BED CALORIMETER AND TENSION-EQUALIZER UNIT. 105 



no chamber and the man breathes into the apparatus by means of a 

 mouthpiece or nosepieces. The same errors apply to the measurements 

 of the various factors, except that with this apparatus the effect of 

 muscular activity on the temperature of the air in the apparatus plays 

 no role. The measurements are affected, however, by changes in the 

 barometric pressure or in the temperature of the air in the apparatus. 



Errors due to variations in barometric pressure and temperature of 

 the apparatus. — The total volume of the spirometer unit is about 10 

 liters; the volume of the tension-equalizer unit is somewhat less. 

 Assuming the volume of the apparatus to be 10 liters, a variation of 

 1 mm. in the barometric pressure would change the total absolute 

 volume 0.013 liter, but a change of 1 mm. in 15 minutes, which is the 

 ordinary length of an experimental period with this apparatus as used 

 in the Nutrition Laboratory, is an enormous barometric change under 

 ordinary conditions. Should there be such a change in the barometric 

 pressure, however, the error in the determination of the oxygen con- 

 sumption would equal but 1 c.c. per minute. 



A change of 1° C. in the temperature of the apparatus results in a 

 change in the volume of 37 c.c, or a little over 2 c.c. per minute in a 

 15-minute period. The total change in the temperature of the appa- 

 ratus is rarely over 1°C, so that omitting both the reading of the barom- 

 eter at the beginning and end of an experiment and the record of the 

 change in temperature of the apparatus would produce an error too 

 small to be ordinarily considered. 



Errors in determining the carbon-dioxide elimination and oxygen con- 

 sumption. — The errors in weighing may be ^O.Ol gm. If the meter 

 is used for measuring the oxygen, the error may be larger than this, 

 i. e., about =*= 1 per cent, which would be equivalent to 2 c.c. per minute 

 with an oxygen consumption of 225 c.c. 



There is also a possibility of error due to the incomplete absorption 

 of carbon dioxide if the apparatus is not properly controlled. If the 

 soda-lime used in the carbon-dioxide absorber is not completely effi- 

 cient, the carbon dioxide may accumulate in the system instead of 

 being absorbed. This source of error has, however, been eliminated 

 during the past two or three years by the test for the presence of car- 

 bon dioxide in the air after it leaves the soda-lime container. 



Occasionally, through carelessness, it may happen that the water 

 absorbers in the lower part of the apparatus are deficient, and some of 

 the water- vapor in the air from the lungs of the man escapes absorption. 

 In this case, it will be absorbed by the sulphuric acid in the water- 

 absorber following the soda-lime container. Since this water-absorber 

 should only absorb the water taken up by the air from the moist soda- 

 lime, and the amount of carbon dioxide produced is determined by 

 noting the increase in the combined weights of the soda-lime container 

 and the following sulphuric-acid container, the record of the carbon 



