of the thermistor with changing temperature 

 (switch in "Temperature Read" position) will 

 unbalance the bridge and the change of current 

 can be read on the meter . 



Oxygen: The voltage divider composed 

 of R-3 and R-4 maintains a constant voltage on 

 the platinum, silver -silver oxide electrode pair. 



The molecular oxygen which will pass 

 the membrane forms hydroxyl ions at the plat- 

 inum surface. The OH ions diffuse through the 

 electrolyte to the silver electrode where they 

 release electrons that flow through the meter 

 circuit, according to the following formula: 



2 Ag + 20H ' -^ Ag2 + H2O + 2 e' 



CALIBRATION OF THE TEMPERATURE SCALE 



With the components listed in this paper 

 the lowest temperature to which the instrument 

 will respond will be about 10° C. which will be 

 near zero microamperes on the meter. This 

 lowest reading is controlled by resistor R-6 

 which is 1800 ohms, the approximate resistance 

 of the thermistor at that temperature. If a 

 lower (or higher) temperature reading is de- 

 sired for zero microamperes, place the ther- 

 mistor in a water bath at the temperature de- 

 sired and with an ohmmeter determine its 

 resistance. A resistor of this value (or near 

 the value) substituted for R-6 will cause the zero 

 microampere reading to correspond with that 

 temperature . The upper range of temperatures 

 will be determined by the setting of resistor 

 R-9. To determine this setting, place the ther- 

 mistor in water at the highest temperature de- 

 sired. Adjust resistor R-9 until approximately 

 full scale reading is obtained, with the selector 

 switch set for "Temperature Read" . Now 

 switch to "Temperature Adjust" position and 

 note the microampere reading. This is the 

 voltage adjustment that should be maintained 

 throughout all subsequent calibration procedures 

 and for the final determination of temperatures, 

 and this reading should be recorded and red- 

 lined on the meter face. 



In order to calibrate the instrument, only 

 two temperatures are needed, one near the low 

 end of the scale of temperatures, and another 

 toward the hig^, say at 12° C. and 30° C. Place 



the electrode in water so that the thermistor is 

 covered, and stir the water vigorously to cir- 

 culate it around the thermistor to assure ther- 

 mal equilibrium. Turn switch to "Temperature 

 Adjust" position and set the needle (with R-9) 

 to indicate the microampere reading previously 

 determined to give full scale deflection of the 

 needle at highest temperature on scale. Switch 

 to "Temperature Read" position. Note the 

 microampere readings at these two temperatures. 

 If these two temperatures are plotted on a graph 

 against the corresponding microampere read- 

 ings and a straight line is drawn between them, 

 the line will pass through the proper microampere 

 reading for other temperatures, as shown in 

 figure 5. Now remove the cover from the 

 meter and write the temperature readings on 

 the face of the meter dial using proper drafting 

 tools and water resistant ink . Be sure to red- 

 line the voltage adjustment reading. 



50 0- 



45 



40 0- 



350 - 



£ 300 



I 



o 25.0- 



5 0- 

 



„/ 



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X 



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— 1 r^ — I r 1 1 1 ' 1 1 ' ' T I ■ I ■ ' 



6-0 80 10 120 140 160 IBO 20 220 240260 2aO 30 32-0 340 36-0 38.0 40.0 

 TEMPERftTURE (DEGREES CENTIGRAOe) 



Figure 5: — Temperatiire plotted against 

 microamperes for temperature cali- 

 bration of the microammeter face. 



CALIBRATION OF THE TEMPERATURE 

 COMPENSATOR 



The oxygen electrode is hi^ly temper- 

 ature sensitive, having a temperature coefficient 

 in the order of 3 - 8 percent per degree C. This 

 temperature coefficient varies both with the size 



