Patent GB-1153230-A (GB)
Title: Gas Analyzer-Controller Equipment
Inventor: LEVY DONALD
IPC: A61M16/10
Filed: 1967-02-20
Abstract: 1,153,230. Anaesthetizing apparatus. MILAB, Inc. 20 Feb., 1967 [18 Feb., 1966], No. 7950/67. Heading A5T. [Also in Divisions G1 and G3] Apparatus for anaesthetizing a patient 3 comprises a machine 1 applying an anaesthetic gas mixture to the patient via coupling circuits 2, 4 and an analysing device which determines the concentration of anaesthetic gas in the mixture and automatically controls the addition of further anaesthetic gas to the mixture. As shown, the closed circuit breathing system includes a cell 10 through which a sample of the anaesthetic gas may be passed, the system also including an ultra-violet source 22 and a photocell 23, 24 whose output controls a vapourizer 18 able to supply additional anaesthetizing halothane gas to the circuit. The machine 1, which provides an oxygen supply and includes a flow meter, breathing bag, and carbon dioxide absorber, is said to be of known type, and the closed circuit breathing system includes solenoid-controlled valves 6, 7 and a pump 11 whereby the system is able to pass air through the sample cell 10 and periodically automatically re-calibrate its anaesthetic metering circuitry. The sample cell 10 may be provided (see also Figure 4, not shown) with fluorescent coatings 25, 26 whereby both visible and ultraviolet light is caused to pass through the cell 10 on to a photocell 23, 24, the ultraviolet light being absorbed by any anaesthetic gas present so that the photocells unbalance an electrical measurement bridge MB, Figure 3 (not shown) which is calibrated to be in balance when air alone is in the sample cell. Alternatively (Figures 5, 6 not shown), fluorescent filters are repeatedly interposed about the sample cell in the path of the ultraviolet source in time with switches (B41, B42, C41, C42) in capacitor measurement bridges. (In the embodiment of Figure 5 only one filter B31 is repeatedly interposed and withdrawn, in that of Figure 6, two such filters C45, C46 are used alternatively). In a final embodiment (Figure 7, not shown), an apertured screen (D50) is rotated around the ultraviolet source so that the light alternately passes direct to the photocell (D36) or via the sample cell 10. In this final embodiment, the photocell (D36) has a fluorescent coating (D53) on a glass cover plate (D29). The wavelength of the ultraviolet light is exemplified, the fluorescent material comprises various silicates, calcium phosphate, or calcium borate on quartz, and the photocells may comprise cadmium sulphide. When anaesthetic gas is present in the sample cell, the measurement bridge MB (Figure 3, not shown) is thrown out of balance to an extent (measured by a meter 28) depending on the anaesthetic gas concentration. Valves are actuated to add anaesthetic gas if the concentration is not at the required level. The measurement bridge periodically recalibrates itself with the aid of a servo motor 31 which moves a rheostat (70) in the bridge and the time intervals between recalibration are determined by a rotating cam (135). The control circuitry (Figure 3, not shown) includes emitter follower stabilizing transistors (60, 62) supplying the measurement bridge, similarly arranged transistors (81, 82) supplying the ultraviolet source 22, and a regulating amplifier (80, 91, 92).