manner that the platinum disc will stand above 

 the edge of the silver tube about 1 or 2 milli- 

 meters (Fig. 2). A piece of scotch tape can 

 then be wrapped around the tube to serve as a 

 mold for the epoxy which holds the platinum 

 disc in place and electrically insulates the 

 electrodes. Care should be exercised in pour- 

 ing this epoxy; do not get it on the platinum or 

 silver . The working surface of the platinum 

 disc can be covered with vaseline, so that any 

 epoxy that is accidentally dropped on it can be 

 easily removed. Do not put vaseline or mole 

 release on any surfaces to be joined by epoxy. 

 After this epoxy has hardened, file off all the 

 excess so that the tip of the electrode will be 

 smoothly rounded as illustrated in figure 2 . This 

 procedure will make an assembly which is about 

 2 1/2 -inches long with the epoxy extending above 

 the silver cylinder about 1 1/2 -inches. 



Now drill a hole (1/8 -inch) somewhat off 

 center into the epoxy to a depth of about one inch 

 or about 1/2 or 3/4 -inch above the top edge of 

 the silver tube . Using a larger bit (3/8), drill 

 from the outside (latterly) through the epoxy into 

 this hole. This procedure forms a cavity about 

 3/8 -inch in diameter to accomodate the tip of the 

 thermistor which can be placed through the 

 vertical hole and positioned in place with plas- 

 ticene or modeling clay (in the 3/8 -inch cavity, 

 not in the vertical hole) . The vertical hole 

 should be filled with epoxy to hole the thermistor 

 in place and insulate the leads . Make sure that 

 the thermistor leads are not in contact with each 

 other before pouring epoxy. When the epoxy has 

 hardened, remove the clay from the cavity 

 around the thermistor tip. It is important to 

 have air-space completely around the tip of the 

 thermistor so that water will circulate efficient- 

 ly and completely to cause the thermistor to 

 respond quickly to any change in water tempera- 

 ture. 



The plastic baby bottle will have a nipple 

 In the lid. This nipple should be cut away so 

 that the outer portion remains as a gasket. The 

 electrode assembly can be slipped through this 

 rubber gasket. When the gasket is placed through 

 the plastic cap, it will act as a seal to prevent 

 the epoxy from running down the electrode sur- 

 face . When the electrode assembly has been 

 placed in the rubber gasket and lid, the four 



leads in the cable should be soldered to the 

 appropriate terminals, making certain that none 

 of these four leads touch each other. The lid 

 can then be placed in a small plastic mold (we 

 used a small plastic glass with the bottom cut 

 out) and cemented into place with epoxy . Some 

 lead (shot) should be added to the epoxy at this 

 point so that the electrode assembly will be 

 heavy enough to straighten the cable in deep 

 water. 



A groove should be filed about 1/8 -inch 

 above the top edge of the silver tube to accom- 

 modate an 0-ring which holds the polyethylene 

 membrane in position. This can be done with a 

 rat-tail file, and any irregularity smoothed with 

 emery paper or steel wool. 



When the electrode assembly is finished, 

 plug it into the instrument case for polarization. 

 Turn the switch to Polarization (Position 5) and 

 place the electrode in a 0.5 N KOH solution and 

 allow it to remain for about 30 seconds. It will 

 turn black or dark brown. Additional instruc- 

 tions are given in the appendix. 



HOW THE CIRCUIT WORKS 



Temperature: There are five switch 

 positions. The first (completely counter clock- 

 wise) is "Off", and no reading should be regis- 

 tered on the meter. The second is "Temperature 

 Adjust". In this position the meter needle should 

 move upscale and can be adjusted by resistor 

 R-9. In this position the oxygen circuitry is 

 also on, but the diffusion current is by-passed 

 through R-2 which allows the electrode to begin 

 equilibrating. The third position is "Oxygen 

 Read" . In this position the temperature circuitry 

 is turned off and the output of the electrodes is 

 switched to the meter. The fifth position is 

 "Polarize" . This position should never be used 

 except when the electrode is bare and ready for 

 polarization as in the instructions in the appendix. 



Battery B-2 and B-3, variable resistor 

 R-9, andthe transistor Tr-1 form a constant 

 current circuit to drive the Wheatstone bridge 

 composed of R-5, R-6, R -7 and the thermistor. 

 The output of the current to this bridge is 

 measured in position two (T. Adj.) by the volt- 

 age drop across R-8. The change in resistance 



