A self-calibrating obscuration smoke detector is provided along with a method for the operation thereof. A light source and a photodetector are mounted in spaced relation to one another with the output of the photodetector being a function of the amount of light sensed by the detector from the light source. The photodetector analog output is converted into digital signals by an A/D converter and digital signals are then delivered to a digital processor adapted to periodically calibrate the detector and to perform sampling operations between calibrations. Other sensing devices may be connected to the system with automatic self-calibrating capabilities with respect thereto including a temperature responsive element the output of which may be used to compensate for changes in ambient temperature. By using a pair of photodetecting cells directly visible to separate light sources or indirectly visible to a single source, a thermally stable system is provided where one cell provides a reference output for the other cell.

A smoke detector comprising a radiation source producing a directed radiation beam and a scattered radiation receiver arrangement having directional characteristics. An evaluation circuit is operatively connected with the receiver arrangement for delivering a signal whenever there is exceeded a predetermined degree of radiation scattered at smoke particles within the radiation beam. Either the radiation source, the radiation receiver arrangement, or both, have such directional characteristic and the evaluation circuit is constructed such that the radiation received from two fields of view of the radiation receiver arrangement are compared with one another. These two fields of view are differently situated relative to the radiation beam and both fields of view contain a markedly different amount of scattered radiation, but receive at least approximately the same spurious radiation from at least approximately the same background.

A method and apparatus for evaluating the end cap round transducer horn assemblies used in debubbling operations wherein the ECR THA can be evaluated off-line at both high and low power and on-line by making electrical measurements on the ECR THA. The electrical measurements are used to characterize the physical condition of the piezoelectric ceramics of the THA. A test box is employed to practice the method. The test box is connected between the THA and a signal analyzer. Power is supplied to the THA and the electrical signals across the THA are sampled. The sampled electrical signals are transmitted to the signal analyzer while maintaining the amplitude and phase relationship thereof. The sampled electrical signals are used to generate an impedance trace for the particular THA. That impedance trace is compared to a model impedance trace. In such manner, it can be determined whether the ECR THA is operational. Further, if the ECR THA is in working condition, the impedance trace can be used to determine how efficiently it is operating. This allows for an ultimate determination to be made of how well a particular ECR THA is functioning.

A photoelectronic smoke detector has walls around a dark chamber to which smoke, but not light, is admitted, and through which light is directed from a source on a path to a limited area of a chamber wall. A first photocell views light scattered from the through path by particles in the path primarily and also receives background light scattered from the chamber wall outside the limited area. A second photocell is disposed to receive background light substantially only from the limited area and other areas outside the source light path. The first cell produces a first signal in response to background and particle-scattered light. The second cell produces a second signal corresponding to background scatter. The first and second cells, preferably photovoltaic photodiodes, are coupled in opposition in a circuit whose output consequently is substantially independent of the background light.

A test means for a smoke detector of light responsive type which includes a light source, a smoke chamber, a light responsive device sensitive to a light beam reflected from a preselected density of smoke in the smoke chamber for actuating an alarm signal, the test means including the same light source and light responsive device and having a separate test light path other than through said smoke chamber with light modifying means in the test light path to simulate a preselected smoke density. The test means includes a normally closed gate for said test light path, the gate being manually opened to transmit light along said test path to the light responsive device to cause actuation of the alarm in the absence of smoke in the smoke chamber.