A refrigeration system , the system comprising: a compressor ; a refrigeration process in fluid communication with the compressor , the refrigeration process including a high pressure line on a high pressure side of the refrigeration system between the compressor and an evaporator , a low pressure line on a low pressure side of the refrigeration system in a refrigerant return path between the evaporator and the compressor , and at least one heat exchanger cooling refrigerant in the high pressure line from refrigerant in the low pressure line; an expansion device receiving high pressure refrigerant from the refrigeration process ; and a bypass circuit for bypassing at least a portion of the refrigeration process , said at least a portion comprising at least one heat exchanger in the high pressure line, the bypass circuit comprising an on/off bypass valve and a flow metering device or a proportional control bypass valve , and being connected to flow refrigerant into a point in the refrigeration process to blend the higher pressure warm refrigerant with low-pressure refrigerant to warm the low pressure flow; the system using a mixed refrigerant to provide refrigeration at temperatures below 183K, the refrigeration system being characterised by further comprising: a control circuit that receives an input signal from an object or fluid that is being cooled by the refrigeration system ; and the bypass circuit being configured to control a temperature of the object or fluid by: in response to receiving a control signal from the control circuit controlling the bypass valve to control the amount of refrigerant that flows through the bypass circuit such that the system performs closed-loop control of the temperature of the object or fluid that is being cooled by the refrigeration system . a compressor ; a refrigeration process in fluid communication with the compressor , the refrigeration process including a high pressure line on a high pressure side of the refrigeration system between the compressor and an evaporator , a low pressure line on a low pressure side of the refrigeration system in a refrigerant return path between the evaporator and the compressor , and at least one heat exchanger cooling refrigerant in the high pressure line from refrigerant in the low pressure line; an expansion device receiving high pressure refrigerant from the refrigeration process ; and a bypass circuit for bypassing at least a portion of the refrigeration process , said at least a portion comprising at least one heat exchanger in the high pressure line, the bypass circuit comprising an on/off bypass valve and a flow metering device or a proportional control bypass valve , and being connected to flow refrigerant into a point in the refrigeration process to blend the higher pressure warm refrigerant with low-pressure refrigerant to warm the low pressure flow; the system using a mixed refrigerant to provide refrigeration at temperatures below 183K, the refrigeration system being characterised by further comprising: a control circuit that receives an input signal from an object or fluid that is being cooled by the refrigeration system ; and the bypass circuit being configured to control a temperature of the object or fluid by: in response to receiving a control signal from the control circuit controlling the bypass valve to control the amount of refrigerant that flows through the bypass circuit such that the system performs closed-loop control of the temperature of the object or fluid that is being cooled by the refrigeration system . <EOS>