182 



reference to their usefulness under conditions normally encountered at sea. 



Despite the simplicity and versatility of most visual instruments, the 

 newer and more expensive photoelectric photometers and spectrophotometers 

 have many advantages to recommend them. When considered as a seagoing in- 

 strument, the ability of a photocell or phototube to ignore differences in opera- 

 tors and to give results which are not a function of the concentration of drama- 

 mine in the operator are real advantages. 



The factor limiting the usefulness of most commercial photoelectric in- 

 struments at sea is the method of registering balance, or indicating the optical 

 density of the test solution. Instruments which use the barrier layer type of 

 photocell generally use a sensitive galvanometer for this purpose. These in- 

 struments are useless at sea. Vacuum phototubes, on the other hand, can be 

 used with relatively simple D. C. amplifiers and the amplified signal made to 

 swing a more rugged mieter. The Beckman Model DU spectrophotometer*, 

 while considerably more refined than is necessary for most routine colorimetric 

 analyses, is of the latter type, and can be used at sea under conditions which 

 make it impractical to carry on the chemical operations which precede colorimet- 

 ric analyses. Beckman instruments have been used on vessels from the Scripps 

 Institution with only routine maintenance and one has been used by the Chesa- 

 peake Bay Institute for over two and a half years with only routine changes of 

 desiccant. 



For most colorimietric analyses filter photometers provide adequate con- 

 trol of spectral band width. Interference filters can be purchased which will 

 give a band width of approximately +^10 m(x with peak transmission at any speci- 

 fied wave length in the visible.** However, the analysis of multi-component 

 systems, such as the analysis for several plant pigments in a single extract, re- 

 quires the use of a spectrophotometer in which the wave length of the incident 

 beam is controlled to approximately +1 m|jL. Such control can be obtained by 

 the use of prisms or gratings. 



Snodgrass et al. (1953) have constructed a self-balancing photoelectric 

 filter photometer by drastic modification of a Lumetron model 402-E.*** The 

 modified instrument has been named Automatic Servo-Operated Photometer or 

 ASOP. In its original form the Lumetron bucks the output of two photocells on- 

 to a sensitive galvanometer as a null indicator. In the modified instrument, the 

 difference in output of the two photocells is chopped and amplified. The ampli- 

 fied signal powers one phase of a two- phase motor which drives a dial (the bal- 

 ancing control on the unmodified instrument) calibrated in per cent transmission 

 and a potentiometer used as a voltage divider across one of the photocells. V 

 When the outputs of both photocells appear the same to the amplifier, the drive 

 motor is not energized and the dial indicates the per cent transmission of the 

 solution in the absorption cell. Micro switches on the door of the absorption 



* - Beckman Model DU Quartz Spectrophotometer, mianufactured by Nation\- 

 al Technical Laboratories, South Pasadena, Calif. 



** - Interference filters can be furnished by: Baird Associates, Inc., Cam- 

 bridge, Mass. , Farrand Optical Co. , New York, N. Y. , and PhoJtovolt Corp. , 

 New York. N. Y. 



*** - Lumetron Photoelectric Colorimeter Model 402-E, manufactured by 

 Photovolt Corp. , 95 Madison Ave. , New York 16, N. Y. 



