1. Field of the Invention
This invention relates to detection apparatus and, more particularly, to apparatus for detecting the presence of smoke or other solids in suspension in a gas by means of optical measurement of the optical scattering coefficient and obscuration of such a gas.
2. Description of the Prior Art
One of the leading causes of fatalities in structural fires is not heat or actual burns but suffocation by smoke inhaled by the sleeping victims. Furthermore, additional fatalities and injuries are caused by the lack of an early warning system during those times or in those locations in which the occupants of a structure are generally unaware or unable to detect a sudden fire.
In response to these hazards, smoke detectors have been devised in order to warn or waken occupants of the accumulation of smoke in order to prevent both suffocation by smoke inhalation and to provide an early warning signal to facilitate safe escape. Prior art devices for the detection of smoke have included various types of electrical discharge devices and ionization devices. However, such devices often entail the use of high voltage sources, complex circuitry and tend to have a sensitivity which is undesirably dependent upon ambient relative humidity and other factors which may affect the ionization or discharge rates of the detecting devices.
The simplest and more common mode for detection of gases and suspended particles in gases by the prior art has been by optical means. Optical detection of smoke densities has generally been accomplished by measuring: (1) the amount of optical obscuration or attenuation of a light beam; (2) the amount of scattering from a light beam caused by the smoke particles; or (3) by a combination of both. In those cases where the measurement of the optical density of smoke has been solely by measuring the obscuration or scattering caused by smoke, the devices have been vulnerable to changes in supply voltage, temperature, dust and grease accumulations, filament age, and detector degradation. The use of two detectors, one balanced against the other, to measure the obscuration and scattering of light from a beam caused by smoke is in large part directed to solve these disadvantages.
In general, one light detection means or photoelectric cell is positioned to measure the amount of light in a beam which traverses a smoke chamber. A second light detection means or photoelectric cell is oriented to measure light scattered in a generally perpendicular direction from the beam when smoke is introduced into the chamber. Thus, by balancing the electrical responses of the two photo cells one against the other, periodic changes in the photo cells and light source as well as degradation caused by aging could be nulled out and the overall operation of the circuitry maintained in a stable mode. However, where two such photoelectric cells have been balanced against each other they have generally been deployed in a "bucking" circuit across a relay coil. See U.S. Pat. No. 2,476,958 and U.S. Pat. No. 2,640,123. Such an arrangement is dependent upon a delicately balanced and adjusted relay. Thus the system is characterized by a lack of long term stability and requires periodic adjustment. Moreover, circuits employing relays are subject to the long term weaknesses and instability inherent to electromechanical devices which depend on mechanical springs and moveable, exposed electrical contacts.
A simpler and more reliable utilization of a two cell system to detect optical densities of smoke is to employ the photo cells in a wheatstone bridge. See for example, U.S. Pat. No. 3,409,885. Although coupling of the photoelectric cells in a wheatstone bridge has led to greater stability and sensitivity, such circuits have in general continued to use electromechanical devices at critical junctions in the circuit and have failed to provide for humidity stabilization of the bridge operation. In addition, such prior art claims have failed to combine the use of a bridge with photocells used in the reflection and obscuration modes. Thus, none of the prior art alarms incorporated any means capable of self-calibrating for the color of smoke.
The typical light source in all such optical mode detectors has been an incandescant bulb which has been installed without regard to vapor condensation on the interior surfaces of the bulb. In such cases where vaporized matter condenses on portions of the bulb used to transmit the principle beam of light, undesirable degradation of the light intensity has occurred over long periods of time.
In most cases prior art devices have failed to incorporate a fail-safe circuit which would generate a signal indicating failure of the optical portion of the system. In those cases where a fail-safe circuit and fail-safe signal is provided, the prior art has usually failed to provide a fail-safe signal which is distinguishable from the general smoke alarm signal, or where such capability has been provided, it has been achieved only by comparatively complex circuitry including electromechanical means.
Finally, prior art devices have generally failed to incorporate a simple means by which the entire circuitry and overall operation of the smoke alarm could be easily and periodically tested by means other than introducing an actual smoke sample into the device, or by means necessitating the removal of a portion of the device.
Therefore, what is needed is a smoke detection and alarm device having long term stability and reliability with high sensitivity irrespective of changes in voltage supply, temperature, dust and grease accumulation, filament age, detector degradation, smoke color, and humidity. In addition, what is needed is a fail-safe circuit, having high reliability simplicity and stability, capable of generating a fail-safe signal which is readily distinguishable from the general smoke alarm signal. What is also needed is a means for easily and periodically testing the overall operation of the device without actually introducing a smoke sample into the device, or without removal of any portion of the device.