182 which measures the rate of flow of cryogen through the line 124 as described in greater detail below this flow rate information is one component upon which the pressure regulation for system 100 is based optionally if the flow rate is too high for the desired pressure in system 100 to be maintained and or for effectively cooling tip 124 gaseous cryogen may be exhausted through a relief valve 107 the gaseous cryogen if not exhausted then passes to the pump 134 which is controlled to maintain the desired pressure in system 100 pump 134 pumps the gaseous cryogen to gaseous cryogen source 102 through the line 132 and then through the two way valve 106 the desired gaseous state of the cryogen upon entering pump 134 is maintained by the heater 122 as previously described optionally the gaseous cryogen may be pumped through the two way valve 103 to the line 131 and hence through the two way valve 130 to the liquid cryogen source 104 through the line 129 this optional flow path may be advantageous in maintaining the desired pressure differential between the liquid cryogen source 104 and the gaseous cryogen source 102 as described in greater detail below system pressure is measured at liquid cryogen source 104 by the pressure gauge 181 and at the gaseous cryogen source 102 by the pressure gauge 184 preferably pressure is higher at the source of gaseous cryogen 102 than at the source of liquid cryogen 104 the pressure regulator 101 and the pump 134 control the overall pressure of system 100.