of the platinum electrode and the thickness of 

 the plastic membrane over the electrode 

 (figures 6 and 7). Theoretically, in a purely 



M.O 120 lU) I40 ISO l«0 170 leo 190 20D 210 22.0 23.0 240 29X1260 270 2BO 290 30O 

 TEMP€RATUfte 1 DEGREES CENTIGRADE) 



Figure 6: — Gurves of the temperatxire 

 coefficient of two platinum elec- 

 trodes with diameters of 1 and 1.2 

 centimeters and covered with 1.0 

 mil Teflon or 1.0 mil polyethylene. 

 The slope of the curve reflects the 

 temperature coefficient and the 

 height the relative output in 

 microamperes . 



diffusion controlled electr6de system the tem- 

 perature coefficient should be on the order of 

 about 1 1/2 to 2 percent per degree C. (Lingane, 

 1958). With this electrode, it appears that the 

 smaller the electrode the smaller the temper- 

 ature coefficient, and the thinner the membrane 

 the smaller the temperature coefficient (figures 

 6 and 7). Therefore, perhaps, if an infinitely 

 small platinum electrode and an infinitely thin 

 monbrane were used, the temperature coeffi- 

 cient would be near the theoretical 1 1/2 or 2 

 percent due to diffusion alone. It is important, 

 therefore, that once the temperature compensa- 

 tion curve has been determined for a particular 

 membrane type that this kind of membrane be 

 used in all future readings, or different temper- 

 ature compensation curves may be determined 

 for different membranes, for example, 1/2 mil 

 Teflon, 1 mil Teflon, 1 mil polyethylene, etc. 



110 12 130 140 130 160 170 IB.O 19.0 200 21.0 22 230 240 230 260 270 290 290 30O 

 TEMPERATURE I DEGREES CENTIGRAOC) 



Figure 7' — The temperature coefficients 

 of a 1.0 centimeter diameter platinum 

 electrode covered with three different 

 membranes. 



In order to calibrate the instrument, place it in 

 water at the highest temperature expected to be 

 encountered in natural waters or in the labor- 

 atory . The exact amount of oxygen in the water 

 for calibration is unimportant; however, it is 

 best that it be an amount that will not be super- 

 saturated at the highest temperature, or it will 

 go out of solution and form bubbles on the mem- 

 brane . The proper oxygen content is most 

 easily achieved by warming water above the 

 high temperature (33° C.) and shaking off any 

 excess oxygen, then place the electrode in this 

 solution and cover with oil before lowering the 

 temperature. When the electrode assemble is 

 in place and a magnetic stirring rod is in posi- 

 tion, cover the water with oil. The oil will limit 

 oxygen diffusion during calibration. It is best 

 to cool the water to the lowest temperature, say 

 10° C, and set the counter dial to 1000 and note 

 the microampere reading at this point. By set- 

 ting the counter dial to its highest resistance, 

 one obtains the greatest sensitivity for any given 

 membrane. Then warm the water by 1° C. in- 

 tervals through the full range of temperatures; 

 about 33° C. - 35° C. is high enough for warm- 

 water fishery work. With each change of tem- 

 perature the counter dial is changed so that the 



