452 



are of different sizes, according to the quantity of water required 

 to be registered. 



PRESSURE GAGES. 



A pressure gage, often called a piezometer, is an instrument 

 for measuring the pressure of water in a pipe. The form most 

 commonly found in the market has a dial and movable pointer, 

 the dial being graduated to read pounds per square inch. The 

 principle on which this gage acts is the same as that of the 

 Richard aneroid barometer and the Bourdon steam gage. Within 

 the case is a small coiled tube closed at one end, while the other 

 end is attached to the opening through which the water is ad- 

 mitted. This tube has a tendency to straighten when under pres- 

 sure, and thus its closed end moves and the motion is communi- 

 cated to the pointer ; when the pressure is relieved the tube as- 

 sumes its original position and the pointer returns to zero. There 

 is no theoretical method of determining the motion of the pointer 

 due to a given pressure, and this is done by tests in which known 

 pressures are employed, and accordingly the divisions of the 

 graduated scale are usually unequal. These gages are liable to 

 error after having been in use for some time, especially so at high 

 pressures, and hence should be tested before and after any im- 

 portant series of experiments. 



VENTURI METERS. 



The Venturi meter is best adapted for measuring the flow in 

 pipes of six inches or more in diameter. It may also be used for 

 smaller pipes, but its cost usually makes its use for small pipes 

 prohibitive. This meter has no interior mechanism, but con- 

 sists of a section, inserted in tlie pipe, in which the cross-section 

 is gradually contracted to a throat, usually one-ninth the area of 

 the pipe, and is then uniformly increased until the original diam- 

 eter of the pipe is reached. The distance between the upper end 

 and the throat is less than the distance from the throat to the 

 lower end. The pressures at the upper end at a point just before 

 contraction begins, and at the throat, are registered by mercury 

 pressure gages. By knowing these differences of pressures, or 

 heads, and the respective areas of the pipe and the throat, the 

 discharge may be computed by a simple formula. The Venturi 

 meter has been thoroughly tested and is now the standard meas- 

 uring device used in all large pipes and conduits. A modifica- 

 tion of the Venturi meter for open ditch measurements has re- 

 cently been experimented with and indications point to its success. 



Another method of gaging the flow in pipes is by means of the 

 Pitot tube, which, in its simplest form, consists of a bent tube, 

 the end of which is placed so as to directly face the current in 

 the pipe. The water pressure forces the water into the tube, and, 

 by measuring this height and applying a simple formula, the ve- 

 locity of the flow can be obtained. The Pitot tube is also used 

 for obtaining the velocity of water in open channels. 



