The present invention generally relates to heat alarms. More specifically, the present invention relates to a heat alarm having improved alarm temperature threshold accuracy and an improved push-to-test, or alarm verification, feature that allows convenient and meaningful testing of the heat alarm at low temperatures.
Heat detectors, for many years, have been constructed using a mechanical temperature sensing element connected to a suitable switch. One technique used to implement such a heat detector utilizes a spring-loaded plunger that is held fast at normal room temperatures by a proprietary mixture of tin, lead and appropriate other elements. This material, similar to solder, melts at a preselected temperature and allows the spring to return to its relaxed state, which in turn causes a switch to close and operate a remote alarm panel or system. While this type of temperature sensing element works well at generating an alarm signal at the desired temperature, it has a useful life of only one actuation and must be replaced after such actuation.
A second type of heat detector in use is constructed using a bimetallic switch. These devices, often called "snap-action thermostats" offer multiple actuations during their lifetime. In this device, varying environmental temperatures cause a bi-metallic element to snap between two bi-stable positions, which in turn causes a switch to be opened or closed as a function of the bimetallic element.
A third type of heat detector that is in current use includes both a fixed temperature sensor and a rate-of-rise heat sensor. In this type of system, in addition to the spring-loaded switch actuator, the actual physical housing of the heat detector acts pneumatically upon the switch. Thus, when a relatively rapid change in temperature occurs, the expansion of the air within the housing causes an integral bellows to expand and actuate the same switch that is used for fixed temperature applications. Slow temperature or pressure changes, such as would be usual in changing environmental conditions, are essentially ignored by the use of a small air-bleed hole which allows the air pressure within the housing to slowly equalize and achieve equilibrium with the outside air pressure. This type of device can be actuated multiple times if the rate-of-rise function is activated, but can be actuated only one time if the fixed temperature feature is activated.
A fourth type of heat detector currently available includes a thermistor to sense the ambient temperature. The resistance of the thermistor changes as the ambient temperature increases and decreases. The varying thermistor is used in a voltage divider circuit as part of a classical Wheatstone bridge network to control the voltage applied to a pin of an integrated circuit. Currently, this type of alarm is offered in a fixed temperature model that generates an alarm when the ambient temperature exceeds a predetermined upper temperature limit, typically around 135.degree. F. The system includes a push-to-test button that can be depressed to electronically test the alarm. The push-to-test button is not simply a button that activates the alarm, but instead electronically tests the operation of the circuit by inserting a component to electronically simulate an elevated temperature, while working in cooperation with the thermistor.
One problem that exists with the push-to-test system described above is that the push-to-test system may not operate when the ambient temperature is below around +20.degree. F. (-7.degree. C.). Below 20.degree. F., the user must utilize an external heat source, such as a hair dryer, to increase the ambient temperature near the heat alarm's sensor. This will cause the heat alarm to emit an "alarm" signal if the sensor is heated above the normal alarm threshold and the alarm device is functioning properly.
Heat alarms are often found in attics, unheated garages, or crawl spaces of homes. In northern climates during the winter the temperature often falls below 20.degree. F., and an actual "push-to-test" event on such a heat alarm may not cause an electrically valid response. This low temperature inoperability may be clearly stated in the user's manual for the product, but many times this user manual is not consulted or not immediately available to the actual user. This inoperability may cause the user to conclude that the heat alarm is not working properly, even though the alarm device may indeed be functioning correctly and the lack of a valid push-to-test response is instead due to the lack of functionality of the push-to-test circuit at low temperatures. The user must then augment the testing procedure with an external heat source to test the heat detector. Further compounding this situation is that some of these remote locations do not typically have convenient outlets wired for 120 volt AC, and utilizing conventional portable external heating sources, such as electric hair dryers, is often difficult.
Therefore, an object of the present invention is to provide a heat alarm that includes an improved testing circuit that allows the heat alarm to be meaningfully tested at substantially lower ambient temperatures. Further, it is an object of the present invention to provide a heat detector that has an improved accuracy alarm temperature threshold, which will subsequently allow the unit to respond to a plurality of specific temperature rate-of-rise parameters required by certain listing agencies. Further, it is an object of the present invention to provide a novel method to safeguard the user from shock hazards in the event of inadvertent contact with the thermistor temperature sensing component of the heat alarm device without detracting from the timely and accurate operation of the alarm device.