It will be apparent from a reading of the specification that the present invention may be advantageously utilized with pressurized containers and vessels intended for many different applications. However, the invention is especially useful when applied to motor vehicle pneumatic tires, and will be described in connection therewith.
A number of pneumatic tire fill valves which include pressure sensing devices have been suggested by the prior art. Typically, such valves have been employed in tire pressure monitors in which a valve is associated with each tire to monitor the air pressure therein and to close or open an electrical switch, thereby actuating a radio transmitter, when the fluid pressure within one of the tires falls below a predetermined level. Frequently, mechanical pop-out or other attention attracting devices have been incorporated within the valve to eliminate the need for interfacing the low tire condition information between the tire and vehicle body.
Prior art schemes frequently have undesirable characteristics in certain applications. The most significant shortcoming of prior art devices is their inability to compensate for temperature variations in the surrounding atmosphere. This is particularly important in applications with pneumatic tires inasmuch as the temperature of the fluid or air within the tire varies over an extremely large range due to heat generated during operation as well as ambient temperature changes, which results in the pressure within the tire also varying greatly. Prior art units merely compare the pressure within the tire with ambient or atmospheric pressure. If the unit has a single pressure set point at which the valve actuates its warning mechanism, the effective calibration will vary with temperature and pressure. This vibration can be so extreme that a tire, filled to its recommended limit during a relatively hot day or after prolonged operation, can give a false low pressure condition signal due entirely to cooling of the atmosphere.
An additional shortcoming of commercially available valves is the inability of the user to manually calibrate or vary the set point once the valve has been installed in the vehicle wheel. This inability results in tire pressure monitors which may have substantially different set points from wheel to wheel. Finally, valves having electrical switches incorporated therein typically provide for interconnection with the rest of the tire pressure monitoring circuit by dressing leads radially outwardly from valve stem. Because of the harsh environment found in a typical automotive wheel, these leads can deteriorate over time and result in either a false low tire condition signal being generated or an open circuit in which no signal will be generated even if such a condition occurs. The commercial units which have been provided to overcome this shortcoming have proven to be extremely expensive and impractical for passenger vehicle application.