CRITICAL DISCUSSION OF RESPIRATION APPARATUS. 243 



exchange, it is sufficient to adjust the counterpoise to an equilibrium 

 with the spirometer bell in a central position. This obviates the 

 necessity of having a water-supply, only sufficient water being required 

 to keep the level of the water inside the tank constant. Care must be 

 taken, however, that the counterpoise is so adjusted that its weight 

 does not exceed that of the drum by such an amount as would 

 produce a decreased tension inside the spirometer which might be 

 sufficient to open the valves and cause a movement of the drum 

 independent of the movements due to the increase in the expired air. 

 This would result in an error in the ventilation figures, although it 

 would not affect the determination of the respiratory exchange. 



The possible errors in the determination of the respiratory exchange 

 by the Tissot method may be divided into two classes : one, those due 

 to factors influencing the readings made in the measurement of the 

 volume of the total air expired, the other due to factors influencing 

 the sampling and the analysis of the expired air. The first two 

 sources of error to be considered in the measurement of the total volume 

 of expired air are those which affect the readings of the barometric 

 pressure and the temperature of the air. The possible inaccuracy in 

 the value for the barometric pressure is extremely small, for with any 

 good barometer readings may be obtained to 0.1 mm.; the error would 

 thus be not more than ^O.l mm., which is well within the limits of 

 error in determining the respiratory exchange. For determining the 

 temperature of the air in the spirometer, a thermometer may be placed 

 in the opening provided at the top and readings made to 0.1° C. It 

 must be considered, however, whether a value thus obtained represents 

 the true temperature of the air inside the spirometer. Errors may be 

 avoided by having the water in the apparatus of the same temperature 

 as the air in the room, so that air collected in the spirometer may be 

 more nearly the temperature of the room than if extremely cold water 

 were used. 



To test the accuracy of the volume measurement, a series of experi- 

 ments was made in which 50 liters of air were collected in a 50-liter 

 spirometer, a 100-liter spirometer, and a 200-liter spirometer and 

 allowed to remain for several days. The temperature was obtained 

 each morning with the siphon automatic device actuating; the baro- 

 metric pressure was also recorded. The volumes were then calcu- 

 lated to 0° C. and 760 mm. pressure. The variations obtained with 

 the three spirometers were 0.7 liter for the 200-liter spirometer, 0.2 

 liter for the 100-liter spirometer, and 0.5 liter for the 50-liter spirometer. 

 The readings may be made more closely on the 50-liter spirometer than 

 on the 200-liter spirometer, as the length of the scale is approximately 

 the same with both apparatus; an increase in volume of 1 liter will 

 therefore produce a greater rise with the 50-liter spirometer than with 

 the 200-liter spirometer. It is quite possible to read to 0.05 liter with 



