I N 1941 [I] the use of helium was introduced for the purpose of estimating functional residual capacity by a closed spirometer circuit technic using a thermal conductivity meter for the analysis of helium in a procedure much like McMichael’s version of the hydrogen method [2]. Subsequently closed helium methods were compared with various procedures by us [3] and by others [4]. Fowler [5’J concludes that in comparison with other methods in common use “greater precision appears possible with closed circuit methods using serial helium or nitrogen analyses.” As described originally our method required a rather elaborate spirometer circuit and employed the technic introduced by Herrald and McMichael [6] to avoid errors believed to be attributable to the “oxygen storage effect” of Lassen, Cournand and Richards [7,8]. One of the valid criticisms of the earlier method was the complexity of the equipment [5]. We have re-examined some of the theoretical and practical considerations in estimating functional residual capacity by the dilution of helium and devised a greatly simplified version of the procedure which yields highly reproducible results with an instrument which is useful for a variety of other purposes and not unduly expensive. Apparatus. A 9 L. respirometer $ was modified slightly (Fig. 1) by obliteration of dead space $ Warren E. Collins Company, 555 Huntington Avenue, Boston, Massachusetts. under the soda lime canister, substitution of a motor driven blower for the internal Sadd flutter valves, introduction of a water level gauge on the spirometer water jacket, provision of a shunt tube to pass some of the gas in the inspiratory tube through the helium meter analytical cell, provision of stopcocks for rinsing tubing and charging the spirometer with helium and oxygen. The entire equipment as well as the small oxygen and helium tanks, the power supply and the helium indicator was mounted on an adjustable hospital bedside table. (Fig. 2.) This apparatus is easy to adjust to the patient whether he is in a prone, sitting or standing position, and constructed so that gas volumes in the spirometer bell may be measured with an accuracy within 10 ml., eliminating the need for a separate gas measuring burette for the helium charge. These modifications in no way impair the usefulness of the spirometer for other purposes. The substitution of the blower reduces resistance materially. f Determination of Dead Space. The blower is turned on, the spirometer is well rinsed with room air, the helium meter “warmed up” and balanced according to instructions, the water 11 Cambridge Instrument Company, Graybar Building, 420 Lexington Avenue, New York 17, New York. Meter calibrated to indicate 0 to 15 per cent helium in air saturated with water vapor. 7 A commercial version of this equipment is available through either the Warren E. Collins Company or The Cambridge Instrument Co.