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IC 
 
 8920 
 
 Bureau of Mines Information Circular/1983 
 
 Characteristics of the OTOX 
 Model CTL Oxygen Sensor 
 
 By J. E. Chilton, G. H. Schnakenberg, Jr., 
 and L. Spinetti 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 

Information Circular 8920 
 
 Characteristics of the OTOX 
 Model CTL Oxygen Sensor 
 
 By J. E. Chilton, G. H. Schnakenberg, Jr., 
 and L. Spinetti 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 James G. Watt, Secretary 
 
 BUREAU OF MINES 
 Robert C. Norton, Director 
 

 This publication has been cataloged as follows: 
 
 Chilton, J. E. 
 
 Characteristics of the OTOX model CTL oxygen sensor. 
 
 (Bureau of Mines information circular ; 8920) 
 
 Includes bibliographical references, 
 
 Supt. of Docs, no.: I 28.27:8920. 
 
 1. Mine gases— Measurement— Equipment and supplies. 2. Oxygen- 
 Analysis— Equipment and supplies. 3. Gas-detectors— Testing. 1. 
 Schnakenberg, George H. II. Spinetti, L. III. United States. Dept. 
 of the Interior. IV. Title. V. Series: Information circular (United 
 States. Bureau of Mines) ; 8920. 
 
 XN^e^TtH [TN305] 622s [622'. 8] 82-600364 
 
CONTENTS 
 
 Page 
 
 Abstract 1 
 
 Introduction 2 
 
 Theory 3 
 
 Experimental systems 4 
 
 Discussion 5 
 
 Stability 5 
 
 Bias over range 7 
 
 Temperature effect 9 
 
 Pressure effect 10 
 
 Pressure surge tests 13 
 
 Response times 14 
 
 Summary 15 
 
 ILLUSTRATIONS 
 
 1 . OTOX oxygen sensor 3 
 
 2. OTOX sensor Initial stability 5 
 
 3. OTOX sensor long-term stability 7 
 
 4. Zero-corrected oxygen response for the OTOX sensor 8 
 
 5. Normalized oxygen response for the OTOX sensor 9 
 
 6 . OTOX sensor temperature response 10 
 
 7. OTOX sensor temperature response for the square root of the absolute 
 
 temperature 11 
 
 8 . OTOX sensor pressure effect 12 
 
 9. OTOX sensor pressure surge effect: Initial pressure +8 In Hg above at- 
 
 mospheric pressure 13 
 
 10. OTOX sensor pressure surge effect: Initial pressure -8 In Hg below atmos- 
 
 pheric pressure 14 
 
 11. Sample time responses for the OTOX sensor 15 
 
 TABLES 
 
 1. Stability of OTOX sensors; sensor response for 20.9 pet O2 6 
 
 2. Linear regression analysis for stability data on OTOX sensors 6 
 
 3 . OTOX sensor oxygea response 8 
 
 4. OTOX sensor normalized oxygen response data 9 
 
 5. Linear regression analysis for OTOX response to oxygen concentration. 9 
 
 6. OTOX sensor temperature response In air 10 
 
 7. Effect of static pressure on sensor response, OTOX CTL 11 
 
 8. Linear regression analysis for OTOX response to change In static pressures 13 
 
 9. Pressure surge response time data , 14 
 
 10. Response times for step change In oxygen concentration to 90 pet of final 
 
 value 14 
 
UNIT 
 
 OF MEASURE ABBREVIATIONS 
 
 USED IN 
 
 THIS REPORT 
 
 cm 
 
 centimeter 
 
 mL 
 
 milliliter 
 
 cm 
 
 cubic centimeter 
 
 mm 
 
 millimeter 
 
 ° C 
 
 degree Celsius 
 
 mV 
 
 millivolt 
 
 g 
 
 gram 
 
 Pa 
 
 pascal 
 
 in 
 
 inch 
 
 pet 
 
 percent 
 
 K 
 
 kelvin 
 
 sec 
 
 second 
 
 min 
 
 minute 
 
 
 
CHARACTERISTICS OF THE OTOX MODEL CTL OXYGEN SENSOR 
 
 By J. E. Chilton, ' G. H. Schnakenberg, Jr., ^ and L. Spinetti -^ 
 
 ABSTRACT 
 
 The Bureau of Mines has examined the operation of an oxygen sensor 
 manufactured by the City University, London, England. The sensor pro- 
 duces a current proportional to the oxygen concentration by reacting 
 oxygen at a nonconsumable cathode. The sensor design is unique in that 
 the primary mode of oxygen transport to the cathode is by diffusion 
 through a capillary. The sensor using this design has a stability of 
 0- to 0.02-pct reading change per day over a 4.7-month test, small tem- 
 perature coefficient of 0.29-pct reading change per ° C, and small 
 pressure coefficient of 0.34-pct reading change per 1,000-ft altitude 
 change. If this sensor were incorporated into an oxygen detector or 
 monitor, this would be a distinct improvement in electrochemical-type 
 oxygen analyzers and would be useful to the mining industry, 
 
 ^Research chemist, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 
 
 ^Supervisory research physicist, Pittsburgh Research Center, Bureau of Mines, 
 Pittsburgh, PA. 
 
 •^Chemist, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. (now 
 retired) . 
 
INTRODUCTION 
 
 An oxygen detector is used in mining 
 operations for verifying that there is an 
 adequate supply of oxygen for life sup- 
 port of miners. The Code of Federal Reg- 
 ulations requires a supply of air con- 
 taining not less than 19.5 vol-pct O2 for 
 underground coal mine ventilation (30 CFR 
 75.301). To verify that ventilation air 
 meets this specification, oxygen in air 
 of underground coal mines should be mea- 
 sured by the use of an oxygen detector 
 with certain properties: 
 
 1. Certified by the Mine Safety and 
 Health Administration (MSHA) (i.e., per- 
 missible) for use in combustible atmos- 
 pheres such as methane in air. 
 
 2. Have a range covering the concen- 
 trations encountered in practice (0 to 
 25 pet O2). 
 
 3. Meet human related factors — size 
 and weight considerations, readability, 
 ruggedness . 
 
 4. Meet certain instrument parameter 
 values — response time, accuracy, preci- 
 sion, and stability. 
 
 5. Meet environmental instrument 
 parameters — temperature, pressure, humid- 
 ity, and electrical and chemical 
 interference. 
 
 The measurement of some of the oxygen 
 sensor properties noted in items 4 and 5 
 are described in this Bureau of Mines 
 report. 
 
 Many commercial oxygen detectors have 
 sensors that operate as a galvanic cell. 
 The cell anode is a reactive metal such 
 as zinc or lead. The cell cathode is a 
 noncomsumable metal or graphite structure 
 catalyzed with silver to promote oxygen 
 reduction. The cell electrolyte is usu- 
 ally a potassium hydroxide solution in 
 water. Oxygen travels to the cell cath- 
 ode through a porous membrane. The po- 
 rous membrane limits the mass transport 
 of oxygen from the air sample to the cell 
 cathode and results in a sensor response 
 
 that is proportional to the oxygen par- 
 tial pressure. For a given oxygen volume 
 concentration, the oxygen response of 
 these cells, because of the membrane- 
 diffusion-limited system, depends direct- 
 ly on oxygen partial pressure (and hence, 
 an ambient pressure) and also is strongly 
 dependent on ambient temperature. Detec- 
 tors using such cells must use an elec- 
 tronic circuit that senses cell tempera- 
 ture and corrects for the temperature ef- 
 fect of the sensor output. Usually no 
 pressure corrections are made electron- 
 ically. The detector output is usually 
 set to read 21 pet in air known to be 
 fresh. From then on altitude or baro- 
 metric pressure changes will affect the 
 reading even if the actual oxygen concen- 
 tration remains at 21 pet. Tests of per- 
 formance of some of these oxygen detec- 
 tors were reported.^ 
 
 A galvanic oxygen sensor has been de- 
 veloped in England that uses a different 
 principle for limiting the diffusion of 
 oxygen to the cathode — that of diffusion 
 through a capillary. This sensor, the 
 OTOX model CTL,^ developed at the City 
 University, Chemistry Department, St. 
 John St., London, was reported to have no 
 dependence on atmospheric pressure and 
 small dependence on temperature. The 
 evaluation of this oxygen sensor was un- 
 dertaken in the quest for improved oxygen 
 detectors for mining use, and five OTOX 
 model CTL oxygen sensors were obtained 
 from the National Coal Board for study. 
 
 The sensor (fig. 1) is assembled in a 
 cylindrical case, 43 mm high by 23 mm in 
 diameter (C flashlight cell size). Al- 
 though the mass transport of oxygen must 
 pass through an air gap and two mem- 
 branes, it is primarily limited by 
 
 '*Ray, R. M. , and F. B. Armstrong. An 
 Evaluation of Several Direct Reading 
 Electrochemical Oxygen Meters. Bartles- 
 ville Energy Research Center (BERC) 
 RI 76/7, 1976, 50 pp. 
 
 ^Reference to specific products does 
 not imply endorsement by the Bureau of 
 Mines. 
 
diffusion through a fine capillary. The in the air at the sensor and is approxi- 
 current produced by the cell is proper- mately 1 ma at 20.9 pet O2 . 
 tional to the oxygen volume concentration 
 
 THEORY 
 
 The OTOX sensor operates by the gas- 
 diffusion-limited mass transport of oxy- 
 gen, and the sensor current is determined 
 by the overall movement of oxygen as 
 follows: 
 
 1 . Oxygen molecules move to the sensor 
 face. 
 
 2. Oxygen molecules diffuse through 
 the capillary into the sensor interior. 
 
 3. Oxygen within the sensor reacts at 
 the cathode surface. 
 
 The mass transport of oxygen (S, g- 
 mole/sec) is limited by diffusion through 
 
 TOP VIEW 
 
 To voltmeter 
 
 47-ohm 
 resistor 
 
 To voltmeter 
 
 the cell capillary. This flow will fol- 
 low Pick's law for steady state diffusion 
 in a single dimension and can be written 
 as follows: 
 
 s=^o. 
 
 (1) 
 
 where D is the diffusion coefficient, 
 cm^/sec, 
 
 A is the surface 
 
 area. 
 
 cm' 
 
 and 
 
 through which diffusion occurs, 
 
 L is the diffusion zone thickness, 
 cm, 
 
 C is the ambient oxygen concentra- 
 tion, g-mole/cm^. 
 
 This equation assumes a rapid reaction 
 of oxygen at the cell cathode. The cur- 
 rent in amperes, i, from the galvanic 
 cell with limiting cathode reaction is 
 related to the oxygen transport rate, S, 
 as follows: 
 
 i = n F S, 
 
 (2) 
 
 where n is the number of g equivalents 
 per g-mole of reaction (for 
 oxygen n = 4) , 
 
 and F is the Faraday constant, 96,480 
 amp-sec/g equivalent. 
 
 The molar concentration (C, g-mole/cm-^) 
 is related to C , the oxygen concentra- 
 tion in percent, as follows: 
 
 C = C 10"2 p/RT, 
 
 where P is the pressure, atm, 
 
 R is the gas constant, 
 82.06 atm cm^ , 
 
 (3) 
 
 FIGURE 1. - OTOX oxygen sensor. 
 
 and 
 
 mole K 
 T is the absolute temperature, K. 
 
By combining the three 
 cell current, i, is equal 
 ing expression: 
 
 equations, the 
 to the follow- 
 
 ^ 100 L R T 
 
 (4) 
 
 temperature and pressure becomes equal to 
 the following expression: 
 
 i = K' t1/2 c«, 
 where K' is a constant. 
 
 (6) 
 
 With the cell 
 rameters fixed, 
 written: 
 
 capillary physical pa- 
 the equation may be 
 
 = Kf C. 
 
 where K is a constant. 
 
 (5) 
 
 The gas diffusion coefficient (D, cm^/ 
 sec) is a function of the average gas 
 molecule velocity and the mean free path 
 of the gas molecule. In free space it is 
 a function of T^^^ P~ ' , and thus the cell 
 current i expressed as a function of 
 
 The equations for gas diffusion are ap- 
 plicable to this system since the OTOX 
 cell has a capillary with a size much 
 greater than the oxygen mean free path. 
 When the oxygen reaching the cathode is 
 limited by gas diffusion rather than mem- 
 brane diffusion, the cell current is a 
 function only of the square root of the 
 absolute temperature, and not of pres- 
 sure. Thus, the OTOX sensor current 
 should be directly proportional to 
 the oxygen percent concentration and not 
 to oxygen partial pressure as in many 
 other oxygen sensors that have been 
 investigated. 
 
 EXPERIMENTAL SYSTEMS 
 
 A load resistor was connected to the 
 sensor leads, and the OTOX sensor current 
 was measured by reading the voltage 
 across the load resistor using a Keithley 
 model 172 DMM digital multimeter. A 147- 
 ohm resistor was chosen for the load 
 since this value of resistance yielded 
 maximum sensor power output and matched 
 the internal sensor resistance. Room air 
 (20.9 pet O2) was used at local tempera- 
 tures and pressures for stability test- , 
 ing. An Aminco environmental chamber was 
 used for controlled-temperature experi- 
 ments. A Boekel chamber was used for 
 controlled-pressure measurement, with 
 differential pressure readings taken from 
 a mercury manometer. Oxygen and nitrogen 
 from cylinders were mixed to form concen- 
 trations of oxygen above 20.9 pet. Cyl- 
 inder nitrogen was used to dilute air to 
 form oxygen concentrations below 20.9 
 pet. A Taylor Servomex model A0272 oxy- 
 gen analyzer and an Edmont-Wilson amper- 
 ometric membrane sensor both were used to 
 measure the oxygen concentrations of the 
 prepared air mixtures. 
 
 A Hewlett-Packard model 7100 strip 
 chart recorder was used to record sensor 
 response for subsequent measurement of 
 response times, as follows: A sensor, 
 initially in air, was plunged into nitro- 
 gen gas flowing at 50 mL/min by placing 
 over the sensor a polyethylene cylinder 
 connected by tubing to a nitrogen cylin- 
 der. The nitrogen flowed past the sensor 
 and exited through 1/8-in-diameter holes 
 in the cylinder to prevent pressure 
 increase at the sensor. The response 
 time for the sensor for a step decrease 
 in oxygen was measured from the strip 
 chart recording as the time from the ini- 
 tial change in response to 90 pet of 
 the final response. By flowing air from 
 a cylinder through a similar fitting, the 
 response time for a step increase in oxy- 
 gen was determined. In this procedure 
 the cell, initially in nitrogen, was 
 rapidly immersed in a flowing air atmos- 
 phere (20.9 pet O2), and the cell current 
 was recorded. 
 
DISCUSSION 
 
 STABILITY 
 
 Five OTOX sensors were tested in room 
 air (20.9 pet O2) for a 13-day period 
 during which the millivolt output of the 
 sensors was monitored under a continuous 
 147-ohm load. The room temperatures and 
 pressures varied during the test. The 
 data obtained are summarized below, in 
 millivolts : 
 
 Sensor 
 
 Initial 
 
 Average 
 
 Std. dev. 
 
 1 
 
 146.0 
 
 145.5 
 
 0.5 
 
 2 
 
 162.0 
 
 163.3 
 
 1.0 
 
 3 
 
 146.0 
 
 144.9 
 
 .4 
 
 4 
 
 151.0 
 
 154.0 
 
 1.6 
 
 5 
 
 163.0 
 
 164.6 
 
 1.4 
 
 The data were taken over 13 days with 
 fluctuations of temperature from 18° to 
 24° C. The maximum coefficient of varia- 
 tion (the standard deviation divided by 
 the average value) is 1.04 pet. The mea- 
 sured values are plotted in figure 2. 
 The data indicated no general increase or 
 decrease of the values of the sensor re- 
 sponse to 20.9 pet O2 for this brief 
 test. 
 
 The OTOX sensor stability was also mea- 
 sured over a 146-day (4.7-month) period. 
 These data were analyzed to obtain infor- 
 mation on the drift or long-term change 
 in sensor output. The drift is expressed 
 as a linear function of time using a 
 least squares regression analysis. The 
 precision of the data can be estimated 
 from the standard deviation of the ob- 
 served data about the calculated drift 
 line. The OTOX sensor outputs for 20.9 
 pet O2 and room temperature (19° through 
 23° C) are summarized in table 1 , and the 
 data are plotted in figure 3. Table 2 
 summarizes the equation parameters for a 
 least squares regression analysis. 
 
 It is apparent from both the graph and 
 the calculated slopes of the stability 
 data that sensor 1 is significantly dif- 
 ferent over long time observations from 
 the other sensors. The measured output 
 of sensor 1 in room air shows almost a 
 ten-fold greater negative drift than that 
 of the other sensors. The average re- 
 sponse of the other sensors gave little 
 variation over the 4.7-month period. The 
 average of the slopes of the curves for 
 
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 1 
 
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 1 
 
 
 1 
 
 1 
 
 
 FIGURE 2. 
 
 6 8 
 
 TIME, days 
 
 OTOX sensor initial stability. 
 
 10 
 
 14 
 
TABLE 1. - Stability of OTOX sensors; sensor response 
 for 20.9 pet O2 , raV 
 
 Day 
 
 No. 1 
 
 No. 2 
 
 No. 3 
 
 No. 4 
 
 No. 5 
 
 
 
 146.34 
 
 161.70 
 
 146.18 
 
 151.46 
 
 162.75 
 
 6 
 
 144.92 
 
 162.89 
 
 144.57 
 
 153.82 
 
 164.74 
 
 7 
 
 145.57 
 
 164.01 
 
 144.93 
 
 154.50 
 
 165.98 
 
 12 
 
 144.90 
 
 165.13 
 
 145.50 
 
 156.23 
 
 166.53 
 
 13 
 
 144.82 
 
 162.14 
 
 143.28 
 
 154.03 
 
 163.77 
 
 39 
 
 139.56 
 
 164.21 
 
 144.49 
 
 153.62 
 
 165.04 
 
 49 
 
 139.30 
 
 163.92 
 
 144.70 
 
 153.24 
 
 166.04 
 
 84 
 
 132.56 
 
 163.94 
 
 144.61 
 
 153.27 
 
 165.52 
 
 88 
 
 132.46 
 
 166.30 
 
 146.46 
 
 155.73 
 
 167.28 
 
 98 
 
 135.76 
 
 164.33 
 
 146.90 
 
 155.76 
 
 165.48 
 
 101 
 
 136.48 
 
 164.16 
 
 147.96 
 
 152.33 
 
 162.63 
 
 103 
 
 136.83 
 
 163.45 
 
 144.69 
 
 152.31 
 
 164.75 
 
 104 
 
 137.63 
 
 16a.92 
 
 145.33 
 
 156.32 
 
 164.63 
 
 Ill 
 
 134.39 
 
 163.97 
 
 147.34 
 
 157.24 
 
 164.88 
 
 115 
 
 133.71 
 
 165.37 
 
 147.47 
 
 156.26 
 
 164.79 
 
 116 
 
 134.38 
 
 164.59 
 
 146.58 
 
 152.76 
 
 165.45 
 
 118 
 
 136.66 
 
 164.32 
 
 148.14 
 
 152.73 
 
 166.49 
 
 119 
 
 134.49 
 
 165.63 
 
 148.08 
 
 153.69 
 
 165.38 
 
 124 
 
 135.35 
 
 164.17 
 
 146.36 
 
 153.60 
 
 165.52 
 
 125 
 
 134.51 
 
 164.04 
 
 146.34 
 
 152.86 
 
 165.00 
 
 126 
 
 133.60 
 
 163.43 
 
 145.69 
 
 152.36 
 
 164.29 
 
 130 
 
 133.99 
 
 163.88 
 
 145.99 
 
 151.50 
 
 164.82 
 
 131 
 
 131.65 
 
 162.57 
 
 145.18 
 
 151.02 
 
 164.12 
 
 136 
 
 129.42 
 
 163.61 
 
 146.24 
 
 151.77 
 
 164.63 
 
 138 
 
 129.35 
 
 163.18 
 
 146.16 
 
 150.98 
 
 164.44 
 
 142 
 
 129.44 
 
 163.89 
 
 146.30 
 
 152.28 
 
 164.74 
 
 144 
 
 126.61 
 
 163.94 
 
 146.73 
 
 152.36 
 
 165.19 
 
 146 
 
 126.62 
 
 164.38 
 
 147.02 
 
 152.21 
 
 165.22 
 
 TABLE 2. - Linear regression analysis for stability data on OTOX sensors^ 
 
 
 Least squares analysis 
 
 Squared 
 regression 
 coefficient, raV 
 
 Standard deviation, mV 
 
 Sensor 
 
 Slope, mVAday 
 
 Intercept, raV 
 
 Slope 
 
 Regression 
 
 1 
 
 -0.10700 
 
 .00525 
 
 .01430 
 
 -.01080 
 
 .00009 
 
 145.8 
 163.5 
 144.7 
 154.5 
 165.0 
 
 0.85200 
 .06740 
 .31900 
 .08800 
 .00002 
 
 0.0088 
 .0038 
 .0041 
 .0068 
 .0041 
 
 2. 18 
 
 2 
 
 .96 
 
 3 
 
 4 
 
 5 
 
 1.02 
 1.70 
 1.03 
 
 
 
 Equation response (mV) = slope x elapsed time + intercept, 
 
 sensors 2 through 5 is nearly zero. Lim- 
 its of the values for true slope 3 may be 
 estimated at a given risk a (say, a = 5 
 
 pet) from the experimental data for cal- 
 culated slope b from sample data as 
 follows: 
 
 'yx 
 
 < 3 < b + t 
 
 'yx 
 
170 
 
 160 
 
 > 
 
 E 
 
 {^'150 
 
 z 
 
 o 
 
 a. 
 
 </) 
 
 UJ 
 
 cr 
 
 140- 
 
 130- 
 
 120 
 
 1 1 1 1 
 
 - 
 
 — ^^7^ 
 
 ^^ 
 
 - 
 
 - 
 
 
 
 - 
 
 ^ 
 
 
 ~'^^:; 
 
 - 
 
 - 
 
 1 1 
 
 1 1 
 
 -.- 
 
 40 
 
 80 120 
 
 TIME, days 
 
 160 
 
 200 
 
 FIGURE 3. - OTOX sensor long-term stability. 
 
 where t is the value taken from the Stu- 
 dent t tables at the n-2 (26) sample lev- 
 
 el, where a = 0.05 and the ratio ( tt — 1 is 
 
 '" ( sf ) '■ 
 
 the standard error of the slope with val- 
 ues given in table 2. The following es- 
 timates for the limits of values of the 
 true slope 3 are obtained: 
 
 Sensor 
 
 3 limits 
 
 
 Upper 
 
 Lower 
 
 1 
 
 -0.860 
 .014 
 .024 
 .0054 
 .0098 
 
 -0.128 
 
 2 
 
 -.0038 
 
 3 
 
 .0045 
 
 4 
 
 -.027 
 
 5 
 
 -.0097 
 
 From data on measurements taken on sen- 
 sors 2, 4, and 5, the true slope could be 
 zero. For sensors 1 and 3 the data taken 
 do not support the hypothesis that the 
 true slope is zero; however, the slope of 
 sensor 3 is small. 
 
 The square of the regression coeffi- 
 cient , a measure of the dependence of the 
 sensor response on time, was large for 
 sensor 1 but approximated zero for the 
 other sensors. The coefficient of varia- 
 tion for the regression line or the 
 
 scatter of the data about the calculated 
 line ranged from 0.6 to 1.5 pet for the 
 five sensors. 
 
 The original response data and the cal- 
 culated intercepts show that the sensors 
 are not identical and could not be 
 directly interchanged in a commercial 
 oxygen detector without electrical 
 compensation for sensitivity and zero 
 offset. This nonuniform response may be 
 due to variation in construction and dif- 
 ferences in dimensions of materials used 
 in the sensor. 
 
 BIAS OVER RANGE 
 
 The response of the OTOX sensors, 
 placed in an environmental chamber, was 
 measured while the atmosphere was changed 
 in steps from to 40 pet O2. The mea- 
 sured sensor responses are listed in ta- 
 ble 3 for different oxygen concentra- 
 tions. Since the sensor responses were 
 not zero in nitrogen, or pet O2 , the 
 data were corrected by subtracting the 
 
 measured response in nitrogen from the 
 measurements taken at the other oxygen 
 concentrations. A graph made from this 
 data is given in figure 4. 
 
 The data are linear for lower values of 
 oxygen and deviate from a straight line 
 above 35 pet O2. This nonlinearity of 
 the complete data was confirmed by a 
 least squares analysis of the data for 
 the five sensors. 
 
 The average sensor sensitivity (milli- 
 volts per percent oxygen) in normal air 
 (20.9 pet O2) follows: 
 
 Sensor 1 6.18 
 
 Sensor 2 7.75 
 
 Sensor 3 6.93 
 
 Sensor 4 7.20 
 
 Sensor 5 7.81 
 
 The sensor response data were normal- 
 ized to yield values of indicated percent 
 oxygen by dividing the zero corrected re- 
 sponse data for each sensor by the corre- 
 sponding sensor average response ratio. 
 
TABLE 3. - OTOX sensor oxygen response, mV 
 
 Oxygen , pet 
 
 O.O.. rrrr 
 
 5.0 
 
 10.0 
 
 15.0 
 
 17.0 
 
 18.0 
 
 20.0 
 
 20.9 
 
 20.9 
 
 22.0 
 
 25.0 
 
 30.0 
 
 35.0 
 
 40.0 
 
 45.0 
 
 No. 1 
 
 1.58 
 
 28.18 
 
 51.24 
 
 85.13 
 
 97.01 
 
 108.42 
 
 117.42 
 
 130.25 
 
 130.87 
 
 130.73 
 
 149.17 
 
 184.21 
 
 222.76 
 
 240.74 
 
 260.64 
 
 No. 2 
 
 1.72 
 32.75 
 63.15 
 105.64 
 120.39 
 133.87 
 145.70 
 164.62 
 163.11 
 162.82 
 185.98 
 229.03 
 270.52 
 286.57 
 302.20 
 
 No. 3 
 
 1.65 
 
 28.90 
 
 56.09 
 
 94.39 
 
 107.69 
 
 119.24 
 
 130.34 
 
 147.12 
 
 146.16 
 
 147.28 
 
 167.24 
 
 207.73 
 
 252.79 
 
 272.48 
 
 292.30 
 
 No. 4 
 
 1.58 
 
 32.12 
 
 59.77 
 
 99.99 
 
 113.21 
 
 126.51 
 
 137.31 
 
 152.84 
 
 151.35 
 
 152.45 
 
 174.30 
 
 211.99 
 
 248.93 
 
 263.25 
 
 277.84 
 
 No. 5 
 
 1.73 
 31.76 
 63.49 
 106.60 
 121.54 
 134.41 
 146.96 
 165.58 
 164.42 
 166.04 
 188.27 
 232.54 
 277.23 
 294.36 
 311.40 
 
 > 
 
 E 
 
 CO 
 
 z 
 o 
 
 Q- 
 CO 
 UJ 
 
 a: 
 
 01 
 
 o 
 if) 
 
 z 
 
 UJ 
 CO 
 
 350 
 
 300- 
 
 250- 
 
 200- 
 
 15 20 25 30 
 
 ACTUAL OXYGEN, pet 
 
 35 
 
 40 
 
 45 
 
 FIGURE 4. - Zero-corrected oxygen response for the OTOX sensor. 
 
Table 4 summarizes this normalized sensor 
 response data, giving the actual percent 
 oxygen as determined by the reference 
 instruments and the indicated percent 
 oxygen values for each sensor. Figure 5 
 is a graph of the normalized responses of 
 the sensors for varying oxygen concentra- 
 tions to 30 pet. Figure 5 also contains 
 the line calculated from the least 
 squares analysis of the data. Table 5 
 summarizes the values of the slope, in- 
 tercept, and standard deviation for the 
 derived curves. The slopes of the calcu- 
 lated curves are close to 1.0, indicating 
 equivalency of the actual and indicated 
 percent oxygen values. The excellent fit 
 of the experimental data to a straight 
 line is indicated by the near-unity value 
 for the square of the regression 
 coefficient. 
 
 TABLE 4. - OTOX sensor normalized 
 oxygen response data, pet 
 
 10 15 20 
 ACTUAL OXYGEN, pet 
 
 FIGURE 5. - Normalized oxygen response for 
 the OTOX sensor. 
 
 Oxy- 
 
 
 
 
 
 
 gen, 
 
 No. 1 
 
 No. 2 
 
 No. 3 
 
 No. 4 
 
 No. 5 
 
 pet 
 
 
 
 
 
 
 0.0. . 
 
 0.00 
 
 0.00 
 
 0.00 
 
 0.00 
 
 0.00 
 
 5.0. . 
 
 4.30 
 
 4.00 
 
 3.93 
 
 4.24 
 
 3.84 
 
 10.0.. 
 
 8.03 
 
 7.92 
 
 7.85 
 
 8.08 
 
 7.90 
 
 15.0.. 
 
 13.51 
 
 13.40 
 
 13.38 
 
 13.66 
 
 13.42 
 
 17.0.. 
 
 15.44 
 
 15.31 
 
 15.30 
 
 15.50 
 
 15.34 
 
 18.0.. 
 
 17.28 
 
 17.05 
 
 16.96 
 
 17.35 
 
 16.98 
 
 20.0.. 
 
 18.74 
 
 18.57 
 
 18.57 
 
 18.85 
 
 18.59 
 
 20.9.. 
 
 20.82 
 
 21.01 
 
 20.99 
 
 21.00 
 
 20.97 
 
 20.9.. 
 
 20.92 
 
 20.82 
 
 20.85 
 
 20.80 
 
 20.83 
 
 22.0.. 
 
 20.89 
 
 20.78 
 
 21.01 
 
 20.95 
 
 21.03 
 
 25.0.. 
 
 23.88 
 
 23.77 
 
 23.89 
 
 23.98 
 
 23.88 
 
 30.0.. 
 
 29.55 
 
 29.33 
 
 29.73 
 
 29.22 
 
 29.55 
 
 TEMPERATURE EFFECT 
 
 The OTOX sensors were placed in the en- 
 vironmental chamber, and the temperature 
 was varied in steps from 5° to 40° C. 
 The sensor responses were measured while 
 in air (20.9 pet O2) at the different 
 temperatures and were corrected to per- 
 cent oxygen readings by dividing by the 
 measured responses value (aiillivolts) at 
 22° C and multiplying by 20.9 pet. Ta- 
 ble 6 lists the data obtained, and fig- 
 ure 6 shows the data. Sensor 1 data are 
 not included in the figure since the 
 curve was not the same as for the other 
 
 TABLE 5. - Linear regression analysis for OTOX response to oxygen 
 
 concentration 
 
 Sensor 
 
 Least squares analysis 
 
 Slope' 
 
 Intercept, pet 
 
 Squared 
 regression 
 
 coefficient , 
 pet 
 
 Standard deviation, pet 
 
 Slope 
 
 Regression 
 
 0.9998 
 .9998 
 
 1.0109 
 .9969 
 
 1.0086 
 
 -0.864 
 -.979 
 
 -1.126 
 -.791 
 
 -1.097 
 
 0.994 
 .993 
 .993 
 .995 
 .993 
 
 0.0245 
 .0264 
 .0274 
 .0231 
 .0263 
 
 Normalized response 
 ^ pct oxygen reading ^ 
 pet gaseous oxygen 
 
 = slope X true oxygen pet + intercept, 
 
 0.687 
 .739 
 .767 
 .648 
 .735 
 
10 
 
 TABLE 6. - OTOX sensor temperature response 
 in air, pet 
 
 Temperature, ° C 
 
 No. 1 
 
 No. 2 
 
 No. 3 
 
 No. 4 
 
 No. 5 
 
 5 
 
 18.66 
 
 19.53 
 
 19.88 
 
 18.49 
 
 19.56 
 
 10 
 
 20.30 
 
 20.26 
 
 20.50 
 
 19.65 
 
 20.22 
 
 15 
 
 20.83 
 20.83 
 
 20.57 
 20.74 
 
 20.81 
 20.93 
 
 20.25 
 20.61 
 
 20.59 
 
 20 
 
 20.74 
 
 22 
 
 20.90 
 20.30 
 
 20.90 
 21.02 
 
 20.90 
 21.13 
 
 20.90 
 21.08 
 
 20.90 
 
 25 
 
 20.98 
 
 30 
 
 19.45 
 
 21.21 
 
 21.30 
 
 21.40 
 
 21.17 
 
 35 
 
 19.12 
 
 21.32 
 
 21.36 
 
 21.62 
 
 21.26 
 
 40 
 
 19.04 
 
 21.25 
 
 21.23 
 
 21.65 
 
 21.20 
 
 four sensors. The maximum changes for 
 the readings are 2 pet of oxygen response 
 increase for a 35° C temperature 
 increase. 
 
 If the principal mechanism limiting the 
 transport of oxygen is by diffusion 
 through a fine capillary, then, as indi- 
 cated in equation 6, a plot of the square 
 root of the absolute temperature versus 
 sensor response should yield a straight 
 line. Figure 7 shows such a plot. This 
 relationship gives a curve that is appar- 
 ently no more linear than that of fig- 
 ure 6. The curvature in this plot could 
 
 be due to the diffusion of oxygen being 
 limited, by transport through a membrane 
 as well as by the capillary. Upon dis- 
 assembly of a sensor, we found two Teflon 
 membranes within the cell separating the 
 capillary from the cathode, which are 
 used with the capillary and dead air 
 space to form the total diffusion trans- 
 port system for the oxygen sensor. 
 
 PRESSURE EFFECT 
 
 The sensors' response to change of air 
 pressure was measured with ambient air 
 (20.9 pet O2) at an average pressure of 
 
 20 
 
 TEMPERATURE, **C 
 
 FIGURE 6. - OTOX sensor temperature response. 
 
11 
 
 22.0 
 
 I8.5L-X- 
 16.6 
 
 17.8 
 
 \/T, CK)^ 
 
 FIGURE 7. - OTOX sensor temperature response 
 for the square root of the absolute temperature. 
 
 97.923 X 10^ Pa (28.92 in Hg). Table 7 
 lists the measured response changes, and 
 figure 8 plots the responses over a range 
 of air pressures. Measurements were also 
 taken of the pressure response of the Ed- 
 mont Wilson membrane-amperometric oxygen 
 analyzer (membrane-limited transport) and 
 of the Taylor Servomex paramagnetic oxy- 
 gen analyzer, model A0272, used as refer- 
 ence analyzers. Each of the reference 
 
 analyzers has a pressure response of 
 3.46 pet of reading per 33.864 x lO^ pa 
 ( 1 in Hg) change in pressure. The pres- 
 sure unit of 1 in Hg was chosen since 
 this is equivalent to 1,000 ft of alti- 
 tude change. The -3.46-pct change in 
 response would also be found on trans- 
 porting either of the reference oxygen 
 analyzers from sea level to a 1,000-ft 
 elevation. 
 
 For sensors with a membrane-limited 
 transport system, the sensor response to 
 pressure changes was given previously in 
 equation 5. For gas diffusion through a 
 membrane, the gas diffusion coefficient D 
 is proportional to T^'^ and not dependent 
 on pressure P; thus, by substitution for 
 D in equation 5 we obtain 
 
 i = K' T" 
 
 1/2 
 
 PC 
 
 (7) 
 
 or 
 
 i = K' PC for constant 
 
 temperature, (8) 
 
 where, again, i is the sensor response 
 
 current , amp , 
 
 and 
 
 P is the pressure, atm, 
 
 C is the oxygen concen- 
 tration, pet. 
 
 The change in response Ai for a change in 
 pressure AP is 
 
 Ai = KAP C , 
 
 (9) 
 
 TABLE 7. - Effect of static pressure on sensor response, 
 OTOX CTL, pet 
 
 Air pressure 
 change, in Hg 
 
 No. 1 
 
 No. 2 
 
 No. 3 
 
 No. 4 
 
 No. 5 
 
 ED^ 
 
 Ti^ 
 
 -8 
 
 -6 
 
 -4 
 
 -2 
 
 19.90 
 20.12 
 20.33 
 20.52 
 20.53 
 20.90 
 21.02 
 21.16 
 21.31 
 
 20.54 
 20.69 
 20.84 
 20.93 
 21.08 
 21.11 
 21.18 
 21.29 
 21.38 
 
 20.26 
 20.46 
 20.70 
 20.92 
 21.09 
 21.23 
 21.31 
 21.49 
 21.58 
 
 20.09 
 20.50 
 20.56 
 20.79 
 20.99 
 21.15 
 21.28 
 21.42 
 21.57 
 
 20.41 
 20.58 
 20.73 
 20.77 
 20.91 
 21.05 
 21.12 
 21.17 
 21.22 
 
 15.1 
 
 16.7 
 
 18.2 
 
 19.4 
 
 20.9 
 
 22.5 
 
 24.0 
 
 25+ 
 
 0) 
 
 15.2 
 16.7 
 18.1 
 19.5 
 
 
 
 20.9 
 
 2 
 
 22.3 
 
 4 
 
 23.8 
 
 6 
 
 25.5 
 
 8 
 
 0) 
 
 ^Edmont Wilson 
 ^Taylor Servon 
 ^Off scale. 
 
 L oxygen 
 lex A027. 
 
 sensor. 
 I oxygen 
 
 analyze! 
 
 r. 
 
 
 
 
12 
 
 Edmont Wilson galvanometric-membrane 
 
 Taylor paramagnetic 
 
 OTOX oxygen sensor-capillary 
 
 1 
 
 -8 
 
 -6 
 
 4-2024 
 MERCURY PRESSURE CHANGE, in 
 
 FIGURE 8. - OTOX sensor pressure effect. 
 
 8 
 
 and the relative percent change in re- 
 sponse is obtained by dividing equation 9 
 by equation 8 and multiplying by 100 as 
 follows : 
 
 ^1 inn ^ ^P 
 -: — • 100 pet = Tjr— 
 
 1 ^ P 
 
 100 pet. (10) 
 
 Thus, the calculated relative percent 
 change in percent sensor response is 
 equal to the percent pressure change. A 
 pressure change of 33.864 x 10^ Pa (1 in 
 Hg decrease) is a 3.46-pct change at our 
 test elevation of 1,000 ft above sea 
 level. 
 
 The paramagnetic analyzer does not rely 
 on oxygen diffusion, and the paramagnetic 
 sensor response is simply proportional to 
 the oxygen concentration (gram-mole per 
 unit volume) and thus proportional to 
 oxygen pressure. The calculated sensor 
 response change per inch of mercury 
 
 pressure for the paramagnetic sensor is 
 3.46 pet, the same as calculated in equa- 
 tion 10. The measured sensor response 
 change per unit pressure of both refer- 
 ence instruments was approximately equal 
 to this calculated value. On the other 
 hand, the OTOX sensors have a pressure 
 response change of only 0.34 pet per 
 33.864 X io2 Pa (1 in Hg). This is 1/10 
 of the change for the reference units, 
 and the deviation of this number from 
 zero again may be due to the presence of 
 the two mechanisms for diffusion control 
 used in the OTOX sensor, that of a capil- 
 lary air space and the membranes within 
 the sensor. 
 
 Table 8 summarizes the results of the 
 statistical tests for the effect of pres- 
 sure on the sensor response. The values 
 for the square of the regression coeffi- 
 cient approach unity and show the excel- 
 lent fit for a linear dependence of 
 
13 
 
 TABLE 8. - Linear regression analysis for OTOX response to change 
 in static pressures' 
 
 
 Least squares analysis 
 
 Squared regression 
 coefficient, pet 
 
 Standard deviation 
 
 Sensor 
 
 Slope, 
 
 Intercept, 
 
 of regression, pet 
 
 
 pct/in Hg 
 
 pet 
 
 
 
 
 1 
 
 0.088 
 
 20.64 
 
 0.99 
 
 
 0.06 
 
 2 
 
 .05 
 
 21.00 
 
 .98 
 
 
 .05 
 
 3 
 
 .08 
 
 21.00 
 
 .98 
 
 
 .07 
 
 4 
 
 .09 
 
 20.93 
 
 .98 
 
 
 .08 
 
 5 
 
 .05 
 
 20.89 
 
 .97 
 
 
 .11 
 
 Taylor 
 
 
 
 
 
 
 Servomex 
 
 .73 
 
 21.01 
 
 1.00 
 
 
 .04 
 
 Edmont 
 
 
 
 
 
 
 Wilson. . 
 
 .71 
 
 20.90 
 
 1.00 
 
 
 .04 
 
 Equation 
 
 Indicated O2 , pct = slope x Ap inch Hg + intercept. 
 
 response on pressure. The OTOX sensors 
 give oxygen response readings propor- 
 tional to the oxygen concentration rather 
 than to the oxygen partial pressure as do 
 the reference detectors. This response 
 to gas concentrations is unique since all 
 other electrochemical sensors, and almost 
 all other sensors using infrared absorp- 
 tion or refractive index measurements, 
 respond to partial pressure of the sensed 
 gas. The ANDROS handheld CO2 or CH4 de- 
 tector using a pressure modulated infra- 
 red absorption technique is the only 
 other sensor we have found with the abil- 
 ity to read the concentration of gas 
 directly.^ 
 
 PRESSURE SURGE TESTS 
 
 The OTOX sensor contains free space 
 or dead volume within the cell interior 
 between the end of the capillary and the 
 membranes . The oxygen in this volume 
 must be reacted before equilibrium oxy- 
 gen concentrations and stable sensor re- 
 sponse values can be obtained. Pressure 
 surges up to 24.90 x 10^ Pa (10-in water 
 pressure) may occur when a miner moves 
 through airlocks (sealed doors) separat- 
 ing aircourses in underground mines. 
 Figure 9 shows a plot of data obtained 
 
 "Chilton, J. E., C. R. Carpenter, and 
 G. H. Schnakenberg. Improvements in Gas 
 Detector Instrumentation. Proc. 5th WVU 
 Conf. on Coal Mine Electrotechnology , 
 Dept. of Elec. Eng. , W. Va. Univ., Mor- 
 gantown, W. Va., July 30-Aug. 1, 1980, 
 pp. 19-1 to 19-17. 
 
 from the response measurements for a sen- 
 sor encountering a pressure change from 
 +270.91 X lo2 Pa (8 in Hg) to atmospheric 
 pressure. The initial response change is 
 due to reduction of oxygen quantity in 
 the capillary by the outward flow of 
 oxygen-def f icient from the space in the 
 cell. Equilibrium diffusion and a stable 
 response are reestablished at atmospheric 
 pressure in 30 sec (time to reach 90 pct 
 of final value). 
 
 Figure 10 shows the reponse change for 
 an air pressure change from -270.91 x 10^ 
 
 20 
 
 60 
 
 80 
 
 40 
 TIME, sec 
 
 FIGURE 9. - OTOX sensor pressure surge ef- 
 fect: Initial pressure +8 in Hy above atmospheric 
 pressure. 
 
14 
 
 240 
 
 120 
 
 1/21.1 pet O2 
 
 
 
 20 
 
 40 
 TIME, sec 
 
 60 
 
 80 
 
 FIGURE 10. ■■ OTOX sensor pressure surge 
 effect: Initial pressure -8 in Hg below atmos- 
 pheric pressure. 
 
 Pa (-8 in Hg) pressure to atmospheric 
 pressure. The increase in cell response 
 is due to the oxygen in air flowing into 
 the sensor internal free space, and the 
 equilibrium sensor response is reestab- 
 lished in 60 sec (time to obtain 90 pet 
 of final value). These experimental 
 pressure surges are greater than would be 
 found in operating mines, and response to 
 
 upset from smaller pressure surges would 
 more quickly return to the original val- 
 ue. Table 9 summarizes the times to re- 
 cover obtained by the pressure surge 
 test. The average time to recover to 90 
 pet of original response value is 22 sec 
 for a negative pressure surge; full re- 
 covery occurs in an average of 55 sec. 
 The average time to recover to 90 pet of 
 the original response value is 61 sec for 
 positive pressure surges; full recovery 
 is in an average of 98 sec. The measured 
 times depend both on quantity of oxygen 
 to be consumed, which is a measure of 
 
 sensor free space volume relative to the 
 surge volume through the capillary, and 
 on the sensor current, which controls the 
 rate of oxygen consumption. This current 
 is set by the value of the load resistor 
 placed across the sensor. 
 
 TABLE 9. - Pressure surge response 
 time data — time in seconds for 
 recovery of response to 90 pet 
 of initial value 
 
 
 Sensor 
 
 Pressure change 
 
 
 Negative 
 (+8 in Hg 
 to in Hg) 
 
 Positive 
 (-8 in Hg 
 to in Hg) 
 
 1 
 
 20 
 16 
 29 
 27 
 21 
 
 45 
 
 2 
 
 62 
 
 3 
 
 52 
 
 4 
 
 80 
 
 5 
 
 65 
 
 RESPONSE TIMES 
 
 Response times were measured for a step 
 decrease in oxygen by first equilibrating 
 the sensor in air and then flooding it 
 with flowing nitrogen at constant ambient 
 pressure. Response times were measured 
 also for a step increase in oxygen con- 
 centration by equilibrating the sensor in 
 nitrogen and then flooding it with flow- 
 ing air at ambient pressure. Figure 11 
 shows the sensor response change for both 
 conditions: The step increase and step 
 decrease in oxygen. Table 10 reports the 
 
 TABLE 10. - Response times for step 
 change in oxygen concentration to 
 90 pet of final value, sec 
 
 
 Sensor 
 
 Test sequence 
 
 
 Step decrease 
 in oxygen 
 
 Step increase 
 in oxygen 
 
 1 
 
 70 
 50 
 26 
 28 
 45 
 
 76 
 
 2 
 
 28 
 
 3 
 
 32 
 
 4 
 
 32 
 
 5 
 
 25 
 
 'For a step decrease, the sensor is 
 originally in air (20.9 pet O2). then im- 
 mersed in nitrogen (0 pet O2); for a step 
 
 -ir»or-£a<aeci ♦•V»£i c £^-r%c r\ir 2.S OlTi 21113.11 V Itl 
 
 mersed in nitrogen 
 
 increase, the sensor _ 
 
 nitrogen, then immersed in air (20.9 pet 
 
 O2). 
 
15 
 
 60 80 
 
 TIME, sec 
 
 FIGURE 11 . = Sample time responses for the OTOX sensor. 
 
 60 
 
 response times in seconds for both condi- 
 tions as times for response to change to 
 90 pet of the final value. The average 
 values of 44 sec for a step decrease and 
 39 sec for a step increase in oxygen con- 
 centration were measured. As noted in 
 the last section, the response times are 
 functions of sensor internal volumes and 
 of the sensor currents generated by reac- 
 tion of the oxygen. The sensor current 
 is a function of sensor load resistance 
 
 value. This current is maximized, and 
 the response time is shortened by the use 
 of smaller load resistors. However, sen- 
 sor life is shortened by using small 
 resistors as loads since the anode con- 
 sumption is proportional to sensor out- 
 put current. An operational amplifier 
 coupled with the sensor to form a 
 current-to-voltage converter can also 
 yield minimum response times for this 
 type of oxygen detector. 
 
 SUMMARY 
 
 a unique 
 The lim- 
 
 The OTOX model CTL sensor is 
 electrochemical oxygen sensor, 
 iting oxygen mass transport process is 
 the diffusion of oxygen through a small- 
 diameter capillary. Oxygen is rapidly 
 consumed at an inert cathode, and the 
 complete galvanic cell also contains a 
 consumable metal anode with an alkaline 
 electrolyte. Because of this design, the 
 sensor has a response with a small tem- 
 perature coefficient and very small pres- 
 sure coefficient. The sensor responds 
 
 to oxygen concentration rather than oxy- 
 gen partial pressure because of the mini- 
 mum pressure dependence. The stated sen- 
 sor life is 6 months in ordinary air 
 atmospheres, but operation in high oxygen 
 concentrations (greater than 30 pet) , 
 with substantial quantities of acid gases 
 (HCl or CO2), or in very dry environ- 
 ments, and the use of electronic cir- 
 cuitry with small load resistances may 
 shorten the operating life. A summary of 
 the measured parameters follows: 
 
16 
 
 1. Average response time to 90 pet of 
 final value by step change of oxygen con- 
 centration is 42 sec. 
 
 2. Average response time to 90 pet of 
 initial value by pressure shock of 
 +270.91 X 10^ Pa to ambient pressure (+8 
 in Hg to ambient pressure) is 61 sec. 
 
 3. Sensor response change to tempera- 
 ture for 5° to 40° C range is +2.9 pet 
 reading change per 10° C. 
 
 4. Sensor response change to pressure 
 change in the range ±270.91 x 10^ Pa (+8 
 
 in Hg) is +0.34 pet reading change per 
 33.864 X 102 Pa (1 in Hg or 1,000 ft of 
 altitude change). 
 
 5. Range is linear from to 30 pet 
 oxygen; the 95-pct confidence limit for 
 indicated percent oxygen at 20.9 pet O2 
 is ±2.84 pet of reading. 
 
 6. Interehangeability of sensors: The 
 OTOX sensors have significantly different 
 output. Sensor replacement with recali- 
 bration would be performed by gain and 
 zero offset adjustment of the detector. 
 
 <'U.S. GOVERNMENT PRINTING OFFICE: 1983-605-015/20 
 
 INT.-BU.OF MINES, PGH., PA. 26659 
 
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