Method for controlling speeds of compressors (V 1, V 2, V 3, V 4) arranged in series that are configured for compressing a fluid, in particular helium, comprising the steps of: - prescribing a setpoint entry pressure (p setpoint) that the fluid is meant to have at an input of the compressor (V 1) arranged furthest upstream, - capturing an actual entry pressure (p actual) of the fluid at the input of the compressor (V 1) arranged furthest upstream, - capturing an actual exit pressure (p 4) of the fluid at an output of the compressor (V 4) arranged furthest downstream, - ascertaining an actual total pressure ratio (π actual), wherein the actual total pressure ratio (π actual) is consistent with the quotient of the actual exit pressure (p 4) of the compressor (V 4) arranged furthest downstream and the actual entry pressure (p actual) of the compressor (V 1) arranged furthest upstream, - determining a proportional integral value from the difference between the actual entry pressure (π actual) and the setpoint entry pressure (p setpoint), - determining a capacitance factor from the proportional integral value and the actual total pressure ratio (π actual), - ascertaining a model total pressure ratio (π model) from the actual total pressure ratio (π actual) and the capacitance factor, - determining a reduced setpoint speed (n 1, n 2, n 3, n 4) for each compressor (V 1, V 2, V 3, V 4), wherein the respective reduced setpoint speed (n 1, n 2, n 3, n 4) is determined as a function value of a control function associated with the respective compressor (V 1, V 2, V 3, V 4), which control function assigns a reduced setpoint speed (n 1, n 2, n 3, n 4) to each pair of values comprising capacitance factor and model total pressure ratio (π model), - converting the reduced setpoint speeds (n 1, n 2, n 3, n 4) into setpoint speeds and adjusting the speed of each compressor (V 1, V 2, V 3, V 4) to the respectively associated setpoint speed. - prescribing a setpoint entry pressure (p setpoint) that the fluid is meant to have at an input of the compressor (V 1) arranged furthest upstream, - capturing an actual entry pressure (p actual) of the fluid at the input of the compressor (V 1) arranged furthest upstream, - capturing an actual exit pressure (p 4) of the fluid at an output of the compressor (V 4) arranged furthest downstream, - ascertaining an actual total pressure ratio (π actual), wherein the actual total pressure ratio (π actual) is consistent with the quotient of the actual exit pressure (p 4) of the compressor (V 4) arranged furthest downstream and the actual entry pressure (p actual) of the compressor (V 1) arranged furthest upstream, - determining a proportional integral value from the difference between the actual entry pressure (π actual) and the setpoint entry pressure (p setpoint), - determining a capacitance factor from the proportional integral value and the actual total pressure ratio (π actual), - ascertaining a model total pressure ratio (π model) from the actual total pressure ratio (π actual) and the capacitance factor, - determining a reduced setpoint speed (n 1, n 2, n 3, n 4) for each compressor (V 1, V 2, V 3, V 4), wherein the respective reduced setpoint speed (n 1, n 2, n 3, n 4) is determined as a function value of a control function associated with the respective compressor (V 1, V 2, V 3, V 4), which control function assigns a reduced setpoint speed (n 1, n 2, n 3, n 4) to each pair of values comprising capacitance factor and model total pressure ratio (π model), - converting the reduced setpoint speeds (n 1, n 2, n 3, n 4) into setpoint speeds and adjusting the speed of each compressor (V 1, V 2, V 3, V 4) to the respectively associated setpoint speed.