbalance in the ratio of the remaining refrigerants. In order to return the system to proper operation, all of the remaining refrigerant must be replaced with a new and potentially costly charge to insure a proper blend ratio. Second, since the blend is proprietary, it is not readily available from the traditional refrigerant supply sources and therefore may be hard to obtain and costly.