fig1 schematically illustrates a cryogenic refrigeration apparatus comprising three refrigerators / liquefiers ( l / r ) arranged in parallel to cool one and the same application 1 . conventionally , each refrigerator / liquefier l / r comprises a working circuit for a working gas which is equipped with at least one working gas flow control valve . each refrigerator / liquefier comprises its own station 2 for compressing the working gas and its own cold box 3 intended to cool the flow 30 of working gas leaving the compression station 2 to a cryogenic temperature at least close to its liquefaction temperature . the flows 30 of working gas cooled by each of the respective cold boxes 3 of the refrigerators / liquefiers l , r are mixed and then placed in a heat exchange relationship with the application 1 in order to give up frigories thereto . the cold working gas having exchanged heat with the application 1 is then split into several return flows 31 distributed respectively across the compression stations 2 . the parallel refrigerators / liquefiers l / r use a working gas of the same nature such as pure gaseous helium . the apparatus 100 preferably comprises electronic control logic 50 comprising for example a microprocessor ( a computer and / or controller ). the electronic logic 50 is connected to measurement members for simultaneous measurement , for each of the refrigerators / liquefiers l / r , of the instantaneous value of at least one and the same operating parameter regarding the working gas in the working cycle of each of the refrigerators / liquefiers l / r . for the sake of simplicity , fig1 does not depict these measurement members ( examples thereof will be illustrated in fig2 to 6 ). the at least one operating parameter measured for each refrigerator / liquefier l / r preferably comprises at least one out of : a flow rate of the return flow of working gas returning to the compression station ( after exchanging heat with the application or a return flow of working gas returning directly to the compression station without passing via the application 1 or certain parts of the cold box 3 ), a flow rate of the flow of cooled working gas at the outlet of the cold box ( after having left the compression station ), a differential in temperature of the working gas between , on the one hand , the flow of working gas in the cold box ( heading toward the application ) and , on the other hand , the return flow of working gas returning to the compression station ( from the application ). the electronic logic 50 is configured ( for example programmed ) to perform real - time calculation of the dynamic mean value of the at least one operating parameter for all the refrigerators / liquefiers l / r and for performing real - time control of the at least one working - gas flow control valve of at least one refrigerator / liquefier l / r as a function of the difference between the instantaneous values of the parameter with respect to said dynamic mean value . more specifically , the electronic logic is configured to cause said instantaneous values of said operating parameter of the various refrigerators / liquefiers r / l to converge toward this dynamic mean value . what that means to say is that each refrigerator / liquefier l / r is controlled in its working cycle as a function of an operating mean of the whole set of refrigerators / liquefiers l / r , so as to cause all the refrigerators / liquefiers l / r to converge toward this mean . this adjustment may be implemented via controllers of the “ proportional integral ” ( pi ) type for controlling the working - gas circuits . for preference , the apparatus performs real - time control of the at least one working - gas flow control valve of at least one refrigerator / liquefier ( l / r ) as a function of the difference between the instantaneous values of the parameter with respect to said dynamic mean value , so as to cause said instantaneous values of said operating parameter of the various refrigerators / liquefiers r / l to converge toward this dynamic mean value . various examples of the control of the apparatus will be described with reference to fig2 to 6 respectively . all or some of these various examples may be implemented cumulatively or alternatively in order to adjust the operation of such an apparatus 100 . as partially illustrated in fig2 , the compression station 2 of each refrigerator / liquefier may comprise two compressors 12 , 22 arranged in series on the working circuit and referred to respectively as the “ low - pressure compressor ” 12 and the “ medium - pressure compressor ” 12 . the low - pressure compressor 12 receives the relatively hot working gas returning at low pressure ( return flow 31 ) having passed or not passed through the cold box 3 . each compression station 2 comprises a bypass circuit 14 for selectively bypassing the low - pressure compressor 12 and which is equipped with a variable - opening controlled bypass valve 4 . the apparatus comprises , for each of the refrigerators / liquefiers l / r , a sensor 13 for measuring the operating parameter consisting of the instantaneous value of the flow rate q of the return flow 31 of working gas returning to the compression station 2 . this measurement sensor 3 is , for example , situated within the cold box 3 , upstream of one or more exchangers 26 which both cool toward the working gas toward the application and heat the working gas returning toward the compression station 2 . the electronic logic 50 may perform real - time calculation of the dynamic mean value of this operating parameter for all the refrigerators / liquefiers l / r . the electronic logic 50 performs real - time control of the opening / closing of each bypass valve 14 as a function of the difference between the instantaneous values of the operating parameter of the refrigerator / liquefier concerned so as to cause said instantaneous values of said operating parameter of the various refrigerators / liquefiers r / l to converge toward this dynamic mean value . for example , the opening / closing of each bypass valve 14 is controlled according to a pressure set point cp according to a formula of the type cp = a - b . δq , where a is a predetermined pressure value , b is a predetermined coefficient ( dimensions = pressure / flow rate ) and δq is the differential ( dimensions = flow rate ) between , on the one hand , the dynamic mean value of the flow rate of the three coolers and , on the other hand , the instantaneous flow rate of the refrigerator / liquefier concerned . in addition , each refrigerator / liquefier l / r may comprise a sensor 15 for measuring the temperature differential dt = t 31 - t 32 of the working gas between the return flow 31 ( returning to the compression station ) and the “ outbound ” flow 32 ( toward the application 1 ) which are situated in the cold box ( 3 ) in a part of the circuit that has one and the same determined temperature range . the expression “ one and the same temperature range in the cold box ” means points on the working circuit at which the outbound flow 32 ( toward the application that is to be cooled 1 ) and return flow 31 ( toward the compression station 2 ) are situated at the same level with respect to the cooling exchangers of the cold box 3 ( for example , the two measurement points are situated in legs of the circuit which are situated between two same cooling exchangers ). what that means to say is that the two points on the circuit have relatively similar temperatures , for example differing by just a few degrees kelvin ( typically between 0 . 1 and 4 ° k . of difference ). the outbound flow 32 is , for example , the flow of working gas leaving a cooling exchanger of the cold box ( for example at the outlet of the first heat exchanger which cools the working gas after it has passed through the compression station 2 ). the return flow 31 in the same temperature range is the part of the working circuit in which the working gas returns toward the compression station 2 before entering this same heat exchanger . according to one advantageous feature , the control of each bypass valve 14 may be corrected as a function of the discrepancy between said temperature differential dt = t 31 - t 32 for the refrigerator / liquefier l / r concerned with respect to the mean of said temperature differential dt = t 31 - t 32 calculated for all of the refrigerators / liquefiers l / r . this temperature differential dt = t 31 - t 32 is indicative of the imbalance in the flow rates of working gas between the return flow 31 ( toward the compression station ) and the outbound flow 32 ( toward the application 1 ). for example , the opening of each bypass valve 14 may be increased when the temperature differential dt = t 31 - t 32 for the refrigerator / liquefier l / r concerned increases ( in terms of absolute value ) with respect to the mean of said temperature differential . this control will have the effect of reducing the imbalance in the flow rates of the working gas between the return flow 31 ( toward the compression station ) and the outbound flow 32 ( toward the application 1 ). as illustrated schematically in fig3 , at the outlet of the compression station 2 , each refrigerator / liquefier l / r may , on the outlet pipe 30 , comprise a variable - opening controlled outlet valve 11 . in addition , each refrigerator / liquefier l / r may comprise a measurement sensor 16 for measuring the operating parameter consisting of the instantaneous value of the flow rate of the flow 30 of gas at the outlet of the compression station 2 . as previously , the electronic logic 50 may be configured to perform real - time calculation of the dynamic mean of this operating parameter for all the refrigerators / liquefiers l / r . the electronic logic 50 may perform real - time control of the opening / closing of each outlet valve 11 according to the difference between the instantaneous values of the operating parameter of the refrigerator / liquefier concerned so as to cause said instantaneous values of said operating parameter of the various refrigerators / liquefiers r / l to converge toward this dynamic mean value . for example , the opening / closing of each outlet valve 11 is controlled according to a pressure set point cp according to a formula of the type cp = c + d . δq , where b is a predetermined pressure value , c is a predetermined coefficient ( dimensions = pressure / flow rate ) and δq is the differential ( dimensions = flow rate ) between , on the one hand , the dynamic mean value of this flow rate for the three coolers and , on the other hand , this instantaneous flow rate for the refrigerator / liquefier concerned . as illustrated in fig4 , the working circuit of each refrigerator / liquefier may , in the cold box 3 , comprise a main pipe 19 comprising an exchanger 20 for cooling the working gas which is immersed in a cryogenic tank 21 of liquefied working gas and a secondary pipe 23 forming a bypass of the main pipe upstream of the cryogenic tank 21 . the secondary pipe 23 opens into this tank 21 into which it delivers the liquefied working gas produced by the cold box 3 . each main pipe 19 comprises a variable - opening controlled downstream valve 5 situated downstream of the cooling exchanger 20 . each apparatus comprises a sensor 24 of the operating parameter consisting of the instantaneous value of the flow rate of the flow of working gas in said main pipe 23 downstream of the flow cooling exchanger 20 . the electronic logic 50 may be configured to perform real - time calculation of the dynamic mean value of this operating parameter for all the refrigerators / liquefiers l / r and to perform real - time control of the opening / closing of each downstream valve 5 as a function of the difference between the instantaneous values of this operating parameter of the refrigerator / liquefier concerned so as to cause said instantaneous values of said operating parameter of the various refrigerators / liquefiers r / l to converge toward this dynamic mean value . for example , the secondary pipe 23 is equipped with a variable - opening distribution valve 25 , the opening of which is increased in the event of increased production of liquefied working gas in the cold box 3 . in addition , control of each downstream valve 5 may be corrected according to the degree of opening of the distribution valve 25 so as to reduce the opening of the downstream valve 5 when the opening of the distribution valve 25 increases , and vice versa . as illustrated in fig5 , the cold box 3 of each refrigerator / liquefier l / r may comprise a plurality of heat exchangers 26 for cooling the working fluid and a bypass pipe 27 bypassing at least some of said exchangers 26 . this bypass pipe 27 bypassing the exchangers 26 provides downstream working gas leaving the cold box 3 . as depicted , the bypass pipe 27 is connected to several portions of the working circuit in a heat exchange relationship with the exchangers 26 via respective controlled bypass valves 6 , 7 , 8 ( valves with variable opening ). each refrigerator / liquefier may comprise a measurement sensor 28 for measuring the operating parameter consisting of the instantaneous value of the flow rate of the flow of gas in said bypass pipe 27 . the electronic logic 50 may comprise a step of real - time calculation of the dynamic mean value of this operating parameter for all the refrigerators / liquefiers l / r and for the real - time control of the opening / closing of at least one of the bypass valves 6 , 7 , 8 as a function of the difference between the instantaneous values and the dynamic mean value of this operating parameter of the refrigerator / liquefier concerned , so as to cause said instantaneous values of said operating parameter of the various refrigerators / liquefiers r / l to converge toward this dynamic mean value . for example , the opening / closing of the bypass valve 7 is controlled according to a pressure set point cp according to a formula of the type cp = g + h . δq , where g is a predetermined pressure value , g is a predetermined coefficient ( dimensions = pressure / flow rate ) and δq is the differential ( dimensions = flow rate ) between , on the one hand , the dynamic mean value of this flow rate for the three coolers and , on the other hand , this instantaneous flow rate for the refrigerator / liquefier concerned . the other bypass valves 6 , 8 allow adjustment of the temperature of the circuit for the refrigerator / liquefier concerned . as illustrated in fig6 , the working circuit may , in the cold box 3 of each refrigerator / liquefier l / r , comprise a plurality of exchangers 26 for warming up the cold working fluid that has exchanged heat with the application 1 . the working circuit additionally comprises a return pipe 29 for the flow 30 of working gas returning to the compression station 2 , the return pipe 29 comprising a portion that is subdivided into two parallel legs 129 , 229 respectively referred to as the “ hot ” and “ cold ” leg . the hot leg 129 does not exchange heat with at least part of the heating heat exchangers 26 . the cold leg 229 itself exchanges heat with several warming up exchangers . the working fluid that has exchanged heat with the application returns to the compression station 2 and is distributed into the hot leg 129 when its temperature is above a determined threshold or into the cold leg 229 when its temperature is below the determined threshold . each hot leg 129 comprises a variable - opening controlled regulating valve 9 . each cold box 3 comprises a measurement sensor 130 for measuring the operating parameter consisting of the instantaneous value of the flow rate of the flow of gas in said hot leg 129 . the electronic logic 50 may be configured to perform real - time calculation of the dynamic mean value of this operating parameter for all the refrigerators / liquefiers and to perform real - time control of the opening / closing of the valve 9 of the hot leg 129 as a function of the difference between the instantaneous values and the dynamic mean value of this operating parameter of the refrigerator / liquefier concerned , so as to cause said instantaneous values of said operating parameter of the various refrigerators / liquefiers to converge toward this dynamic mean value . for example , the opening / closing of each valve 9 of the hot leg is controlled according to a pressure set point cp according to a formula of the type cp = i + j . δq , where i is a predetermined pressure value , j is a predetermined coefficient ( dimensions = pressure / flow rate ) and δq is the differential ( dimensions = flow rate ) between , on the one hand , the dynamic mean value of this flow rate for the three coolers and , on the other hand , this instantaneous flow rate for the refrigerator / liquefier concerned . similarly , each cold leg 229 comprises a variable - opening controlled regulating valve 10 and a measurement sensor 131 for measuring the operating parameter consisting of the instantaneous value of the flow rate of the flow of gas in said leg 229 . the electronic logic 50 may be configured to perform real - time calculation of the dynamic mean value of this operating parameter for all the refrigerators / liquefiers and to perform real - time control of the opening / closing of the valve 10 of the cold leg 229 as a function of the difference between the instantaneous values and the dynamic mean value of this operating parameter of the refrigerator / liquefier concerned , so as to cause said instantaneous values of said operating parameter of the various refrigerators / liquefiers r / l to converge toward this dynamic mean value . as before , the opening / closing of each valve 10 of the cold leg may be controlled according to a pressure set point cp according to a formula of the type cp = k + l . δq , where k is a predetermined pressure value , l is a predetermined coefficient ( dimensions = pressure / flow rate ) and δq is the differential ( dimensions = flow rate ) between , on the one hand , the dynamic mean value of this flow rate for the three coolers and , on the other hand , this instantaneous flow rate for the refrigerator / liquefier concerned . it will be understood that many additional changes in the details , materials , steps and arrangement of parts , which have been herein described in order to explain the nature of the invention , may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims . thus , the present invention is not intended to be limited to the specific embodiments in the examples given above .