for purposes of orienting the present invention with respect to known devices , particularly by referring to fig1 in which a single broken line rectangle is shown , which corresponds to the double broken line rectangle in fig2 the apparatus illustrated therein is very similar to a double pump , high volume evacuator , such as shown in prior u . s . pat . no . 3 , 964 , 112 , in which a single pump is illustrated and , more particularly , prior u . s . pat . no . 4 , 245 , 989 over which the present invention to a large extent constitutes an improvement by simplifying the apparatus shown in said patent . both of said patents are assigned to the assignee of the instant invention . the structure shown within the aforementioned broken line rectangles comprises a pair of similar liquid seal vacuum pumps 10 , which are similar to pumps 10 and 12 in the 4 , 245 , 989 patent . these pumps are connected by a tee 12 to an inlet conduit 14 , which receives &# 34 ; gray water &# 34 ; from operatory inlets 16 , the conduit 14 including a solids collector 18 , similar to the collector 26 in u . s . pat . no . 4 , 245 , 989 . an important purpose of the present invention is to always provide sufficient water within the pumps 10 that there is no need to prime the same at startup . moreover , the system of the present invention provides means by which the so - called &# 34 ; gray water &# 34 ; cannot enter the fresh water supply introduced into the system and thereby risk contaminating the same . another tee 20 commonly receives discharge from pumps 10 and conducts the same through conduit 22 to the air - water separator unit 24 . the separator unit 24 preferably is mounted vertically and is arranged to maintain a reservoir of &# 34 ; gray water &# 34 ; 26 at a suitable level controlled by the u - shaped discharge conduit 28 , the conduit 28 actually constituting a double u - shaped configuration , the second u - shaped portion of the conduit being at a level commensurate with the amount of water desired to be maintained in the reservoir 26 , and then having a depending discharge section 32 which discharges into a drain 34 . an essential feature of the present invention constitutes the fact that the lower portion of the &# 34 ; gray water &# 34 ; reservoir 26 discharges through conduit 36 by gravity to the inlet conduit 14 and tee 12 into the inlet of the respective pumps 10 . by means of the gravity flow , and the maintenance of a satisfactory level in the reservoir 26 , there will always be ample sealing water for the pumps 10 , in accordance with the present invention . the upper portion of the separator unit 24 has an air discharge conduit 38 extending outwardly to atmosphere . because of evaporation and otherwise , it is essential that the desired amount of water be maintained in the &# 34 ; gray water &# 34 ; reservoir 26 for gravity flow to the pumps 10 and to accomplish this , regardless of the amount of water obtained from the operatory inlet 16 , and also for the safety purpose of not contaminating fresh water supply with said &# 34 ; gray water &# 34 ;, the present invention provides a fresh water inlet conduit 40 . the fresh water passes through certain regulating elements , commonly encompassed by a bracket 42 , details of which are described in aforementioned u . s . pat . no . 4 , 245 , 989 , the fresh water exiting from the regulating elements 42 by a conduit 44 to a three - way valve 46 , having two outlets 48 and 50 , outlet 48 being connected to a solenoid - operated valve 52 , controlled by a timer 54 and the other outlet 50 constituting a bypass to the discharge conduit 36 , for purposes to be described . in fig2 which is somewhat diagrammatic , it will be seen that the conduit 44 includes a pair of solenoid valves 56 , which are part of 42 in fig1 and actually constitute valves which are turned on at the beginning of day &# 39 ; s operation , for example , and remain on during the entire period of operation , the same being closed at the end of the day , whereby they are somewhat in the nature of master control solenoid valves . the discharge from these valves communicate with a vacuum breaker 58 , which is included in conduit 44 prior to the three - way valve 46 . discharge conduit 60 leads from the timer actuated solenoid valve 52 , and the fresh water therein passes through another vacuum breaker 62 which also is shown in fig1 . in fig1 the outlet 48 from the three - way valve 46 actually is part of the conduit 44 , which leads to the vacuum breaker 62 and discharges through another portion of conduit 44 into the lower portion of the separator unit 24 at a level above the discharge conduit 36 , which is also connected to the separator unit , as clearly shown in both fig1 and 2 . the primary purpose of the present invention is to introduce in a very conserving manner , fresh water to the reservoir 26 at spaced intervals of very short duration . by way of example , the intervals may be of the order of forty seconds but the valve remains on only four seconds at each of these intervals , whereby it will be seen that a relatively small amount of water is required to furnish the makeup for the water normally required by the pumps . it also is to be understood that the time example set forth above is essentially illustrative rather than being of a restrictive nature since , depending upon operating conditions , variations in the &# 34 ; on &# 34 ; and &# 34 ; off &# 34 ; periods may be necessary , but the specific illustration is primarily set forth to illustrate the possible economy in consumption of fresh water . this economy is effected by a substantially more simple system than that of prior u . s . pat . no . 4 , 245 , 989 , due to the elimination of the tanks 76 and 78 thereof and employing only the air - water separator unit which is used as a reservoir for the &# 34 ; gray water &# 34 ; and thus , it is a distinction over the use of the separator in u . s . pat . no . 4 , 245 , 989 . as an example of water economy over that of the earlier stram u . s . pat . no . 3 , 482 , 313 , conventional operation thereof required a consumption of approximately thirty gallons of water per hour . the average consumption by the recycler of the present invention is of the order of about one - tenth of that of the stram system , or approximately three gallons per hour . also , another recommended time period for use of the present invention , based upon a per hour rate is to have the valve open for periods of six seconds each hour , which constitutes a certain amount of saving of operation of the valve in mechanical wear . it also is obvious that the system of the present invention is substantially more simple than that of prior plowman u . s . pat . no . 3 , 964 , 112 , in addition to the fact that the plowman patent is concerned with recycling cuspidor water , whereas the present invention primarily is concerned with recycling &# 34 ; gray water &# 34 ;, resulting from flushing oral cavities of patients to remove cooling and flushing water therefrom . in the event either the solenoid valve 52 or timer 54 should malfunction in a manner so as not to supply water to the separator unit 24 , from which water for sealing the pumps 10 is received , the system normally would lose it prime after a period of time , such as about one hour , and would thereafter no longer supply vacuum to the system . should such situation occur , the three - way valve 46 can be moved to activate outlet 50 , which will cause fresh water to be delivered to conduit 36 and discharge such fresh water into inlet conduit 14 , which leads to tee 12 and pumps 10 . such an arrangement will at least permit the operatory system to function until suitable restoration of the normal function of solenoid valve 52 and timer 54 can be resumed , following repair or replacement . it will be understood that the foregoing operation is merely of the nature of an emergency and normally will not result in water economy . another attribute and advantage of the present invention as illustrated in fig1 resides in the fact that the solenoid valve which is actuated by timer 54 constitutes the only moving member in the fresh water supply system for the air - water separator unit 24 . especially in the system of prior u . s . pat . no . 4 , 245 , 989 , &# 34 ; gray water &# 34 ; circulates through certain of the control valves , and in the event solid waste material becomes lodged between the movable valve member and its seat , closing of the valve is not possible and leakage will occur , sometimes resulting in substantial damage to the floor of a dental operatory or otherwise . in contrast to this , the inclusion of only a single inlet control valve for introducing makeup water to the pumps , it will be seen that only clean water , free of contamination of any kind , especially solid particles or the like , will pass through the control valve and thereby eliminate the possibility of the supply valve remaining open , thus constituting a safety factor . from the foregoing , it will be seen that the present invention provides a relatively simple system to supply makeup water of a clean fresh nature , free of contamination , to one or more vacuum pumps , the supply being operated by a controlled cycle of minimum periods occurring in cycles of much longer periods of time , but being operable to insure the constant presence of sealing fluid in the air - water separator unit for delivery by gravity to the pumps , thereby insuring the presence of an adequate supply of water , which essentially is &# 34 ; gray water &# 34 ; to provide suitable sealing for the pumps and thereby insure the production of required vacuum to dental operatories , resulting in substantial economy in fresh water consumption automatically controlled by a timer , which actuates the sole movable element in the fresh water supply line to the air - water separator from which the sealing water is derived . the foregoing description illustrates preferred embodiments of the invention . however , concepts employed may , based upon such description , be employed in other embodiments without departing from the scope of the invention . accordingly , the following claims are intended to protect the invention broadly , as well as in the specific forms shown herein .