24 MANUAL OF CURRENT OBSERVATIONS 
ratios will be necessary. It is important that the recording ratio used for the record 
shall be clearly stated on each chart. The cam in the accumulator relay should be 
turned to its zero position each time a new chart is installed on the recorder in order 
that the first contact mark will correctly indicate the full accumulated count from the 
time of the installation. 
62. Rating table.—All Price current meters in use by this Survey are calibrated from 
time to time by the National Bureau of Standards and formulas are furnished which 
indicate the velocity of the current in terms of the number of revolutions of the meter 
wheel in a specified interval of time. From these data rating tables may be prepared 
which will give directly in a convenient tabular form the velocity corresponding to the 
number of revolutions of the meter wheel. The manufacture of the Price meter has 
now become so standardized that a single rating table may be prepared which will be 
applicable without material error to many instruments of this type. The following 
rating formulas were derived by averaging the ratings for 26 different meters in recent 
use by this Survey and they may be used generally for any of the instruments of this 
type unless information to the contrary is received. It is the general practice of the 
office to check each new calibration of any meter with the standard table in order that 
any material difference may be taken into account. The rating formulas follow: 
Let V=velocity of current in. knots, and 
N=number of revolutions of the meter wheel per minute. 
Then V=0.0216 N-+0.021 when JN is less than 30 and 
V=0.0219 N-+0.012 when JN is greater than 30. 
Rating tables (Tables 2 and 3) based upon the above formulas are given on pages 48, 49. 
63. Different methods may be employed for the interpretation of the graphs of 
the automatic recorder. Table 3, which has been especially prepared for the purpose, 
may be conveniently used when the recording ratio is either 1:10 or 1:50. For the pur- 
poses of this table the contact marks in any 12-minute (0.2 hr.) period are counted and 
the number used as the argument for entering the table. The corresponding tabular 
velocity will be the average velocity over the 12-minute period and may be attributed 
to the middle of the period. Similar tables may be readily prepared to provide for a 
period of any other length and for any other recording ratio. The period for the count 
may be taken at such a length that the number of contact marks will directly express 
the velocity of the current without any table. For example, if a 11-minute period is 
taken when the recording ratio is 1:50, the number of contact marks occurring in this 
period will be substantially the same as the velocity expressed in tenths of a knot. If 
the recording ratio is 1:10, the number of contact marks in a 2.2-minute period will 
express the velocity in tenths of a knot, and the number of contact marks in a 22-minute 
period will give the velocity in hundredths of a knot. 
Ekman Current Meter 
64. The Ekman current meter (figs. 17 and 18) was developed by Dr. V. Walfrid 
Ekman, a Swedish scientist, who explains the instrument in an article entitled “An 
Improved Type of Current Meter,” which was published in the Hydrographic Renew 
for November 1932. It is also explained in the British Admiralty Manual of Hydro- 
graphic Surveying (1948) page 314. The manufacture of this meter has not been as 
completely standardized as that of the Price meter and the following description refers 
