as shown in the drawings for purposes of illustration , the present invention is concerned with a volume - limited delivery means 10 for the delivery of a preselected volume of breathable gas mixture to a critically ill patient , as part of a life support system , or to a patient receiving periodic respiration therapy treatments . in volume - limited respiration systems , a preselected volume of air or other breathable gas mixture is delivered to the patient during the respiration phase of each breathing cycle of the patient . typically , and as shown in the drawings , the preselected volume is measured by means of a cylinder 12 and a piston 14 movable axially within the cylinder . the breathable gas mixture to be delivered to the patient is admitted to the cylinder 12 on one side of the piston 14 , the upper side as illustrated in the drawings . as shown , the breathable gas is admitted to the cylinder 12 through a line 16 , and is ultimately delivered to the patient through another line 18 . it will be appreciated that , in the diagrammatic view of fig1 various necessary valves , such as a check valve in line 16 to prevent backflow to the supply of breathable gas , have been omitted for clarity . some form of low friction sealing means is used between the piston 14 and cylinder 12 , typically , as shown in fig1 - 4 , a rolling seal 20 . the rolling seal 20 is essentially a folded , nearly cylindrical sleeve of resilient , usually elastomeric material , slightly tapered from one end to the other . the seal 20 is circumferentially affixed by its ends to both the piston 14 and the cylinder 12 , and has sufficient length to allow free movement of the piston over its full displacement . the rolling seal 20 and the piston 14 effectively divide the cylinder 12 into two chambers , an upper chamber 22 used to contain the gas mixture to be delivered to the patient , and a lower chamber 24 , into which a flow of driving gas , usually air , is admitted through a line 26 , under the control of a flow - control valve 28 and a solenoid valve 30 . when it is desired to deliver the preselected volume of gas to the patient , driving gas is admitted into the lower chamber 24 , and the piston 14 is thereby raised to deliver the gas to the patient from the upper chamber 22 . after delivery of the gas to the patient , the lower chamber 24 is vented , the piston 14 is allowed to fall to its starting position , and the upper chamber 22 is again filled with the breathable gas mixture , supplied through the line 16 . the basic volume - limited gas delivery system as described thusfar suffers from a significant volume error due to the resilience of the rolling seal 20 , and there is an inherent time lag between the time of admission of the driving gas and the beginning of delivery of gas to the patient , as driving gas pressure builds up in the lower chamber 24 . in accordance with the present invention , a flow of driving gas into the chamber 24 is established and maintained before it is desired to start delivering gas to the patient . the apparatus of the invention includes an exit port 32 in the cylinder 12 , connecting with the lower chamber 24 , a valve seat 34 affixed to the exit port , and a valve seal 36 affixed to the bottom of the piston 14 and located to cooperate with the valve seat 34 . also included is an exit port valve 38 positioned to cooperate with another valve seat 40 located at the outer end of the exit port 32 . in the embodiment shown in fig1 the initial flow of driving gas , i . e ., prior to the movement of the piston 14 to deliver gas to the patient , is provided through a separate bleed - flow line 42 into the lower chamber 24 . prior to the delivery of gas to the patient , the exit port valve 38 will be open , and the bleed - flow supplied through line 42 will exit through the exit port 32 . however , the weight of the piston 14 will tend to keep the valve seal 36 seated on the valve seat 34 , and thus block the exit port 32 . as the pressure in the lower chamber 24 builds up because of the incoming bleed flow on line 42 , the piston 14 is raised slightly to allow an exit flow around the seal 36 and through the exit port 32 . an equilibrium point is reached at which the cylinder 14 is raised just sufficiently to permit an exit flow equal to the input bleed flow , and to maintain a substantially constant pressure in the chamber 24 . when it is desired to begin delivery of gas from the upper chamber 22 to the patient , a full flow of driving gas is admitted to the lower chamber 24 by opening the solenoid valve 30 , and simultaneously closing the exit port valve 38 to seal the lower chamber 24 . since the piston 14 was previously in a state of equilibrium , delivery of gas from the upper chamber 22 will begin almost instantaneously on the admission of the full flow of driving gas to the lower chamber 24 . furthermore , volume errors due to the resilience of the rolling seal 20 are practically eliminated , since the pressure in the lower chamber 24 due to the bleed - flow also serves to maintain the rolling seal in a prestressed condition prior to delivery of the breathable gas . the embodiment illustrated in fig2 is similar to that shown in fig1 except in one important respect . full flow , rather than bleed flow , of the driving gas into the lower chamber 24 is utilized to float the piston 14 to its equilibrium position , and to inflate and stress the rolling seal 20 . it will be appreciated that the full flow of driving gas will , in most respiration systems , be adjustable according to patient requirements . accordingly , the piston 14 will not always float at the same axial position prior to the delivery of gas to the patient from the upper chamber 22 . clearly , this factor introduces an error into the determination of the volume of gas delivered to the patient , since the starting position of the piston 14 is variable , and is dependent upon the value of the full flow of the driving gas through the line 26 . however , this variable starting position of the piston 14 can be easily compensated for by means of conventional electrical measurement techniques . delivery of gas to the patient is initiated in the fig2 embodiment by closing the exit port valve 38 . as in the fig1 embodiment , the resultant increase in pressure in the lower chamber 24 immediately causes delivery of gas from the upper chamber 22 , and the delivery response is extremely rapid . it will also be apparent that the fig2 embodiment has another advantage in that the full flow of driving gas is maintained uniformly at all times . this tends to eliminate pressure transients in the system , and provides for much smoother operation of the system . fig3 is a more detailed sectional view of a cylinder 12 and piston 14 embodying the invention in accordance with the bleed - flow principal described in connection with fig1 . in this particular form of the invention , the bleed - flow line 42 and the full - flow line 26 are routed through a two - position valve 44 , by means of which either the bleed flow or the full flow may be selected for communication with the lower chamber 24 through line 46 . the connection shown at 48 is provided to allow pressure measurement in the upper chamber 22 . the rolling seal 20 of fig1 is modified in the fig3 embodiment , to the extent that two rolling seals 20a and 20b are used . the cylinder 12 has an upper portion 12a , and a lower portion 12b of substantially smaller diameter than the upper portion , and the piston 14 similarly has an upper portion 14a comparable in size to the upper portion 12a of the cylinder , and a lower portion 14b comparable in size with the lower portion 12b of the cylinder . the first rolling seal 20a is positioned between the upper portions 12a and 14a of the cylinder and piston , and the lower rolling seal 20b is similarly positioned between the lower portions 12b and 14b of the cylinder and piston . it can be seen that this construction creates an intermediate chamber 50 between the two rolling seals 20a and 20b . this intermediate chamber 50 is connected by a passage 52 to atmospheric , or lower than atmospheric pressure , and this relatively low pressure therefore acts to keep the rolling seals 20a and 20b inflated toward the intermediate chamber at all times . the exit port valve 38 is of a conventional pneumatic type having an inflatable element which is compressed by inflation against the seat 40 . fig4 is a detailed sectional view of the full - flow embodiment illustrated diagramatically in fig2 . again , the connection shown at 48 is for pressure measurement in the upper chamber 22 of the cylinder 12 . in this version , only a single rolling seal 20 is employed . as mentioned earlier , when the preselected volume of gas has been delivered to the patient , the piston 14 is allowed to fall to its starting position , the exit port valve 38 is opened , and a flow of gas is admitted to the lower chamber 24 to re - establish an equilibrium position for the piston . for some designs of the piston 14 , the descending piston may bounce when the seal 36 impacts against the seat 34 , and the bouncing may continue for a short period of time until equilibrium is again reached . unfortunately , in some designs , this bouncing may continue until it is time to deliver another volume of gas to the patient from the cylinder 12 . the modification illustrated in fig5 has been found to be useful in damping out these bouncing vibrations of the piston 14 , so that a stable equilibrium condition is reached well before the time of the next patient inspiration phase . fig5 shows a portion of a cylinder 12 and a bellows 54 for delivering the gas to the patient . the lower chamber 24 is formed by the space below and outside the bellows 54 and the upper chamber 22 is formed by the space inside the bellows . the illustrated cylinder 12 has an exit port 32 , an exit port valve 38 , valve seats 34 and 40 , and a valve seal 36 , all similar to corresponding elements in the previously described embodiments . in addition , however , affixed to the valve seal 36 is a snubber 56 comprising a generally conically shaped element attached to and depending from the seal . more specifically , the snubber 56 includes a base frusto - conical section tapered to an angle of approximately 10 ° with respect to the axis of the cylinder 12 , and an adjoining outer or distal frusto - conical section tapered to a steeper angle , approximately 30 ° in the illustrative embodiment . the snubber 56 is centrally mounted on the seal 36 , and , as the seal approaches the seat 34 , the snubber enters the opening of the exit port 32 with a substantial clearance . as the distal section of the snubber 56 enters the exit port 32 there is an immediate but gradual increase in the resistance to the downward movement of the bellows 54 , because of the increased resistance to the flow of air through the exit port 32 . consequently , the deceleration of the descending bellows 54 is gradually and moderately increased , without sudden shock loads or resultant oscillations . then , as the base portion of the snubber 56 enters the exit port 32 , the smaller angle of taper of that portion provides a slower increase in the deceleration . ideally , the base portion of the snubber will provide a zero rate of change of deceleration , i . e ., a uniform deceleration , and the seal 36 will be smoothly decelerated to its equilibrium position , substantially without any bouncing motion . in effect , the snubber 56 provides a smooth two - stage deceleration of the descent of the bellows 54 , and thereby damps out any vibrations resulting from a too rapid descent . it will be appreciated that the snubber 56 , while shown installed in apparatus of the bellows type , could be employed equally well in apparatus of the rolling seal type . it will be appreciated from the foregoing that the present invention has substantially advanced the art of volume - limited respirators . in particular , it provides novel means for minimizing errors in the volume of gas delivered to the patient , and for increasing the speed of response of the volume delivery apparatus . it will also be appreciated that , although particular embodiments of the invention have been described in detail for purposes of illustration , various modifications may be made without departing from the spirit and scope of the invention . accordingly , the invention is not to be limited , except as by the appended claims .
an improved respirator gas delivery means of the type having a pneumatically - actuated piston slidably mounted in a cylinder , and separated therefrom by a resilient seal , such as a rolling seal or a bellows . an initial flow of air used to actuate the piston is established prior to delivery of a desired volume of breathable gas to a patient . the piston has a valve seal attached to its lower side , cooperating with a valve seat at an exit port in the cylinder , and tending to close the exit port under the weight of the piston . the initial flow of actuating air supports the piston at an equilibrium position allowing the flow to escape through exit port , and prestressing the resilient seal to minimize errors in the delivered volume . closure of an external exit port valve halts the exit of actuating air , and allows the piston to move upwardly immediately in response to the continued admission of actuating air . alternative embodiments are disclosed in which the initial flow of actuating air is either a relatively small bleed flow , or the full actuating flow .