Maximum rates of cooling by three programmable freezers, and the potential relevance to sperm cryopreservation.

Maximum rates of cooling for the Asymptote EF 100 and the Cryologic CL8800 temperature controller with either a standard or fast chamber were determined and viewed in the context of sperm cryopreservation. All three systems use liquid nitrogen to cool the plate or chamber which would hold the sample, opposed by a variable amount of heat from an internal heater. Maximum rates of cooling for all systems were a function of the temperature gradient between the liquid nitrogen and the plate/chamber and at a plate/chamber temperature of 15 degrees C were 16.5 degrees C/min, 13.3 degrees C/min and 8.0 degrees C/min for the Asymptote EF100, Cryologic fast and slow chambers respectively. These machines are not suited to the freezing of sperm from species requiring rapid rates of cooling, an important consideration when planning to purchase a piece of equipment for this application, and scientists are advised to discuss specifically their requirements with prospective suppliers.