44 COMPARISONS OF RESPIRATORY EXCHANGE. 



The kymograph is next set in motion and the three-way valve opened 

 between the small hand spirometer and the large spirometer. Regular 

 movements up and down are then made with the small spirometer, care 

 being taken that the beginning of the movement at the bottom and the 

 end of the movement at the top are made slowly so as not to jar the 

 ventilation adder wheel. This is continued until the wheel has 

 revolved a number of times. The kymograph record is then coated 

 with a fixative and when it is dry a number of measurements of the 

 records of the strokes are made, using a pair of dividers and a milli- 

 meter scale, and estimating to about 0.1 mm. The average of ten 

 measurements is then multiplied by the value per millimeter of the 

 bell of the spirometer (21.33 c.c. in the example given) and the total 

 number of strokes. This gives the total volume required to move the 

 ventilation adder wheel the number of revolutions which has taken 

 place. Dividing this volume by the number of revolutions gives the 

 "apparent" volume per revolution. A sample calculation and calibra- 

 tion is given : 



Length of movement of the bell of spirometer, 26.1 ram. 



Number of movements, 82. Number of revolutions of the alumi- 

 num wheel, 9.10. 



Calculation: (82X26.1X21.33)^9.10 = 5.02 liters, volume per revo- 

 lution. 



Back-lash. — In the actual use of the ventilation adder wheel there 

 is a certain amount of backward movement each time that the spiro- 

 meter bell moves in an upward direction. This is due to the fact that 

 the edge of the wheel is milled and the transverse grooves are wide 

 enough to permit some backward motion before the pawl fits firmly 

 into the groove. In order to determine the amount of this backward 

 movement, calibrations of the ventilation adder may be made with 

 two different lengths of stroke. If the same number of complete 

 revolutions are obtained, the value per stroke for the back-lash may then 

 be calculated from the difference in number of strokes and the difference 

 in total volume for the complete number of revolutions. This has been 

 done in a number of calibrations and the results are as follows : 



Calibration with 7.04 mm. movement of the bell of the spirometer 

 gave, as a result, 5.49 liters per revolution of the ventilation 

 adder wheel. 



Calibration with 26.06 mm. movement of the bell of the spirometer 

 gave, as a result, 5.05 liters per revolution of the ventilation 

 adder wheel. 



The number of movements of the bell for 9 revolutions of the venti- 

 lation adder wheel, with 7.04 mm. per movement, was 248 

 greater than that with 26.06 mm. movement. 



The difference in volume for 9 revolutions amounted to 3.96 liters. 



Therefore, the amount of backward movement of wheel at each 

 stroke was 3.96-r- 248 = 0.016 liter per movement. 



