CHAPTER 12 

 WATER TRANSPAKENCY AND LIGHT ABSORPTION MEASUREMENTS 



12-1 GENERAL REMARKS. The phys- 

 ical relationships governing tlie penetration and 

 absorption of light, the color, and the trans- 

 parency of the sea are of prime importance to 

 physical and biological oceanography. For 

 such measurements, three general types of 

 instruments are in use: The submarine photom- 

 eter, the hydrophotometer, and the vSecchi disc. 



The submarine photometer detects and re- 

 cords directly, in footcandles, the light existing 

 at the surface and all depths down to approxi- 

 mately 150 meters. Through the use of filters, 

 observations are made in the infrared ultra- 

 violet ranges of the spectrum. The instrument 

 consists of two 2-pen Brown potentiometer 

 recorders and a detecting unit of 3 photo- 

 electric cells and a Statham strain gage. 



The hydrophotometer has a self-contained 

 constant light source which allows greater 

 latitude in observation. It may be used at 

 any time of day or night and will measure finer 

 graduations of transparency. 



The Secchi disc with its simplicity of theory 

 becomes difficult in operation owing to currents, 

 drift, and light reflection from the sea surface 

 and provides only an approximate index of 

 transparency of sea water. 



Color measurements by use of the Forel 

 scale are only approximate because of practical 

 allowances concerning operation. 



12-2 TRANSPARENCY MEASURE- 

 MENTS WITH THE SECCHI DISC— The 

 Secchi disc is a circular plate, usually of wood, 

 having a diameter of 30 centimeters. It is 

 covered with a flat white paint. A ring at- 

 tached at the center of the upper surface allows 

 a graduated line to be secured. Opposite the 

 ring on the under surface is attached a lead 

 weight so that the disc will sink rapidly and 

 vertically. This weight should be at least 5 

 pounds and may be increased when conditions 

 of drift and current warrant. Secchi discs used 

 on Navy surveys are provided with 5- and 7%- 

 pound weights. The line attached to the 

 Secchi disc should be marked off in 1-meter 



intervals to at least 50 meters. It is recom- 

 mended that heavy cotton sash cord or heaving 

 line be used. 



The Secchi disc is designed to measure 

 transparency and is dependent upon the avail- 

 able illumination which varies with the time of 

 day, cloud formation, and amount of cloud 

 cover. The Secchi disc observations are re- 

 corded at the top of the A-sheet and must be 

 taken at the same time the associated meteoro- 

 logical data is taken for that sheet. Complete 

 instructions for recording these data are given 

 in chapter 14. 



12-3 Obtaining the Secchi Disc Observa- 

 tions. — The Secchi disc should be lowered into 

 the water from the shaded side of the ship until 

 the disc is just perceptible, and the depth in 

 meters noted. The lowering is then continued 

 until the disc is no longer visible. The disc 

 is then slowly raised until it is again barely 

 visible. The depth reading of this point is then 

 averaged with the reading obtained on lowering 

 and is recorded as given above. 



12-4 DETERMINING WATER COLOR 

 WITH THE FOREL SCALE.— The Forel scale 

 used on Navy surveys consists of a series of 1 1 

 small vials containing ammonical copper sul- 

 phate and neutral potassium chromate in such 

 proportions that a different graduation of color 

 is imparted to each vial. These vials are nu- 

 merically designated and are compared directly 

 with the water in the manner described below. 



The water color is most easily determined in 

 conjunction with the Secchi disc. After com- 

 pletion of the transparency measurement de- 

 scribed in 12-3, raise the white Secchi disc 

 until it lies one meter below the surface. The 

 number of the vial that blends most closely 

 with the water color against the Secchi disc is 

 recorded on the A-sheet. The whiteness of the 

 disc provides the background to which the 

 color is referred; this color may not be the color 

 of the sea surface visible away from the ship. 

 The vials must be shaded from open sunlight 

 when the determination is being made. 



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