Abstract: Some cyclic variables include within each cycle a value that can be determined extrinsically. In such case, the sensed value may differ from the known value by an amount that is a combination of long-term drift of the sensor and random measuring error. The drift component can be evaluated and eliminated by the following method. Once each cycle, for a number of cycles, the sensor measures the variable at a time when its value should equal the extrinsically-known value. The differences are plotted versus time, and a best-fitting straight line is determined, which indicates the drift. Throughout the next cycle as the variable is continuously sensed, the drift determined from the best-fitting straight line is continuously applied to correct the sensed value.

Abstract: A compact low-cost fire detector responds quickly by detecting an increase in the concentration of carbon dioxide gas in the ambient air. The detector also calculates the rate of build-up of carbon dioxide. The detector avoids the use of moving parts by employing a differential temperature black body source of infrared radiation in conjunction with a dual pass band filter. One of the pass bands is located at the 4.26 micron absorption band of carbon dioxide gas and the other pass band is located at 2.20 microns at which none of the atmospheric gases has an absorption band. The latter channel serves as a reference and renders the detector immune to false alarms caused by dust or smoke particles in the air or due to deterioration of certain components. The fire detector makes use of a sample chamber that consists of a serpentine passage in a block of material, the walls of the serpentine passsage being highly reflective so as to act as a light pipe.

Abstract: A system for detecting fires uses at least two carbon dioxide sensors positioned at spaced locations in a room. Each sensor produces an electrical output signal representative of the carbon dioxide concentration in its vicinity. A computer calculates the ratio of the concentration sensed by each sensor to the concentration sensed by each of the other sensors, and any imbalance in the distribution of carbon dioxide will be reflected in these ratios. Random variations prevent the ratios from being equal, and the magnitude of the random variations is quantized by calculating the standard deviation of the ratios. The ratios are then normalized and compared to a threshold level that corresponds to a chosen false alarm rate.

Abstract: A sample chamber for use in measuring the absorption of radiation as it passes through a gas within the chamber includes a block having an extended serpentine passage through it. The walls of the extended passage are coated with a highly reflective material so that the extended passage acts as a light pipe for transmitting the radiation. A number of smaller passages permit gases in the space surrounding the sample chamber to diffuse into the extended passage through which the radiation is conducted. The sampling chamber is made by joining two halves, each of which has a planar face in which an elongated groove is produced. The halves are molded of plastic and in quantity the chamber is quite inexpensive.

Abstract: A fire detector that responds more rapidly than the widely used smoke detectors senses increases in the concentration of carbon dioxide associated with a fire. The build-up of carbon dioxide is sensed by measuring the concomittant increase in the absorption of a beam of radiation whose wavelength is located at a strong absorption band of carbon dioxide. The device is considerably simplified by the use of a window to the sample chamber that is highly permeable to carbon dioxide but which keeps out particles of dust, smoke, oil, and water. This permits the fire detector to be built of very inexpensive materials.

Abstract: An instrument for determining the concentration of a particular gas that might be present in a sample has no moving parts and is extremely compact. The instrument uses as a source of radiation a device that has a radiating element whose temperature is alternated between T.sub.1 and T.sub.2 and whose spectrum approximates that of a blackbody. Radiation from this source is passed through a dual pass band filter that has two non-overlapping pass bands, one of which is centered at a wavelength at which the gas absorbs and the other of which is centered at a wavelength at which the sample does not absorb radiation. After passing through this filter, the radiation passes through the sample chamber and then is intercepted by a detector which produces an electrical signal determined by the radiation intercepted. The electrical signal is processed to provide an indication of the concentration of the gas.