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Control apparatus for producing a control signal indicating that a physical variable has reached a userselected level. A transducer generates for the control apparatus electrical pulses having a repetition period which is a function of the physical variable. The control apparatus repeatedly counts c

Control apparatus for producing a control signal indicating that a physical variable has reached a userselected level. A transducer generates for the control apparatus electrical pulses having a repetition period which is a function of the physical variable. The control apparatus repeatedly counts clock pulses in response to the transducer pulses to provide a first digital signal representative of the repetition period of the transducer pulses. The apparatus also repeatedly generates a second digital signal corresponding to the user-selected level of the physical variable by adding to a preset binary code a binary amount related to a userselected voltage. The first and second digital signals are compared to produce the control signal for a switching circuit which responds thereto. The comparison is overriden if an analysis of most significant bits of the first digital signal shows that the physical variable varies beyond a predetermined level. In control apparatus for a heat transfer system the comparison is made with a high fluid pressure level, and a low charge pressure level, as well as a userselected low fluid pressure level. Subject to predetermined time delays, the system is actuated when the high pressure is reached, and deactuated when the selected low pressure is reached. Abnormally low fluid charge is indicated when an excessive rate of change from the high pressure level to the low charge pressure level repeatedly occurs. An automotive air conditioning system and methods for controlling such a system are also described.

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1. Apparatus for controlling the operation of an air conditioning system having a compressor, electrically energizable means for driving the compressor, a pressure line for conducting compressed refrigerant from the compressor, an evaporator, a blower operable at different blower speed selections fo

1. Apparatus for controlling the operation of an air conditioning system having a compressor, electrically energizable means for driving the compressor, a pressure line for conducting compressed refrigerant from the compressor, an evaporator, a blower operable at different blower speed selections for moving air over the evaporator thereby to cool the air, and a suction line for returning expanded refrigerant to the compressor, the apparatus comprising: means for producing an electrical signal responsive to the pressure of the refrigerant in the suction line; means for providing a selected one of a plurality of electrical reference signals corresponding to a selected low pressure level in response to a respective one of the blower speed selections, and for providing another electrical reference signal corresponding to a higher pressure level; means for comparing the electrical signal representing the refrigerant pressure in the suction line to the higher pressure electrical reference signal and to the selected one low pressure electrical reference signal; and means responsive to said comparing means for electrically energizing the compressor driving means when the refrigerant pressure exceeds the higher pressure level and for electrically deenergizing the compressor driving means when the refrigerant pressure falls below the selected low pressure level. 2. Apparatus as set forth in claim 1 wherein said electrical reference signal providing means also provides another electrical reference signal corresponding to a low charge pressure level and said comparing means also compares the electrical signal representing the refrigerant pressure in the suction line to the low charge electrical reference signal and wherein the apparatus further includes low charge indicating means responsive to the refrigerant pressure falling from the higher pressure level to the low charge pressure level within a preestablished period of time during at least some of a plurality of successive periods of energization of the compressor driving means for indicating the existence of a low refrigerant charge. 3. Apparatus as set forth in claim 2 which includes means associated with said low charge indicating means for deenergizing the compressor driving means when the refrigerant pressure falls from the higher pressure level to the low charge pressure level within the preestablished period of time during at least some of a plurality of successive periods of energization of the compressor driving means. 4. Apparatus as set forth in claim 1 wherein said means for electrically energizing the compressor driving means is responsive to said comparing means for electrically energizing the compressor driving means only after a first predetermined time period following deenergization of the compressor driving means and is responsive to said comparing means for electrically deenergizing the compressor driving means only after a second predetermined time period following energization of the compressor driving means. 5. Apparatus as set forth in claim 1 wherein said electrical signal producing means comprises: transducer means for generating a series of pulses having a repetition period which is a function of the refrigerant pressure; and means responsive to the series of pulses for generating a first digital signal, which is representative of the repetition period, the first digital signal comprising the pressure responsive electrical signal. 6. Apparatus as set forth in claim 5 wherein said electrical reference signal providing means comprises: means responsive to the selection of any one of a plurality of voltage levels indicative of the blower speed selections for generating a second digital signal, the second digital signal comprising the selected one low pressure electrical reference signal; and means for supplying to said comparing means the second digital signal when the compressor driving means is energized and for supplying to said comparing means a third digital signal, the third digital signal comprising the electrical reference signal corresponding to the higher pressure level when the compressor driving means is deenergized. 7. Apparatus as set forth in claim 6 wherein said electrically energizable means for driving the compressor comprises an electromagnetic clutch, and said means for electrically energizing the compressor driving means comprises: means responsive to said comparing means for generating a clutch energizing signal when the refrigerant pressure exceeds the higher pressure level after a first predetermined time period following deenergization of the clutch and for terminating the clutch energizing signal when the refrigerant pressure falls below the selected low pressure level after a second predetermined time period following energization of the clutch. 8. Apparatus as set forth in claim 7 wherein said comparing means comprises: means for digital comparison of the first digital signal with the second digital signal and of the first digital signal with the third digital signal; and means responsive to said digital comparison means for providing a low pressure comparison signal upon the refrigerant pressure falling below the selected low pressure level when the clutch energizing signal is being generated and for providing a high pressure comparison signal when the refrigerant pressure exceeds the higher pressure level after the clutch energizing signal is terminated, the low pressure comparison signal resulting from the digital comparison of the first and second digital signals, the high pressure comparison signal resulting from the digital comparison of the first and third digital signals; said clutch energizing signal generating means responding to the high pressure comparison signal and generating the clutch energizing signal only after the first predetermined time period, and responding to the low pressure comparison signal and terminating the clutch energizing signal only after the second predetermined time period. 9. Apparatus as set forth in claim 8 wherein said clutch energizing signal generating means comprises: timed control means for providing a first pulse when the first predetermined time period has elapsed after the clutch energizing signal has been terminated and for providing a second pulse when the second predetermined time period has elapsed after the clutch energizing signal has been initiated; flipflop means having a first output state during which the clutch energizing signal is generated and a second output state during which no clutch energizing signal is generated; and means for coupling the low pressure comparison signal and the high pressure comparison signal to said flipflop means, said timed control means enabling said coupling means by the first pulse so that the high pressure comparison signal is able to set said flipflop means to the first output state, said timed control means enabling said coupling means by the second pulse so that the low pressure comparison signal is able to set said flipflop means to the second output state. 10. Apparatus as set forth in claim 9 wherein said timed control means is responsive to a time base signal, and said first digital signal generating means comprises counter and frequency dividing means, said counter and frequency dividing means producing the time base signal. 11. Apparatus as set forth in claim 6 for use with a low charge indicator wherein said supplying means further supplies a fourth digital signal corresponding to a low charge pressure level; said comparing means providing a low charge pressure comparison signal when a comparison of the first and fourth digital signals indicates the refrigerant pressure has fallen below the low charge pressure level by the time a third predetermined period of time has elapsed following energization of the compressor driving means; said apparatus further comprising means for actuating the low charge indicator in response to repeated occurrences of the low charge pressure comparison signal during different periods of energization of the compressor driving means. 12. Apparatus as set forth in claim 1 wherein said electrically energizable means for driving the compressor comprises an electromagnetic clutch and said means for electrically energizing the compressor driving means comprises: means responsive to said comparing means for generating a clutch energizing signal when the refrigerant pressure exceeds the higher pressure level after a first predetermined time period following deenergization of the clutch and for terminating the clutch energizing signal when the refrigerant pressure falls below the selected low pressure level after a second predetermined time period following energization of the clutch. 13. Apparatus as set forth in claim 1 wherein said electrical signal producing means comprises: a housing with fluid pressure responsive means movable therein defining an expansible chamber adapted for communication with the suction line for sensing the magnitude of the refrigerant pressure in the suction line; a body of magnetoelastic material, the magnetic properties of which change as a function of the stress to which said body is subjected, said body being stressed in response to expansion and contraction of said expansible chamber; and means responsive to the stress-induced changes in the magnetic properties of said body for producing the electrical signal responsive to the refrigerant pressure in the suction line. 14. Apparatus for controlling the operation of an air conditioning system having a compressor, energizable clutch means for driving the compressor during repeated intervals, a pressure line for conducting compressed refrigerant from the compressor, an evaporator, a blower for moving air over the evaporator thereby to cool the air, and a suction line for returning expanded refrigerant to the compressor, the apparatus comprising: transducer means comprising a body of material having a physical property which changes as a function of the stress to which said body is subjected, said transducer means being stressed in response to the refrigerant pressure in the suction line, said transducer means further comprising means responsive to the stress-induced changes in the physical property of said body for producing an electrical signal which is a function of the refrigerant pressure in the suction line; electronic means for comparing a signal indicative of a low refrigerant charge pressure limit with the electrical signal at the end of a predetermined time period after initiation of energization of the clutch for driving the compressor; and means for producing a signal indicative of a low charge condition in response to said comparing means when the refrigerant pressure is less than the low charge pressure limit at the end of at least one such predetermined time period. 15. Apparatus as set forth in claim 14 wherein the apparatus further comprises: means responsive to the low charge signal for deenergizing the clutch means thereby to deactivate the compressor. 16. Apparatus as set forth in claim 14 wherein the low charge pressure limit is greater than a suction line pressure at which the clutch driving the compressor would be deenergized. 17. Apparatus as set forth in claim 14 wherein the pressure responsive electrical signal produced by the transducer means is a series of pulses having a repetition period which is a function of the refrigerant pressure, and wherein said electronic comparing means comprises: means for generating clock pulses higher in frequency than the transducer pulses; digital counter means for generating time pulses from the clock pulses, the time pulses being lower in frequency than the transducer pulses; means responding to the time pulses and the transducer pulses for causing the digital counter means to periodically reset and count clock pulses to provide a count indicative of the repetition period of the transducer pulses; means for timekeeping using the time pulses so as to generate a first timekeeping output during the predetermined time period in each of the intervals of clutch energization and ceasing the first timekeeping output otherwise; means for generating a low charge pressure comparison signal only when the first timekeeping output coincides with the transducer pulse period count reaching at least a predetermined value. 18. Control apparatus for use with a transducer for generating electrical pulses having a repetition period which is a function of a physical variable and a switching circuit responsive to a control signal indicating that the physical variable has reached a specified level, the control apparatus comprising: means for generating a series of clock pulses; means for counting the series of clock pulses in response to the transducer pulses to provide a first digital signal representative of the repetition period of the transducer pulses, for generating timing pulses having a repetition period greater than the transducer pulses, and for repeating the counting in response to at least some successive ones of the timing pulses; means for sensing any one of a plurality of voltage levels respectively indicative of a selected one of a plurality of levels of the physical variable and, in response to the timing pulses, repeatedly generating a second digital signal representing the selected one level by providing a third digital signal representing the highest selectable level of the plurality of levels of the physical variable and adding to the third digital signal a binary amount related to the sensed voltage level thereby to generate the second digital signal; and means for comparing the first and second digital signals to produce the control signal. 19. Control apparatus as set forth in claim 18 wherein said means for sensing and repeatedly generating the second digital signal comprises means for up-counting from the third digital signal for a period of time related to the sensed voltage level, whereby the adding is achieved. 20. Control apparatus as set forth in claim 18 wherein said counting, generating, and repeating means comprises: means for counting the series of clock pulses and for frequency-dividing the series of clock pulses to generate the timing pulses; means for generating pulses in response to the timing pulses to reset the counting; and means for generating other pulses in response to the reset pulses and the transducer pulses so that each other pulse respectively commences later than each reset pulse by a period of time indicative of the repetition period of the transducer pulses and temporarily prevents the counting and frequency-dividing means from counting during each other pulse. 21. Control apparatus as set forth in claim 20 wherein the control apparatus further comprises: timer means for preventing response by the switching circuit to the control signal until a predetermined time period has elapsed, said timer means utilizing as a time base the timing pulses. 22. Control apparatus as set forth in claim 18 wherein the specified level is a low level for the measured physical variable and the control apparatus further comprises: means for substituting a fourth digital signal representing a high limit for the measured physical variable in place of the second digital signal at said comparing means, said substituting means being responsive to at least one signal from the switching circuit thereby to alternately and sequentially provide the control signal in a manner adapted for control of the physical variable through the switching circuit relative to both the low level and the high limit. 23. Apparatus for controlling a heat transfer system having a working fluid exhibiting a pressure comprising: an assembly comprising a body having a changeable physical property as a function of the pressure to which said assembly is subjected; means responsive to the pressure-induced changes in the physical property of said body for producing an electrical digital signal responsive to the fluid pressure; means for generating a selected one of a plurality of electrical digital reference signals corresponding to a selected low pressure limit and for generating another electrical digital reference signal corresponding to a high pressure limit; means for comparing the electrical digital signal responsive to the fluid pressure with each electrical digital reference signal; and means responsive to said generating means and said comparing means for actuating the heat transfer system when the fluid pressure reaches the high pressure limit and for deactuating the system when the pressure reaches the selected low pressure limit. 24. Apparatus as set forth in claim 23 wherein said means for actuating and deactuating the system comprises timed control means and means responsive to said timed control means and to said comparing means for performing the actuating and deactuating only after a predetermined time period following each immediately preceding deactuation and actuation respectively. 25. Apparatus as set forth in claim 24 wherein said means for producing the electrical digital signal responsive to fluid pressure comprises: means for producing oscillations having a repetition period functionally related to changes in the physical property of said body; means for generating clock pulses; and means for counting the clock pulses during the repetition period thereby to produce the electrical digital signal, the counting means also frequency dividing the clock pulses so as to provide a time base signal for the timed control means. 26. Apparatus as set forth in claim 23 wherein said means for actuating and deactuating the system provides a signal indicative of operational state of the heat transfer system and said generating means comprises: means for receiving information indicative of the selected low pressure limit and digitally up-counting from a preset digital code in response to the information so as to provide the selected one of the plurality of electrical digital reference signals corresponding to the selected low pressure limit; and means responsive to the signal indicative of operational state for alternately providing as a reference signal at least the electrical digital reference signal corresponding to the high pressure limit and the selected one electrical digital low pressure reference signal; and said comparing means comprises means for digitally comparing the pressure-responsive electrical digital signal with the reference signal. 27. Apparatus as set forth in claim 23 wherein said means for producing the electrical digital signal responsive to fluid pressure comprises: means for producing oscillations having a repetition period responsive to changes in a magnetoelastic physical property of said body; and means for counting clock pulses during the repetition period thereby to produce the electrical digital signal. 28. Apparatus as set forth in claim 23 wherein the electrical digital signal responsive to the fluid pressure has digital bits including at least two most significant bits and said comparing means is additionally responsive to the at least two most significant bits so as to provide a compare signal if the pressure reaches a predetermined high pressure and if the pressure reaches a predetermined low pressure, regardless of the result of comparing the electrical digital signal with each electrical digital reference signal. 29. Apparatus as set forth in claim 28 wherein said means for actuating and deactuating the system provides a signal indicative of operational state of the heat transfer system and said generating means comprises: means for receiving information indicative of the selected low pressure limit and digitally augmenting a preset binary number in response to the information so as to provide the selected one of the plurality of electrical digital reference signals corresponding to the selected low pressure limit; and means responsive to the signal indicative of operational state for alternately providing as a reference signal to the comparing means at least the electrical digital reference signal corresponding to the high pressure limit and the selected one electrical digital low pressure reference signal. 30. Apparatus as set forth in claim 28 wherein said comparing means comprises logic means responsive to a binary value 11 as the two most significant bits of the pressure-responsive electrical digital signal for providing a first logic level as the compare signal, and is responsive to "0" as the most significant bit for providing the logical complement of the first logic level as the compare signal. 31. Apparatus as set forth in claim 28 wherein said means for actuating and deactuating the system comprises timed control means and means responsive to said timed control means and to said comparing means for performing the actuating and deactuating only after a predetermined time period following the next-previous deactuation and actuation respectively. 32. Apparatus as set forth in claim 31 wherein said means for producing the electrical digital signal responsive to fluid pressure comprises: means for producing oscillations having a repetition period responsive to changes in the physical property of said body; means for generating clock pulses; and means for counting the clock pulses during the repetition period thereby to produce the electrical digital signal, the counting means also frequency dividing the clock pulses so as to provide a time base signal for the timed control means. 33. An air conditioning system comprising: a compressor; electrically energizable means for driving the compressor; an evaporator; means for conducting refrigerant from the compressor to the evaporator; a blower operable at different blower speed selections for moving air over the evaporator thereby to cool the air; means for returning refrigerant to the compressor from the evaporator; and control apparatus comprising: means for producing an electrical signal responsive to the pressure of the refrigerant in said returning means; means for providing a selected one of a plurality of electrical reference signals corresponding to a selected low pressure level in response to a respective one of the blower speed selections, and for providing another electrical reference signal corresponding to a higher pressure level; means for comparing the electrical signal representing the refrigerant pressure in said returning means to the higher pressure electrical reference signal and to the selected one low pressure electrical reference signal; and means responsive to said comparing means for electrically energizing said compressor driving means when the refrigerant pressure exceeds the higher pressure level and for electrically deenergizing said compressor driving means when the refrigerant pressure falls below the selected low pressure level. 34. Air conditioning system as set forth in claim 33 wherein said electrical reference signal providing means also provides another electrical reference signal corresponding to a low charge pressure level and said comparing means also compares the electrical signal representing the refrigerant pressure in said returning means to the low charge electrical reference signal wherein said control apparatus and which further includes means responsive to the refrigerant pressure falling from the higher pressure level to the low charge pressure level within a pre-established period of time during at least some of a plurality of successive periods of energization of said compressor driving means for indicating the existence of a low refrigerant charge. 35. Air conditioning system as set forth in claim 34 which includes means associated with said low charge indicating means for deenergizing said compressor driving means when the refrigerant pressure falls from the higher pressure level to the low charge pressure level within a preestablished period of time during at least some of a plurality of successive periods of energization of said compressor driving means. 36. Air conditioning system as set forth in claim 33 wherein said means for electrically energizing said compressor driving means is responsive to said comparing means for electrically energizing said compressor driving means only after a first predetermined time period following deenergization of said compressor driving means and is responsive to said comparing means for electrically deenergizing said compressor driving means only after a second predetermined time period following energization of said compressor driving means. 37. Air conditioning system as set forth in claim 33 wherein said electrical signal producing means comprises: transducer means for generating a series of pulses having a repetition period which is a function of the refrigerant pressure; and means responsive to the series of pulses for generating a first digital signal, which is representative of the repetition period, the first digital signal comprising the pressure responsive electrical signal. 38. Air conditioning system as set forth in claim 37 wherein said electrical reference signal providing means comprises: means responsive to the selection of any one of a plurality of voltage levels indicative of the blower speed selections for generating a second digital signal, the second digital signal comprising the selected one low pressure electrical reference signal; and means for supplying to said comparing means the second digital signal when said compressor driving means is energized and for supplying to said comparing means a third digital signal, the third digital signal comprising the electrical reference signal corresponding to the higher pressure level when said compressor driving means is deenergized. 39. Air conditioning system as set forth in claim 38 wherein said compressor driving means includes an electromagnetic clutch, and said means for electrically energizing said compressor driving means comprises: means responsive to said comparing means for generating a clutch energizing signal when the refrigerant pressure exceeds the higher pressure level after a first predetermined time period following deenergization of said clutch and for terminating the clutch energizing signal when the refrigerant pressure falls below the selected low pressure level after a second predetermined time period following energization of said clutch. 40. Air conditioning system as set forth in claim 39 wherein said comparing means comprises: means for digital comparison of the first digital signal with the second digital signal and of the first digital signal with the third digital signal; and means responsive to said digital comparison means for providing a low pressure comparison signal upon the refrigerant pressure falling below the selected low pressure level when the clutch energizing signal is being generated and for providing a high pressure comparison signal when the refrigerant pressure exceeds the higher pressure level after the clutch energizing signal is terminated, the low pressure comparison signal resulting from the digital comparison of the first and second digital signals, the high pressure comparison signal resulting from the digital comparison of the first and third digital signals; said clutch energizing signal generating means responding to the high pressure comparison signal and generating the clutch energizing signal only after the first predetermined time period, and responding to the low pressure comparison signal and terminating the clutch energizing signal only after the second predetermined time period. 41. Air conditioning system as set forth in claim 40 wherein said clutch energizing signal generating means comprises: timed control means for providing a first pulse when the first predetermined time period has elapsed after the clutch energizing signal has been terminated and for providing a second pulse when the second predetermined time period has elapsed after the clutch energizing signal has commenced being generated; flipflop means having a first output state during which the clutch energizing signal is generated and a second output state during which no clutch energizing signal is generated; and means for coupling the low pressure comparison signal and the high pressure comparison signal to said flipflop means, said timed control means enabling said coupling means by the first pulse so that the high pressure comparison signal is able to set said flipflop means to the first output state, said timed control means enabling said coupling means by the second pulse so that the low pressure comparison signal is able to set said flipflop means to the second output state. 42. Air conditioning system as set forth in claim 41 wherein said timed control means is responsive to a time base signal, and said first digital signal generating means comprises counter and frequency dividing means, said counter and frequency dividing means producing the time base signal. 43. Air conditioning system as set forth in claim 38 for use with a low charge indicator wherein said supplying means further supplies a fourth digital signal corresponding to a low charge pressure level; said comparing means providing a low charge pressure comparison signal when a comparison of the first and fourth digital signals indicates the refrigerant pressure has fallen below the low charge pressure level by the time a third predetermined period of time has elapsed following energization of said compressor driving means; said system further comprising means for actuating the low charge indicator in response to repeated occurrences of the low charge pressure comparison signal during different periods of energization of said compressor driving means. 44. Air conditioning system as set forth in claim 33 wherein said compressor driving means includes an electromagnetic clutch and said means for electrically energizing said compressor driving means comprises: means responsive to said comparing means for generating a clutch energizing signal when the refrigerant pressure exceeds the higher pressure level after a first predetermined time period following deenergization of said clutch and for terminating the clutch energizing signal when the refrigerant pressure falls below the selected low pressure level after a second predetermined time period following energization of said clutch. 45. Air conditioning system as set forth in claim 33 wherein said electrical signal producing means comprises: a housing with fluid pressure responsive means movable therein defining an expansible chamber adapted for communication with said returning means, for sensing the magnitude of the refrigerant pressure in said returning means; a body of magnetoelastic material, the magnetic properties of which change as a function of the stress to which said body is subjected, said body being stressed in response to expansion and contraction of said expansible chamber; and means responsive to the stress-induced changes in the magnetic properties of said body for producing the electrical signal responsive to the refrigerant pressure in said returning means. 46. A method for controlling the operation of an air conditioning system having a compressor, clutch means for driving the compressor, a pressure line for conducting compressed refrigerant from the compressor, an evaporator, a blower operable at different blower speed selections for moving air over the evaporator thereby to cool the air, and a suction line for returning expanded refrigerant to the compressor, the method comprising the steps of: producing an electrical signal responsive to pressure of refrigerant in the suction line; selecting one of a plurality of electrical reference signals corresponding to a selected low pressure limit in response to a respective one of the blower speed selections; comparing the electrical signal representing the refrigerant pressure in the suction line to the selected one electrical reference signal; and engaging the clutch when the refrigerant pressure exceeds a high pressure limit and responding to the comparing step so as to disengage the clutch when the refrigerant pressure falls below the selected low pressure limit, whereby the operation of the air conditioning system compressor responds to each of the blower speed selections. 47. The method as set forth in claim 46 wherein the engaging step is timed so that the clutch is engaged only after a first predetermined time period following disengagement, and so that the clutch is disengaged only after a second predetermined time period following engagement. 48. The method as set forth in claim 47 wherein the method further comprises the intermediate steps of: comparing the electrical signal representing the refrigerant pressure with another electrical signal corresponding to a low charge pressure limit when the clutch is engaged and a third predetermined time period is elapsing; and disengaging the clutch in response to the comparing when repeated instances of refrigerant pressure falling below the low charge pressure limit within the third predetermined period of time have occurred. 49. The method as set forth in claim 46 wherein the selecting step comprises the steps of sensing any one of a plurality of voltage levels indicative of the blower speed selections; and converting the any one voltage level to a digital signal which is the selected one electrical reference signal. 50. The method as set forth in claim 48 wherein the producing step comprises the steps of: generating electrical oscillations having a repetition period which is functionally related to the refrigerant pressure; and counting clock pulses during the repetition period thereby to produce an electrical digital signal which is the pressure-responsive electrical signal, said counting also frequency-dividing the clock pulses so as to provide a time base for the first, second, and third predetermined time periods for the engaging and disengaging steps, respectively. 51. A method for generating a control signal for apparatus having a transducer for generating pulses having a repetition period which is a function of a physical variable and also having means for controlling the physical variable in response to the control signal, the method comprising the steps of: generating a series of clock pulses; counting the series of clock pulses in response to the transducer pulses to provide a first digital signal representative of the repetition period of the transducer pulses; generating timing pulses having a repetition period greater than the transducer pulses; repeating the counting in response to the timing pulses; sensing any one of a plurality of electrical levels respectively indicative of a selected one of a plurality of limits for the physical variable; repeatedly generating in response to the timing pulses a second digital signal representing the selected one limit by providing a third digital signal representing a highest limit of the plurality of limits and adding to the third digital signal a binary amount related to the sensed electrical level thereby to generate the second digital signal; and comparing the first and second digital signals to produce the control signal. 52. The method as set forth in claim 51 wherein the first digital signal has two most significant bits and the method further comprises producing the control signal in response to any occurrence of at least one predetermined combination of the two most significant bits regardless of the result of comparing of the first and second digital signals. 53. The method as set forth in claim 51 wherein the step of adding a binary amount related to the sensed electrical level comprises up-counting from the third digital signal in response to the clock pulses during a period of time related to the sensed electrical level. 54. The method as set forth in claim 51 wherein the method further comprises the step of signalling an abnormal condition of the apparatus when there occurs a rate of change in value of the first digital signal in excess of a predetermined rate of change. 55. A method for controlling a heat transfer system having a working fluid exhibiting a pressure, comprising the steps of: producing an electrical digital signal which is functionally related to the fluid pressure; generating a selected one of a plurality of electrical digital reference signals corresponding to a selected low pressure limit when the heat transfer system is on; generating another electrical digital reference signal corresponding to a high pressure limit when the heat transfer system is off; comparing the electrical digital signal related to the fluid pressure with each electrical digital reference signal; and turning on the heat-transfer system when the comparing indicates that the fluid pressure has reached the high pressure limit and turning off the heat transfer system when the comparing indicates that the fluid pressure has reached the selected low pressure limit. 56. The method as set forth in claim 55 wherein the method further comprises the intermediate step of constraining the operation of the system to change between on and off only after a predetermined time interval from the next previous change. 57. The method as set forth in claim 56 wherein the producing step comprises the steps of generating electrical oscillations having a repetition period which is a function of the fluid pressure; generating clock pulses; and counting the clock pulses during the repetition period thereby to produce the electrical digital signal, said counting also frequency-dividing the clock pulses so as to provide a time base for the constraining step. 58. The method as set forth in claim 55 wherein the low pressure limit digital signal generating step comprises digitally augmenting a preset binary code by an amount which is a function of a selected electrical analog level corresponding to the selected low pressure limit to provide the selected one of the plurality of electrical digital reference signals. 59. The method as set forth in claim 55 wherein the electrical digital signal responsive to the fluid pressure has digital bits including two most significant bits and the method further comprises the step of turning off the heat-transfer system when at least one predetermined combination of the two most significant bits occurs, regardless of the result of the comparing of the electrical digital signal related to the fluid pressure with each electrical digital reference signal. 60. The method as set forth in claim 59 wherein the low pressure limit digital signal generating step comprises: digitally augmenting a preset binary code by an amount which is a function of a selected electrical analog level corresponding to the selected low pressure limit to provide the selected one of the plurality of electrical digital reference signals.