Above, inverted painted bulb-type unit, and 

 below, the shielded-type unit, both measur- 

 ing one half the total radiation reflected from 

 the ocean surface. The boom is extended 

 while underway. 



A pyrheliometer mounted in gimbals and shielded from reflected radiation. 



Figure B-29. Shipboard lastallation of pyrheliometer cells. 



clean at least once each day, and more often if 

 necessary. Since numerous recorders can be used 

 to collect pyrheliometer data the operator should 

 be familiar with the calibration, maintenance, 

 and troubleshooting of the particular instru- 

 ment being used. 



The pyrheliometer assembly is durable 

 enough to withstand fairly heavy sea condi- 

 tions; however, it is advisable to rig in and 

 secure the equipment if bad storms and abnor- 

 mally heavy seas are encountered. On naval 

 vessels, the pyrheliometer should be secured and 

 the bulbs removed and stored before any large 

 caliber guns are fired as the bulbs will be shat- 

 tered by the concussions. 



Chart paper should be checked daily to in- 

 sure sufficient paper for the day's operation, and 

 the ink reservoir should be checked occasionally 

 to insure a sufficient supply of ink to the pen. 



B-21 Storing and Shipping Pyrheliometer 

 Records. — Used chart paper rolls are labeled 

 indicating the ship, cruise number, and dates of 

 beginning and ending of the roll. If the original 

 roll boxes are retained, the rolls may be stored 

 and shipped in them. If there are no such boxes 

 available, the rolls should be fanfolded, inserted 

 in heavy manila envelopes, and shipped in cover- 

 ing envelopes or heavy wrapping paper. These 

 records should be forwarded at the end of a 

 cruise to: U.S. Naval Oceanographic Office, 

 Washington, D.C. 20390. 



B-22 Water Transparency and Light Ab- 

 sorption Measurements. — The physical rela- 

 tionshiijs governing the penetration and absor^D- 

 tion of light and the color and transparency of 

 the sea are of prime importance to physical and 



B-14 



biological oceanography. For such measure- 

 ments, four general types of instruments or de- 

 vices are in use: The submarine photometer 

 (irradiance or K meter), the hydrophotometer 

 (transmissometir or a meter), the Secchi disc, 

 and the Forel scale. 



The submarine photometer detects ambient 

 luminous flux in foot-candles and records a ratio 

 of surface illuminance to the illuminance exist- 

 ing at various depths down to approximately 

 150 meters. From this ratio, the diffuse attenua- 

 tion (K) constant per unit length is obtained. 



The hydrophotometer has a self-contained 

 constant light source and measures the attenua- 

 tion coefficient (a) of a beam of light per unit 

 length. 



From these two hydrological factors (K and 

 a) obtained with the irradiance meter and the 

 transmissometer, the range at which a sub- 

 merged swimmer can detect certain underwater 

 objects can be p edicted. 



The Secchi d sc with its simplicity of theory 

 becomes difficumn operation owing to currents, 

 drift, and light reflection from the sea surface 

 and provides only an approximate average index 

 of transparency of sea water. 



Color measurements with the Forel scale pro- 

 vide a color index which gives an indication of 

 the transparency of sea water. 



B-23 Transparency Measurements With the 

 Secchi Disc— The Secchi disc (fig. B-30) is a 

 circular plate, having a standard diameter of 

 30 centimeters. One side is white and the other is 

 black. A ring attached at the center of the disc 

 allows a graduated line to be secured. A 5 to 7^2- 

 pound lead weight is attached to the disc so the 

 device will sink rapidly and vertically. The line 



