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 (202, 206, 208, 212) cooling refrigerant in the high pressure line from refrigerant in the low pressure line; an expansion device (124, 126) 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 (206, 208, 212) 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 (505,605) 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 (202, 206, 208, 212) cooling refrigerant in the high pressure line from refrigerant in the low pressure line; an expansion device (124, 126) 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 (206, 208, 212) 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 (505,605) 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.