SPIROMETER UNIT. 43 



A specimen calibration of the bell of a spirometer follows: 



Height of bell at start, 42 mm.; at end, 175 mm. 



Oxygen admitted, 2.935 liters; factor of meter, 0.9623; temperature 



of meter, 18.8° C; temperature of water in spirometer, 19.2° C. 

 (2.935 X (273.0 + 19.2) X 0.9623) 4- (175 - 42) X (273.0 + 18.8) = 



21.28 c.c. per mm. 



The volume represented by each millimeter rise of the bell is there- 

 fore 21.28 c.c. 



Calibration of the ventilation adder. — The periphery of the wheel of 

 the ventilation adder is milled. The pawl above the wheel is triangular 

 at the end and engages in this milling as the bell moves in an upward 

 direction. Notwithstanding this arrangement, however, there is some 

 slight backward movement. Theoretically the value in c.c. of one revo- 

 lution of the ventilation adder wheel should be equivalent to the circum- 

 ference of the wheel in millimeters multiplied by the value in c.c. of 

 a millimeter of the bell of the spirometer. The calibration can be 

 carried out in a number of different ways. The bell of the spirometer 

 may be filled with air or oxygen and readings taken of the level of the 

 spirometer and of the ventilation adder wheel; the bell may then be 

 pushed down until it is empty and a second reading taken of the level 

 of the spirometer bell and of the ventilation adder wheel. As this 

 method does not take into account any backward movement, the cali- 

 bration should be carried out under as nearly the same conditions as 

 possible as those which are present when the subject is breathing into 

 and out of the apparatus. This may be accomplished by connecting 

 a bulb to the opening of the three-way valve, this bulb being connected 

 to another bulb filled with water, the upper and lower portions of the 

 bulb being marked. The first bulb may be alternately filled and 

 emptied to the upper and lower marks by raising and lowering the 

 second bulb. An up and down motion of the spirometer bell is thus 

 produced, simulating respiration. If the exact volume between the 

 two marks on the first bulb is known, also the number of movements 

 or strokes and the number of revolutions of the ventilation adder, the 

 value per revolution may then be calculated. 



This method was used in the development of the apparatus, but 

 recently the ventilation adder has been calibrated by a more convenient 

 method in which the small hand spirometer, described in detail on 

 page 79, has been used. This hand spirometer consists of an inverted 

 cylinder which moves in a bath between two concentric cylinders on 

 the same principle as the spirometer of the respiration apparatus. A 

 handle is fastened to this inverted cylinder by which it can be moved 

 up and down in a rigid framework, the length of the stroke (vertical 

 movement) being adjusted by a set-screw. The general method of 

 calibrating the ventilation adder with this apparatus is as follows: 

 The small spirometer is connected to the three-way valve, the ventila- 

 tion adder wheel is set at zero, and the kymograph drum is brought 

 near the writing-point of the spirometer on the respiration apparatus. 



