36 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 62 



was used in all tests. The alcohol gage was fitted with a straight 

 glass tube 0.15 inch in diameter. The tube was clean and dry. The 

 velocity calculated from the alcohol gage was found to be 12 per cent 

 low. The tube was then wet by blowing the liquid to the top of the 

 scale and then allowing it to settle back to zero. Readings taken 

 subsequently were only 4 per cent low. The experiment was repeated 

 using a glass tube 0.17 inch in diameter, clean and wet. The velocity 

 recorded was 4 per cent low. A different tube, but of same diameter, 

 was then put in the alcohol gage. Its average readings were found 

 to be 10 per cent low. Examination of the glass tube showed two 

 minute cracks in the glass hardly to be seen with the naked eye. A 

 glass tube 0.2 inch in diameter was then tested and read 2 per cent 

 low. A tube 0.22 inch in diameter read 1.5 per cent low. A tube 

 0.25 inch in diameter could not be used on the 3-degree pitch, as the 

 alcohol would not form a meniscus. 



In all, some 1,000 check observations were made, and the following 

 conclusions drawn : 



(i) The inclined type of liquid gage as commonly employed in 

 ventilation work is not an instrument of precision. 



(2) For consistent results, the glass tubing used must be free from 

 all slight flaws on the inner surface which might cause changes in 

 capillarity throughout the bore. 



(3) The tube must be uniform in diameter. 



(4) The tube must be as large as it is possible to use and still get 

 a good meniscus. 



(5) For alcohol at 3 degrees inclination an internal diameter of 

 0.22 inch is suitable. 



(6) The maximum precision with such a gage used to measure 

 air speeds from 4 to 40 miles per hour is about 1.5 per cent on 

 velocity. 



(7) The alcohol gage properly constructed is consistent and very 

 sensitive. 



(8) The alcohol gage may be used as an instrument of precision 

 when calibrated against a standard. 



In its final form with 0.22-inch tube, this alcohol gage was found 

 to measure speeds within 1.5 per cent. Such precision is ample for 

 engineering work, and this type of gage is recommended for a cheap 

 portable instrument. For a laboratory standard, however, an error 

 of 1.5 per cent cannot be accepted. Since the gage responded to 

 changes of velocity of less than | per cent, its sensitivity is such that 

 it may be calibrated against a better manometer, and when calibrated, 

 may be as precise as the standard. 



