OXYGEN SUPPLY. 



49 



Accuracy in filling the meter for the several calibrations is also an 

 important consideration. The meter should always be filled to within 

 at least 1 mm. of the same level each time and, if the other observations 

 are made with sufficient accuracy and uniformity, the only cause for 

 variation in the mechanical factor should be the level. Calibrations 

 independently made by two observers after emptying and refilling the 

 meter each day are given in table 6. 



That the difference in level of the water inside the meter makes a 

 difference in the factor of the meter is shown by some experiments 

 which were carried out by Dr. E. P. Cathcart, 1 of the London Hospital 

 Medical College. In these tests approximately 4 liters of oxygen were 

 passed through the meter in from 2 to 3 minutes. The volume at 0° and 

 760 mm. as measured by the meter was computed from the meter read- 

 ings and the records of the temperature and barometer; the true volume 

 was computed from the loss in weight of the oxygen cylinder. The 

 correction factors, which are given in table 7, were calculated by 



Table 7. — Results of Cathcart' s experiments on the effect 

 of varying levels of water in the meter. 



dividing the true volume of gas leaving the cylinder by the amount 

 computed from the meter readings. It will be seen from table 7 that 

 there was a marked change in the correction factor of the meter when 

 the water-level was increased in height. It is also quite possible to 

 have the level of the water so low that the meter will not record at all. 

 It has been pointed out that the meter must be calibrated under 

 exactly the same conditions as used in the experiment. One of these 

 conditions is rapidity of admission. If the oxygen is admitted at the 

 rate of 1 liter in 3 or 4 minutes, it should be calibrated at that rate; 

 if more rapidly, it should be calibrated at the higher rate. The effect 

 of the rate of admission upon the correction factor is clearly shown in 

 the series of results which were obtained by Dr. Cathcart in connection 

 with an experiment on muscular work. (See table 8.) The time 

 varied from the rate of 4 liters in 21 seconds to the rate of 4 liters in 

 9 minutes 30 seconds. It will be seen that up to the rate of 4 liters 

 in 2 minutes 6 seconds, the correction factor varies with the rate that 

 the gas passed through the meter. During the calibrations particular 



Research Associate of the Nutrition Laboratory in 1911-12. The results of these tests have 

 been previously published in a description of the spirometer unit. (Benedict, Deutsch. Archiv 

 klin. Med., 1912, 107, p. 183.) 



