an apparatus and method for automatic operation of a refrigeration system to provide refrigeration power to a catheter for tissue ablation or mapping . the primary refrigeration system can be open loop or closed loop , and a precool loop will typically be closed loop . equipment and procedures are disclosed for bringing the system to the desired operational state , for controlling the operation by controlling refrigerant flow rate , for performing safety checks , and for achieving safe shutdown . the catheter - based system for performing a cryoablation procedure uses a precooler to lower the temperature of a fluid refrigerant to a sub - cool temperature at a working pressure . the sub - cooled fluid is then introduced into a supply line of the catheter . upon outflow of the primary fluid from the supply line , and into a tip section of the catheter , the fluid refrigerant boils at an outflow pressure of approximately one atmosphere , at a temperature of about − 88 ° c . in operation , the working pressure is computer controlled to obtain an appropriate outflow pressure for the coldest possible temperature in the tip section .