A method for operating a refrigeration unit comprising a compressor, an economizer solenoid valve and a flash tank, the method comprising: measuring a first characteristic of the refrigeration unit, wherein the first characteristic comprises a flash tank pressure (P ft), a compressor suction pressure (P suc), and a discharge port pressure (P dis); calculating a compressor middle stage pressure (P mid) based on the compressor suction pressure (P suc) and the discharge port pressure (P dis); determining if a first comparison (C 1) and a second comparison (C 2) are satisfied based on the first measured characteristic and the calculated compressor middle stage pressure (P mid); characterized in that: the economizer solenoid valve is opened when both the first comparison and the second comparison are satisfied, the economizer solenoid valve is closed when either the first comparison or the second comparison is not satisfied, and the first comparison (C 1) is a comparison of the compressor middle stage pressure (P mid) against the critical pressure (P crit) of the refrigerant modified by a first pressure constant (dP 1) wherein P mid < (P crit - dP 1), and the second comparison (C 2) is a comparison of the flash tank pressure (P ft) against the critical pressure (P crit) of the refrigerant, wherein the first comparison is satisfied when the middle stage pressure (P mid) is below the critical pressure (P crit) of the refrigerant modified by the first pressure constant (dP 1) and the second comparison is satisfied when the flash tank pressure (P ft) is below the critical pressure (P crit). measuring a first characteristic of the refrigeration unit, wherein the first characteristic comprises a flash tank pressure (P ft), a compressor suction pressure (P suc), and a discharge port pressure (P dis); calculating a compressor middle stage pressure (P mid) based on the compressor suction pressure (P suc) and the discharge port pressure (P dis); determining if a first comparison (C 1) and a second comparison (C 2) are satisfied based on the first measured characteristic and the calculated compressor middle stage pressure (P mid); characterized in that: the economizer solenoid valve is opened when both the first comparison and the second comparison are satisfied, the economizer solenoid valve is closed when either the first comparison or the second comparison is not satisfied, and the first comparison (C 1) is a comparison of the compressor middle stage pressure (P mid) against the critical pressure (P crit) of the refrigerant modified by a first pressure constant (dP 1) wherein P mid < (P crit - dP 1), and the second comparison (C 2) is a comparison of the flash tank pressure (P ft) against the critical pressure (P crit) of the refrigerant, wherein the first comparison is satisfied when the middle stage pressure (P mid) is below the critical pressure (P crit) of the refrigerant modified by the first pressure constant (dP 1) and the second comparison is satisfied when the flash tank pressure (P ft) is below the critical pressure (P crit). the economizer solenoid valve is opened when both the first comparison and the second comparison are satisfied, the economizer solenoid valve is closed when either the first comparison or the second comparison is not satisfied, and the first comparison (C 1) is a comparison of the compressor middle stage pressure (P mid) against the critical pressure (P crit) of the refrigerant modified by a first pressure constant (dP 1) wherein P mid < (P crit - dP 1), and the second comparison (C 2) is a comparison of the flash tank pressure (P ft) against the critical pressure (P crit) of the refrigerant, wherein the first comparison is satisfied when the middle stage pressure (P mid) is below the critical pressure (P crit) of the refrigerant modified by the first pressure constant (dP 1) and the second comparison is satisfied when the flash tank pressure (P ft) is below the critical pressure (P crit).