794 APPENDIX III 



in error due to some local cause or other condition, as indicated by its reading, 

 after the addition of the velocity head, giving a head in excess of the initial 

 available head corresponding to the elevation of the surface of headwater, 

 the source of the discrepancy shall be found and removed, or the piezometer 

 eliminated. 



When stilling boxes are used in the tailrace the communication between 

 the box and channel shall consist of one or more piezometer openings in a 

 plane surface parallel to the flow, in order to avoid velocity effects. When 

 board gauges are used at the side of the channel, they shall be flush with 

 the wall surface. 



The effective head on the turbine is to be taken as the difference between 

 the elevation corresponding to the pressure in the penstock near the entrance 

 to the turbine casing, and the elevation of the tailwater at the highest point 

 attained by the discharge from the unit under test, the above difference being 

 corrected by adding the velocity head in the penstock at the point of measure- 

 ment and subtracting the residual velocity head at the end of the draft tube. 

 The velocity head in the penstock shall be taken as the square of the mean 

 velocity at the point of measurement, divided by 20; the mean velocity 

 being equal to the quantity of water flowing in cubic feet per second, divided 

 by the cross-sectional area of the penstock at the point of measurement 

 in square feet. The residual velocity head at the end of the draft tube shall 

 be taken as the square of the mean velocity at the end of the draft tube, 

 divided by 2g, the mean velocity being equal to the quantity flowing in 

 cubic feet per second, divided by the final cross-sectional discharge area 

 of the closed or submerged portion of the draft tube in square feet. 



(6) Open Flume Setting. In the case of turbines set in open flumes, the 

 head is to be measured by board, rod or float gauges located immediately 

 above the center of the turbine, and by board, rod or float gauges in the 

 tailrace, all gauges being placed at points reasonably free from local dis- 

 turbances, and not less than two gauges being installed in the flume and not 

 less than two in the tailrace. 



Such gauges are to be free of velocity effects,a nd if this is not obtainable 

 when the gauges are setjn the open channel, they shall be placed in properly 

 arranged stilling boxes. When stilling boxes are used, the communication 

 between the box and channel shall consist of one or more piezometer openings 

 in a plane surface parallel to the flow, in order to avoid velocity effects. 

 When board gauges are used at the side of the channel, they shall be flush 

 with the wall surface. 



The effective head on the turbine is to be taken as the difference be- 

 tween the elevation of the free water surface immediately above the center 

 of the turbine, and the elevation of the tailwater at the highest point attained 

 by the discharge from the unit under test, the above difference being cor- 

 rected by subtracting the residual velocity head at the end of the draft tube. 

 The residual velocity head at the end of the draft tube shall be taken as the 

 square of the mean velocity at the end of the draft tube, divided by 2g; the 

 mean velocity being equal to the quantity flowing in cubic feet per second, 

 divided by the final cross-sectional discharge area of the closed or sub- 

 merged portion of the draft tube, in square feet. 



