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DODKIN—FIELD CHECKS OF THE SALT-VELOCITY METHOD 675 E d w i n A. T a y l o r .8 The results of three check tests at Sorocaba show that the salt velocity varied from volumetric by 0.03, 0.56, and 0.13 per cent, respectively. These results have a maximum spread for any one run of 0.5 per cent and an average difference between the two methods of 0.25 per cent. The results of two check tests at Parahyba show that, for the wide range of the tests, salt-velocity and Gibson methods check within less than 0.1 per cent. For each unit, the two curves of discharge on gate opening coincide at full gate and differ only slightly at half gate. 8 Consulting Engineer, W orcester, Mass. A check test at Serra No. 5, comparing discharge results, using three different salt-velocity test sections, shows a maximum spread of less than 1 per cent, 0.5 per cent plus and minus. For the whole test, the average difference was less than 0.1 per cent. These comparative tests covered 24 runs and 130 salt-velocity shots. Similar check tests, with equally close comparative results, were made at Serra Nos. 3 and 7. The writer has made a number of pump- and turbine-efficiency tests, using the salt-velocity method of water measurement. Several times the results by salt velocity have been checked by other engineers, using other methods of water measurement,
676 TRANSACTIONS OF THE A.S.M.E. NOVEMBER, 1940 and frequently the salt-velocity results have been checked by the same method, using different test apparatus and test sections at the same plant. Tables 7 and 8 show the results of fourteen such check tests on salt velocity, two by current meter, two by pitometer, two by Gibson, two by volumetric, and six by interchecking with various salt-velocity test sections. Details of tests, apparatus, and procedure for the check tests in Tables 7 and 8 are omitted. The engineers conducting these tests were experienced in their lines and the tests were made in accordance with the latest accepted practice in each method. A summary of the comparative results, shown in Table 7 of this discussion, indicates a close agreement between salt velocity and current meter, once in Germany and once in Pennsylvania. A comparison of the discharge values by pitometer in Germany and in New York and by Gibson in Quebec and in New York shows a close agreement with the salt-velocity values for discharge at the same plants. A comparison of salt velocity with the five volumetric check runs in Table 7 shows the results have an average spread for all runs of 0.75 per cent and an average weighted difference for the five runs of 0.6 per cent. While the values for average difference for the Pacific Coast volumetric checks are plus 0.6 per cent, the Brazilian volumetric checks show an average difference of minus 0.25 per cent. The average mean difference, weighted for signs, for the eight check tests in Table 7 is +0.05 per cent. In analyzing and comparing the results in Table 7, the spread and the deviation for runs should be given more weight than the average difference in values for the whole test. This average difference may mean very little alone. For instance, there could be a spread or run deviation in discharge values of 5 or even 10 per cent and the mean difference between methods for the whole test could still be zero. In that case, attaching much importance to the mean difference for the whole test becomes very significant of the accuracy of the method. This is important in field tests for pump and turbine efficiency because, in computing final results, curves are drawn to fit the test points and these curves do graphically what the average differences in Table 7 do arithmetically. Table 8 of this discussion shows the results of six check tests at four different power plants, using various salt-velocity test sections. These test sections varied in length from 300 ft to 3.5 miles. The average spread for all runs shown in Table 8 was 0.33 per cent. The average mean difference, weighted for signs, for the six check tests shown in Table 8 is minus 0.06 per cent. The mean difference for all the check tests shown in both Tables 7 and 8, which involve over 100 salt-velocity runs and 500 shots, is zero.' A u t h o r ’s C l o s u r e Messrs. Strowger and Taylor have presented additional data showing verifications of the salt-velocity and Gibson methods. The assembly of such data is valuable in appraising the degree of accuracy which may be expected by these methods. Further testing results, where the agreement was less satisfactory, would also be valuable. Undoubtedly, attempts to extend the field of these methods have been responsible for the majority of tests where agreement was relatively poor. A record of these attempts, and of the experience gained in overcoming difficulties, is desirable. To present one such case the following is submitted: Prior to the tests of Parahyba unit 4 described in the paper, preliminary tests were made on this unit with both salt-velocity and Gibson methods. The electrode setup for the salt velocity was the same as in the later tests. The injection station consisted, however, of only 12 pop valves placed in the vertical plane of the stop-log grooves. The form of the resulting curves was decidedly poor, each curve showing 3 peaks resulting from the unequal paths from the 3 horizontal rows of 4 pop valves each. The results differed from the final results given in the paper by an amount varying from 0 per cent at 180 mm guide-vane opening to +1.6 per cent at 345- mm guide-vane opening. The use of the salt-velocity method in this manner was an obvious but unwitting abuse. Notwithstanding, there is comfort in the relative closeness of agreement with the later tests made with an improved layout. The preliminary Gibson tests agree very closely with the final ones reported in the paper. The suggestions given in the paper regarding mixing length resulted principally from thought on conditions of testing at the Parahyba plant. Mr. Mason, in his study6 and in his discussion of this paper, and Professor Hooper, in his paper® have contributed further to this question. The determination of conditions providing a satisfactory mixing and distribution of the salt solution in the water channel between the injection valves and the upper electrodes would be a suitable laboratory study. It is apparent, however, that a requirement relating the number of injection valves to the cross-sectional area of the waterway is entirely inadequate without further stipulations. These probably should include factors for the channel dimensions, form, roughness, and possibly for the velocities expected. For ordinary commercial testing in hydroelectric plants the low velocity limits brought out by Professor Hooper are rarely, if ever, encountered.
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