24 



HYDRAULICS AND ITS APPLICATIONS 



alcohol and water of specific gravity, say *85, and kerosene of specific 

 ity, say '82, the coefficient becomes 1 -f- | j(#'+ 8) + (S 1 S)\. 



Thus in the latter case taking -j = T V> tne coefficient becomes 7'43, 



while with water and kerosene in the same tube the coefficient is 6'1. 



Errors of Fluid Differential Gauges. Any error made in estimating 

 the specific gravity of one of the fluids in a differential gauge will lead to 

 an error in the gauge coefficient, and this error will be proportionately 

 greater the larger the coefficient. Where the heavier fluid is water, an 

 error of one-tenth of 1 per cent, in the estimation of S will lead to 

 the following percentage errors in the value of C. 



In a gauge which may be required to record under widely differing 

 pressures, an appreciable error may be introduced, unless allowance is 

 made for the different compressibilities of the two fluids. In such a 

 gauge as is shown in Fig. 9, where air and water are the fluids, the 

 gauge reading is usually taken as giving the true pressure difference in 

 feet of water, although, strictly speaking, this is the difference between the 

 pressures due to this same height of columns of air and of water. At 

 atmospheric pressure the error thus introduced is negligible, but at 

 higher pressures it may become important, as indicated in the following 

 table. 



The percentage error is smaller where two liquids are used, owing to 

 the smaller difference in their compressibilities, although with toluene and 



