TM No. 377 



recorder to the final presentation. These include errors in editing, data 

 tape reading, and possible biasing associated with interpolation and final 

 computations . Errors in velocity measurement incurred by the wave meters 

 or induced by the strip chart recorder are more fittingly considered in 

 chapter V after we have seen some of the field results. 



Data Editing — The hand editing of the primary raw data tapes (see 

 figure II-7) was a straightforward procedure. This step was critical, how- 

 ever, in that it provided information needed by the person reading the tapes 

 on the film reader. Explicit directions were written on the tapes concerning 

 the sense of the voltage pulses in relation to the sign of the associated 

 velocity values and the tape speeds. 



The most serious error that can be introduced into the wave data is 

 associated with the reading of the raw data tapes using the Telerecordex 

 film reader. The crux of the problem lies in the fact that the linear 

 spacing of the voltage spikes on the data tape varies inversely with the 

 fluid flow driving the impeller. This is shown from a calibration curve 

 (figure III-5) where voltage pulse separation is plotted against flow speed. 

 This -inverse or hyperbolic relationship occurs because the freqtiency of the 

 impeller varies directly with fluid velocity (as shown by the calibration 

 curves in figure (EI-21). Hence , the period of impeller rotation 'or the 

 time spacing between consecutive voltage pulses) is inversely related to 

 the fluid flow. At increasingly higher flow speeds, the difficulty in 

 resolving and measuring the spacing of the pulses rapidly increases. The 

 strip chart recorder speed (l, 5, 20 or 100 mm sec" 1 ) was adjusted to give 

 the pulse resolution, while at the same time not wasting chart paper. 



Lack of concentration and precision on the part of the person running 

 the hairline marker over the tape reader can result in serious timing 

 errors; hence, corresponding serious velocity errors. The problem is 

 compounded by the fact that, in the range where high repetition rate of 

 pulses renders the reading most .difficult, the velocity values are the 

 largest, Also, since the high velocity values give rise to the high 

 frequency voltage spikes, and since the paper feed is run at maximum 

 speed in this range, the result is a lowering in the optical density of 

 the trace due to the high pen speed acress the tape. This makes resolution 

 of the voltage spikes even more difficult . 



This increase of errors with higher velocities occurred in the early 

 data series of BBELS-5 and BBELS-7 (see chapter IV). In the tabulations of 

 velocity data, a small percentage of points displayed a disturbingly large 

 scatter at peak velocity values above 60-30 cm sec"" 1 ,, 'These peak values 

 occur on wave records obtained both during high wind speeds and with the 

 wave meter positioned near the surface. The scatter was predominantly at 

 the peaks of the wavelike oscillatory motions. The few spurious values were 

 as much as 10-15 percent above or below the normal fluctuation of the velo- 

 cities. The punch-cards containing these spurious values were removed from 

 the card deck of the uninterpolated data. 



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