US3522789A

US3522789A - Tire pressure alarm device

Info

Publication number: US3522789A
Application number: US680922A
Authority: US
Inventors: Marion A Garrison
Original assignee: MARION A GARRISON
Current assignee: MARION A GARRISON
Priority date: 1967-11-06
Filing date: 1967-11-06
Publication date: 1970-08-04
Anticipated expiration: 1987-08-04

Description

United States Patent 72 Inv r; Marion A. Garrison 2325 Monaco Parkway, Denver, Colorado 80207 [21] Application No.: 680,922

[22] Filed: Nov. 6,1967

[45] Patented: Aug. 4, 1970 [54] TIRE PRESSURE ALARM DEVICE 15 Claims, Drawing Figs.

Primary Examiner Louis J. Capozi Att0rneyMcGrew and Edwards ABSTRACT: A device for indicating change of pressure in a pneumatic tire for a vehicle comprises a cylinder having a piston movable therein between normal and abnormal pressure positions. The piston is urged away from the abnormal position by a biasing spring. When moving to the abnormal position the piston moves an indicating member into indicating position in which the member engages a signal producing member to warn the driver of the vehicle. A permanent magnet is mounted adjacent the indicating position of the piston and as the piston approaches urges it into the indicating position and locks it there; this provides a positive movement of the piston into indicating position and its secure retention in that position.

Patented Aug. 4, 1970 Sheet FIG. 2

INVENTOR. Marion A. Garrison FIG. 3

ATTORNEYS Patented Aug. 4, 1970 3,522,789

Sheet 2 of 5 INVENTOR. Marion A. Garrison ATTORNEYS Patented Aug. 4, 1970 3,522,789

Sheet of 3 FIG.I5

109 H2 @4330]! 15 INVENTOR.

Marion A. Garrison ATTORNEYS SPECIFICATION This invention relates to air pressure alarms for the tires of motor vehicles and particularly to an improved change of pressure device of the expansible chamber type.

Many devices have been designed heretofore for the purpose of indicating a change in pressure in the pneumatic tires of vehicles. These devices have been provided for the most part to detect loss of pressure in tires and have included many forms of mechanism for giving alarms or indications in various ways. It is desirable that the alarm device be simple in construction and reliable in operation under all road and weather conditions. Accordingly, it is an object of this invention to provide an improved abnormal tire pressure warning device which has a minimum number of moving parts and is simple, rugged and reliable in operation.

It is another object of this invention to provide an improved abnormal pressure indicator for the pneumatic tires of vehicles.

It is another object of this invention to provide an improved abnormal pressure indicator for tires which remains effective after long periods of normal pressure operation of the tires of a vehicle.

Briefly, in carrying out the objects of this invention in one embodiment thereof, a block or body having a cylinder therein is mounted on the rim of a pneumatic tired vehicle wheel and connected in communication with the interior of the tire. A piston is arranged in the cylinder and a spring urges the piston towards its low pressure position. When tire pressure is normal the piston is held in its high pressure position; if the pressure drops the piston moves away from its high pressure position and as it approaches its low position is urged forward by a permanent magnet and is finally snapped into its low pressure position and locked there. The movement of the piston to its low position projects a rigid contact finger into position to engage a flexible switch finger mounted on a fixed part of the vehicle and to operate the switch on each rotation of the wheel. The warning signal is given by a lamp or a buzzer in the circuit of the switch. In another embodiment an arrangement is included whereby the pressure in the tire is util' ized to assist the movement of the piston to its low pressure position. In a further embodiment an arm pivoted on the block constitutes the contact finger.

The features of novelty which characterizes this invention are pointed out with particularity in the claims annexed to and forming a part of this specification. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following description taken in connection with the accompanying drawings in which:

FIGURE l is a diagramatic plan view of an automobile chassis provided with an air pressure warning system embodying the invention;

FIGURE 2 is a schematic diagram of the electric circuit of the system of FIGURE l;

FIGURE 3 is an enlarged sectional elevation view of a wheel of the vehicle of FIGURE 1 lshowing the pressure sensing and signal switch devices of the system;

FIGURE 4 is a side elevation view of the tire and devices of FIGURE 3;

FIGURE 5 is an enlarged side elevation view partly in section of the pressure sensing device of FIGURE 3;

FIGURE 6 is a sectional view along the line 6 6 of FIGURE FIGURE 7 is a longitudinal sectional view of the switch device of FIGURE 3;

FIGURE 8 is an end elevation view of the switch of FIGURE 7;

FIGURE 9 is a longitudinal sectional view of a second sensing device embodying the invention;

FIGURE 10 is a sectional elevation view along the line 10- 10 of FIG. 9;

FIG. 11 is a side elevation view of the device of FIG. 9 turned about its longitudinal axis;

FIG. 12 is a plan view of a third embodiment of the invention;

FIG. 13 is a sectional view taken along the line 13-13 of FIG. 12;

FIG. 14 is an enlarged sectional view taken along the line 14-14 ofFIG. 13; and

FIG.- 15 is a sectional view similar to FIG. l3 illustrating another modified form of the device.

Referring now to the drawings, the vehicle chassis shown in FIG. l comprises a frame 10 mounted on front wheels 11 and rear wheels 12. The engine which has not been shown is connected to drive the wheels 12 with a differential 13. The

wheels

11 and 12 are provided with pneumatic tires and each of the four tires illustrated is provided with a sensing device 14 and a flexible switch assembly 15 which is actuated upon an indication of low pressure made by the sensing device 14 and completes a circuit to a pilot light 16 on the dashboard of the vehicle. A circuit has been illustrated as including the usual storage batteries 17 grounded to the frame 10 by a cable 18 and connected to the light through a lead 19, and thence to each of the switches 15. When anyone of the switches is closed the circuit is completed and the lamp turned on.

The electrical circuit as shown schematically in FIG. 2 also includes a fuse 20 arranged in the line 19 between the battery and lamp 16. As shown in these figures, the four switches are connected in parallel between the frame 10 and a line 21 constituting a continuation of the line 19 to the line 16 to the switches. It will be apparent that operation of any one of the switches will energize the lamp 16. When one of the sensing devices 14 has been operated a finger projects toward the switch 15 and strikes the switch each time the wheel completes a revolution this produces a flashing signal at the lamp 16.

The arrangement of the sensing devices is such that when it operates it strikes the flexible switch as clearly shown in FIGS. 3 and 4. As shown in these figures, the wheel indicated as wheel 12 is provided with a pneumatic tire 22 of the tubeless type. A sensing element is connected in communication with the interior of the tire through a tube 23. The switch assembly 15 is secured on the rear axle housing indicated at 24 by a clamp comprising two

halves

25 and 26 clamped together by bolts 27. In FIG. 3 the sensing device 14 has been shown with its indicating finger 28 projected in indicating position and lies in a path to strike a flexible operating portion 30 of the switch assembly 15. This portion being made flexible so that it may be bent as it is struck by the projecting finger 28. The bent position of the member 30 being indicated by dotted lines in FIG. 4; this bent portion makes it possible for the vehicle to be reversed without injury to the switch which is held out of its closing position upon reverse movement of the wheel.

The details of construction of the sensing device 14 are shown in FIGS. 5 and 6. As illustrated in these figures, the sensing device 14 is securely mounted on a plate 31 which is bolted, welded or otherwise secured to the rim of the wheel 12. The sensing element comprises a cylindrical member 32 in threaded engagement with a plate 31. A cylinder 33 is pro vided within the body or block 32 and a piston 34 is mounted for axial movement in the cylinder. A portion 28 is in the form of a closed end tube having its open end rigidly secured to the piston 34 and having its cylindrical interior in alinement with and constituting an extension of a cylindrical bore 35 in the piston 34. The piston 34 is urged outwardly by a spring assembly comprising an outer spring 36 and an inner spring 37. The

springs

36 and 37 urge the piston upwardly and when the pressure is lower in the cylinder 33 above the piston, the piston rises and moves the portion 28 to the dotted line position. The piston is provided with an annular groove 38 within which a sealing gasket 40 is seated thereby sealing the piston against leakage when in its lowermost position in cylinder 33. Leakage between body 32 and the plunger or projection 28 is prevented by an O-ring gasket 41 seated in a groove formed in the inner upward end of body 32 about the projection 28 the O-ring being retained by an annular ring which is a permanent magnet 42. Air from the tire 12 is supplied through the connection 23 and a fitting 43 to a duct 44 through the wall of the body 32. This duct leading into the top of the cylinder 33 below the ring 42 is shown in FIG. 6. The opening at the bottom of the body 32 is closed by the plate 31, thin wall portion 35 of which extendsacross a threaded opening in which the sensing element 14 is mounted. A portion of the plate 45 is provided with a minute vent 46 to release air from the chamber below the piston 34 as it moves into its lower position against the plate 45. A longitudinal groove 47 is provided in the wall of the body 32 to equalize pressure on the two sides of the piston 34 after it passes the lower end of the groove. This facilitates the rapid movement of the piston under the force of the

springs

36 and 37. As the piston rises it comes under the influence of the magnet 42 and snaps into position against the magnet. This holds the projection 28 securely and firmly in its extended position and assures positive engagement and operation of the switch portion 30. The vent 46 preferably has a very slow rate of discharge of air and may, for example, be equivalent to a slow leak by which the tire would become essentially flat in say, three to four hours. When pressure is again applied to the tire 12 the air enters the upper portion of the cylinder 33 and passes to the other side of the piston through the groove 47 which equalizes the pressure on both sides of the piston. The piston is then pressed by hand against the force of the magnet and of the

springs

36 and 37 into its lower position closing the groove 47 whereupon the pressure forces the piston downward and the remainder of its movement is cushioned somewhat depending on the rate of flow of air through the vent 46.

The action of the magnet 42 in attracting and locking the piston in its upper position increases the effectiveness of the pressure sensing element and provides additional force for assuring the setting of the piston projection 28 in its indicating or outside position.

The

springs

36 and 37 are selected to provide the required compressive force for the pressure range under which the sensor is to operate and the number and size of springs is selected accordingly.

The mechanism of the switch is shown in FIGS. 7. and 8; the body of the assembly 15 comprises a cylindrical portion 48 of rigid plastic material having a solid plug 50 closing the end remote from the flexible finger 30. The switch comprises two

flexible blades

51 and 52 mounted in a block of insulating material 53 in which they are securely held by two rivets 54. The rivets being surrounded by respective insulating sleeves and passing with the sleeves through circular openings in the

plates

51 and 52. The plug 50 carries leads 55 and 56 which connect the switch between the ground and the supply lead 21 of FIG. 2 An operating rocker arm 57 is pivoted at 58 in the end of the insulating tube 48 remote from the block 50 and has an offset end or projection 60 which engages an extended portion 61 of the plate 52. It will thus be seen that when the arm 30 is moved downwardly the arm 57 is rotated and closes the switch. This closing contact occurs each time the projection 28 engages the flexible finger 30 of the switch. The flexible finger 30 is formed integrally with a skirt portion 62 which flts closely over the tube 48 and encloses the switch contacts and the actuator. The switch assembly 15 may be provided with a mounting bracket 63 for facilitating its mounting between the

clamps

25 and 26. I

The embodiment of the invention illustrated in FIGS. 9, l0 and l l is arranged for mounting in the curved flange of the wheel indicated at 70 in FIG. l0 The sensing member of this embodiment comprises a body 71 having a longitudinal cylinder 72 formed therein and a piston 73 fitted for sliding movement in the cylinder. The piston is biased toward its right hand position by a spring 74 which is a compression spring exerting pressure between the spring and a block or plug 75 threaded into the outer end of a sleeve 76 which forms an extension of the cylinder. The block 75 may be adjusted in position by first removing a cotter pin 77 a suitable number of holes being provided in the .sleeve 76 to afford adjusted positions of the pin. Adjustment of the plug 75 changes the pressure exerted by the spring 74 on the piston 73. The sleeve 76 provides an annular shoulder 78 which acts as a limit or stop to determine the extreme movement of the piston 73 to the left. The right hand end of the piston is provided with an extension 80 of reduced cross section which is threaded to receive a nut 81 which is provided to clamp an annular sealing gasket 82 against the piston to prevent leakage of air past the piston. A permanent magnet 83 is provided to perform the same function as the magnet 42 of the first embodiment. The magnet 83 is securely attached to a plug 84 which holds it in position in the body 71. The piston in FIG. 9 is shown in its low pressure or right hand position. When pressure is admitted from the tire through a duct 85 corresponding to the duct 23 of the first embodiment, the piston moved toward the left as soon as sufficient pressure is built up to break the magnetic armature 81 from the magnet 83 and the spring 74 is compressed. This movement of the piston rotates an indicator arm 86 about a pivot pin 87, the movement of the arm 86 being effected by the engagement of a pin 88 in a slot 90 in the arm 86. At the end of its movement toward the left, the arm 86 lies in a retracted position indicated by dotted lines in FIG. l 1. Whenever the pressure within the cylinder 72 falls below a predetermined value, the piston moves toward the right rotating the arm 86 in a clockwise manner so that it projects into the path ofa switch element such as the finger 30 of FIG. 7.

The embodiment of the invention illustrated in FIGS. l2, l3 and 14 is similar to that of FIG. 5 and 6 but includes a further feature whereby the pressure remaining in the tire is used to aid the compression spring in moving the sensing device to its indicating position. The device of this embodiment comprises a base plate 92 having a cylindrical bore 93 in which a cylinder housing or block 94 is secured in threaded engagement. A piston 95 is slidably mounted in a cylinder 96 within the block 94 and is sealed with respect to the cylinder wall by a gasket 97. A cylinder projection 98 is formed integrally with the piston 95 and passes through a circular opening in a head or closure member 100 which is welded, brazed or otherwise secured to the cylinder block 94. The projection 98 is sealed for sliding engagement in the head 100 by an annular gasket 101 fitted within an annular recess about the opening in the head 100. The piston is urged to the right by a compressed spring 102. The pressure within a tire is communicated to the cylinder 96 through an inlet connection 102 by means of a tube (not shown) such as the tube 23 of the first embodiment. Air from the inlet 103 reaches the cylinder through a duct 104 a recess 105 being provided in the piston 95 so that there is direct communication between the inlet 103 and the interior of the cylinder 96 to the right of the piston 95 when the piston is in its far left position as illustrated in full lines in FIG. l3.

During the operation of the tire pressure detector when the indicator cylinder 98 is in its far right hand position as indicated by dotted lines in FIG. 13. the tire is in communication with the cylinder 96 on the left hand side of the piston and air leaks out through a very small vent 106; in order to place the sensor in operation, the projecting cylinder 98 is pressed to the left thereby compressing the spring and finally opening the communication between the right hand side of the cylinder and inlet 103. The sensor then remains in this position, the air to the left of the piston 95 being slowly exhausted through the vent 106. The piston 95 remains in its left hand position as long as the pressure of the tire is above a predetermined value. In the event that the pressure falls or reaches a value sufficiently low so that the force of the spring 102 moves the piston to the right the gasket 105 then closes the port 104 and on further movement of the piston, the port 104 is opened to the cylinder on the left hand side of the piston whereupon the left hand side of the cylinder is placed in communication with the inlet 103 and the pressure of the tire aids the spring 102 in moving the piston to the right. A minor amount of leakage occurs through the inlet 103 through the cylinder to the vent 106. The vent is made sufficiently small that the leakage is minor or of the order of a slow leak in a tire. In order to speed the movement of the piston to the right when the tire pressure falls a groove 107 is provided in the outer wall of the cylinder 98 and extends about the length of the cylinder chamber when the piston is in its far left hand position. This groove as soon as the piston has moved a distance of about the thickness of the head 100 provides communication between the cylinder 96 and the atmosphere and facilitates the movement to the right. Similarly this groove lessens somewhat the force required to press the piston 95 to the left when setting the sensor.

The head 100 is a permanent magnet like the magnet 42 of the first embodiment and performs the same function in locking the indicator piston 98 in its outermost position.

A further embodiment of the invention is illustrated in FIG. which is a sectional view through a sensing element similar to the view in FIG. 13. This embodiment differs from the last embodiment in that an arrangement is provided for sealing the sensor against leakage of air from the tire through the sensor after operation thereof. As shown in FIG. 15, the sensor is mounted on a base 109 and comprises a cylinder block 110 threadly attached to the base and having a cylinder 111 formed therein; a piston 112 is slidably mounted in the cylinder 111 and an extension 113 of the piston is slidably mounted in a circular opening in the head of the cylinder block 110. The piston 112 is sealed against leakage by an annular gasket 114 mounted in an annular groove or seat in the outer wall of piston 112; the portion 113 of the piston is similarly sealed by a gasket 116 seated in an annular groove 117 in the head of the block 110 and around the opening through which the piston 113 projects. An annular magnet 118 is securely mounted adjacent the head of the cylinder and performs the same function as magnet 42 of the first embodiment. The tire pressure is communicated to the cylinder 110 through a passage 120 opening into the upper end of the cylinder and connected to the tire through a fitting 121 and-a suitable tube (not shown) corresponding to the tube 23 of the first embodiment. When the piston is in the high pressure position in which it is illustrated high pressure is also admitted to the upper side ofthe gasket 114 through a duct 122. As shown the gasket 114 is a generally U-shaped gasket and the pressure within the tire thus urges the two sides of the gasket respectively against the bottom wall of the groove 115 and the side wall of the cylinder 110; this assures good sealing when the tire pressure is normal.

Should the pressure in the sensing device of FIG. 15 fall below a predetermined value, the piston 112 will start to move upwardly under the force of a compression spring 123 mounted within the piston portion 113 and pressing against the closed end of this portion and against an annular boss 124 formed on the base 109.

Within the piston 113 a small hollow closed end cylinder block 125 is threaded into a cylindrical opening in the boss 124 and a cylinder 126 is provided within the block 125; a slidable guide or piston 127 is mounted in the cylinder 126 and is urged upwardly by a relatively strong spring 128 mounted within the piston 127 and exerting pressure between the base or plate 109 and piston 127. The hollow interior of the piston 127 is in communication with the outside through a small vent 130 and with the cylinder 126 through a passage 131 the piston 127 has an extension rod or actuating bar 132 extending upwardly through the block 125 and contacting the interior top wall of the piston 113. When the piston 113 moves upwardly the piston 127 thus follows it until a soft O-ring gasket 133 seated in an annular recess at the top of the piston 127 engages the inner upper wall of the cylinder 126. This gasket then seals the cylinder 126 from the interior of the piston 113 with which it communicates through clearance 134 about the rod 132. As the piston 113 moves farther away from the plate 109 the gasket 114 moves past the port of the duct 122 and pressure from the tire enters the interior of the piston extension 113, however, the leakage of this air to the at mosphere through the duct 130 is prevented by the engagement of the gasket 133 with the wall of the cylinder 126. After the O-ring gasket 133 has engaged the top wall of the cylinder and has been compressed to effect sealing of the clearance about the rod 132 it remains in position and the top wall of the piston 113 moves away from the rod 132 under force of the spring 123. The upward movement of the piston is facilitated by the equalization of air pressure, air from the tire passing through duct 122 to the underside of the piston 112 which has moved upwardly, as well as filling the cylinder above the piston.

It will be noted that the gasket 116 is also of the U type; this gasket is held in good sealing relationship with the wall of the groove 117 and the wall of the piston 113 by air pressure from the tire which reaches the space below the gasket 117 through a clearance 135 between the annular magnet 118 and the piston extension 113. The sensing unit is thereby effectively sealed against leakage when the piston is in its normal position seated against the plate 109 as illustrated in FIG. 15.

During the operation of all the sensing devices disclosed, a contact finger or plunger is moved into the path of a detecting element such as a switch 15 of FIG. 7. The movement of the detecting arm or plunger to its indicating position is positive and forcibly effected and its operation is effective even in very cold weather. Construction of the device is simple and rugged and the sensing device operates effectively even though it may remain in its set position for an extended period of time.

While the invention has been illustrated and described in connection with specific structural arrangements other modifications and applications will occur to those skilled in the art; therefore, it is not desired that this invention be limited to the details illustrated and described and it is intended by the appended claims to cover all modifications which fall within the spirit and scope ofthe invention.

Iclaim:

1. A device for indicating a change of air pressure in a vehicle tire and the like comprising a rigid body having a pressure chamber therein, a movable air pressure responsive member mounted in said chamber, means for admitting to said chamber air under pressure from a tire or the like for urging said member in a first direction upon an increase in pressure, a spring for biasing said member in the opposite direction, means connected to said member and effective upon movement of said member to a predetermined position in one of said directions to produce a pressure indicating signal, said member having at least a portion thereofof magnetic material, and a permanent magnet secured to said body and effective upon movement of said member near said predetermined position to attract and engage and lock said member in said position.

2. A device for indicating a change of air pressure in a vehicle tire and the like as set forth in Claim 1 wherein said chamber comprises a cylinder and said member is a piston reciprocable in said cylinder.

3. A device for indicating a change of air pressure in a vehicle tire or the like as set forth in Claim 2 wherein said pressure indicating means includes a stop element mounted on said body and connected to said piston, said stop element upon movement of said piston to said predetermined position being moved to a position projecting outwardly from said body for producing said signal.

4. A device for indicating a change of air pressure in a vehicle tire or the like as set forth in Claim 2 wherein said piston is movable between a high pressure position and a low pressure position, said means for. admitting air under pressure to said chamber including a port in the wall of said chamber adjacent said high pressure position, said piston upon decrease of the pressure in said chamber and movement under the force of said spring toward said low pressure position uncovering said port and admitting air under the decreased pressure to said chamber on the opposite side of said piston whereby the air under decreased pressure assists said spring in moving said piston toward said low pressure position.

5. A device for indicating a change of air pressure in a vehicle tire or the like as set forth in Claim 2 wherein said predetermined position is the extreme position of said piston in said opposite direction whereby said magnet locks said piston in its low pressure position.

6. A device for indicating a change of air pressure in a vehicle tire or the like as set forth in Claim 3 wherein said stop member comprises an arm pivoted on said body and connected to said member by a lost motion linkage affording rotation of said arm and straight line movement of said member.

7. A device for indicating a change of air pressure in a vehicle tire or the like as set forth in Claim 5 including by-pass means for substantially equalizing the pressure on the two sides of said piston for a substantial portion of the stroke of said piston when moving toward and away from said low pressure position.

8. A device for indicating a change of air pressure in a vehicle as set forth in Claim 5 including a vent for removing air from the low pressure side of said piston and sealing means for preventing the discharge of air from said low pressure side through said vent when said piston is in its low pressure indicating position.

9. A device for indicating a predetermined decrease of air pressure in vehicle tires and the like comprising a block having a cylinder therein, a piston slidably mounted in said cylinder, means for supplying air under pressure from a tire to said cylinder on one side of said piston for moving said piston toward a high pressure position, spring means for urging said piston toward a low pressure position, means for venting air from said cylinder on the other side of said piston, an indicating device, means dependent upon movement of said piston toward said low pressure position for actuating said indicating device, and means dependent upon a predetermined movement of said piston toward said low pressure position for providing a bypass for air around said piston to equalize the pressure on the two sides thereof and facilitate the further movement of said piston toward said low pressure position.

10. A device for indicating a predetermined decrease of air pressure in a vehicle tire and the like as set forth in Claim 9 wherein said piston includes a concentric cylindrical extension, said block having an opening for accommodating movement of said extension from a position within said block to a position at least partway outside thereof, a first means for sealing said piston and the wall of said cylinder to prevent the leakage of air across said piston, a second means for sealing said extension and the wall of said opening for preventing leakage of air from said cylinder and wherein said bypas means provides a passage directly around one of said sealing means.

11. A device for indicating a predetermined decrease of air pressure in a vehicle tire and the like as set forth in Claim 9 including a sealing means effective upon a predetermined movement of said piston toward said low pressure position for preventing the discharge of air from said cylinder through said vent.

12. A device for indicating a change of air pressure in a vehicle tire and the like comprising a rigid body having a cylinder therein and a piston reciprocable in said cylinder, means for admitting to said cylinder on one side of said piston air under pressure from a tire or the like for urging said piston in a first direction upon an increase in pressure, a spring for biasing said piston in the opposite direction, means connected to said piston and effective upon movement of said piston to a predetermined position in one of said directions for producing a pressure indicating signal, and means dependent upon decrease of the pressure in said cylinder and movement of said piston under the force of said spring toward said low pressure position for admitting air under the decreased pressure to said cylinder on the other side of said piston to asist said spring in moving said piston toward said low pressure position.

13. A device for indicating a change of air pressure in a vehicle tire and the like as set forth in Claim 12 wherein said last mentioned means includes a passage extending through the wall of said cylinder and having a port opening into the cylinder adjacent said high pressure position of said piston.

14. A device for indicating a change of air pressure in a vehicle tire and the like as set forth in Claim 13 wherein initial movement of said piston toward its low pressure position closes said port and terminates the admission of air to the high pressure side of said piston and wherein further movement of said piston toward its low pressure position uncovers said port.

15. A device for indicating a change of air pressure in a vehicle tire and the like as set forth in Claim 12 including means effective upon a predetermined movement of said piston toward its low pressure position for providing communication between the interior of said cylinder and the atmosphere for facilitating the movement of said piston toward its low pressure position.