Patent Number: 039873030
Section: summary

BACKGROUND OF THE INVENTION The use of infrared gas analyzers is becoming of increasing importance in several different fields. For example, in the medical field, infrared gas analyzers may be used to monitor concentration of gases in the blood or in a sample of expired air. In conjunction with pollution control, infrared gas analyzers may be used to monitor air pollutants deleterious to the environment. A particular method for monitoring CO.sub.2 in the blood is by means of a transcutaneous measurement at the surface of the skin. Typically, the outermost layer of a small area of skin on a patient is removed, e.g., by repeated application of a strip of adhesive tape. A small pill box type device may then be sealed onto the skin so that the atmosphere in the box can come into pressure equilibrium with the body fluids in the skin. The concentration of CO.sub.2 in the pill box may then be measured by a suitable infrared gas analyzer which is preferably included within the pill box. An effective transcutaneous CO.sub.2 measurement of the kind described above requires that the infrared gas analyzer be capable of providing a very rapid response time while utilizing only a small sample volume. Furthermore, the detector output should be insensitive to extraneous variables such as the intensity of any light sources employed, detector bias, and aging effects. Additionally, in medical applications of this kind the detector will be exposed to various sterilizing agents as well as contaminants present in the body fluids. The detector output should therefore be insensitive to contamination from these sources. SUMMARY OF THE INVENTION In accordance with one of the illustrated preferred embodiments, the present invention provides an infrared gas analyzer which is particularly suited for use in a transcutaneous CO.sub.2 measurement device. The analyzer uses a single infrared source and a single detector illuminated through a single interference filter. Positioned in the optical path between the source and the detector is a rotating wheel containing two reference cells and a sample cell. The two reference cells are each enclosed between a pair of window surfaces. One of the reference cells is filled with a gas containing a standard quantity of CO.sub.2, which may be mixed with an inert gas. The other reference cell contains no CO.sub.2. In accordance with one embodiment of the invention, the sample gas is circulated in a region surrounding the rotating wheel. The sample cell is specially constructed to be an "open" cell directly accessible to the circulating sample gas. For example, the specially constructed sample cell may comprise a cell-like opening of the same volume as the reference cells, but having a pair of sapphire windows closely spaced to each other on only one side of the cell. Thus, a sample gas admitted to the region surrounding the rotating wheel will freely flow also into this sample cell region. In operation, the wheel is rotated to sequentially present the two reference cells and the sample cell in the optical path between the infrared source and detector. Three signal outputs are thereby generated. The detected signal outputs during the intervals when the two reference cells are in the optical path provide two standard readings, e.g., zero and full-scale output readings. During the interval when the sample cell is in the optical path the CO.sub.2 in the sample gas absorbs some radiation so that the signal amplitude is a function of the partial pressure of CO.sub.2 in the sample. As will be described in more detail below, these three signals may be utilized to produce an associated output signal which is indicative of the partial pressure of CO.sub.2 in the sample, and is also essentially independent of variations in the source intensity, detector efficiency, or contamination of the optical windows by contaminants in the sample. In accordance with another of the illustrated embodiments of the invention, an analyzer is provided in which the sample cell is an enclosed cell isolated from the rotating reference cells. This embodiment of the invention is particularly suited to applications in which there is a high probability of contaminating fluids being present in the gas, e.g., in measurements of CO.sub.2 in air expired from the lungs. The isolated sample cell can be easily cleaned or replaced. In the preferred embodiments of the invention, the rotating wheel is driven by the interaction of a number of permanent magnets positioned in the wheel with electromagnetic coils mounted in the fixed housing. The electromagnetic coils are activated sequentially in response to optoelectronic signals generated from a number of optical timing marks positioned around the periphery of the wheel; very precise rotational frequency is thereby maintained. The timing marks may also be used to control external signal-processing circuitry.