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1. Test solutions were prepared by diffusing mixtures of the oxygen and nitrogen gas through tap water The gas mixtures had certified oxygen content of 0 00 8 80 15 30 and 28 80 respectively By dividing these values by the percentage of oxygen in air the theoretical percentage saturation dissolved oxygen level could be calculated These were 0 00 42 0 73 1 and 137 5 A fifth level 100 air saturation was achieved by bubbling air through tap water Prior to the test the dissolved oxygen concentrations were verified by Winkler determination SCA 1979 To reduce the effects of temperature variation between the various test solutions all tests were carried out at ambient room temperature However in order to allow subsequent comparison of the data the temperature of each test solution was noted Prior to the test the instrument was calibrated for 100 air saturation dissolved oxygen in distilled water in accordance with to the manufacturer s instructions The sensor was placed in each of the test solutions in ascending order of dissolved oxygen concentration and allowed to stabilise before the readings were taken The sensor was then placed in each of the test solutions in descending order allowed to stabilise and further readings taken This test sequence was repeated five times The sensor was returned to its transit container for a period of at least 5 minutes between each successive set test solutions Readings were t2. The Evaluation of a pHOX Model 962 Hand Held Dissolved Oxygen Meter WRe R amp D 220 28 T THE EVALUATION OF A pHOX MODEL 962 HAND HELD DISSOLVED OXYGEN METER D A Neville M M I Harman P D Whalley and A Chappell Research Contractor WRc plc Henley Road Medmenham PO Box 16 Marlow Buckinghamshire SL7 2HD National Rivers Authority Rivers House Waterside Drive Almondsbury Bristol BS12 4UD Evaluation Report 220 28 T National Rivers Authority Rivers House Waterside Drive Almondsbury BRISTOL BS12 4UD Tel 0454 624400 Fax 0454 624409 National Rivers Authority 1993 All rights reserved No part of this document may be reproduced stored in a retrieval system or transmitted in any form or by any means mechanical photocopying recording or otherwise without the prior permission of the National Rivers Authority Dissemination Status Internal Restricted External Restricted Statement of use This evaluation report is one of a series produced by the NRA National Evaluation Facility in conjunction with WRc The performance of an individual item of equipment is described in terms of both laboratory and field tests Information concerning the reliability applicability and cost of ownership of instruments 1s provided where possible Research Contractor This document was produced under R amp D Contract 220 by WRc plc Henley Road Medmenham PO Box 16 Marlow Buckinghamshire SL7 2HD Tel 0
3. 120 99 180 99 300 100 600 100 1200 102 220 28 T 17 Table 6 20 Power consumption Meter Setting V mA mW 8 51 0 61 5 19 8 29 16 78 139 11 7 26 40 70 295 48 NOT STATED Rechargeable Pack 7 2 V 1 Ah NOT STATED Instrument Setting mg 17 E 9 8 17 3 9 8 17 3 98 17 4 9 8 17 4 9 8 17 4 9 8 17 4 Display Blank OFF ON Backlight on Table 6 2b Battery characteristics Battery Make Battery Type Battery Voltage e Battery Capacity Replacement Interval Table 6 3 Effects of different power supply voltages on instrument readings Power Supply Volts sat 8 51 106 7 99 106 7 49 106 6 99 106 6 51 106 6 01 106 5 49 220 28 T 18 Toble 6 4 Instrument readings for different flows at the sensor surface Water Temperature 10 3 C Flow Dissolved Temp Dissolved Rate Oxygen C Oxygen mS mgr sat 0 35 10 7 11 2 100 0 30 10 6 11 2 100 0 24 10 5 11 2 100 0 16 10 5 11 2 99 0 10 10 5 11 2 100 0 05 10 4 11 2 98 0 00 10 2 11 2 96 0 04 10 6 11 2 100 0 09 10 8 11 2 102 0 13 10 8 11 2 102 0 19 10 9 11 2 104 0 29 11 0 11 3 104 0 37 11 1 11 3 106 Table 6 5 Instrument readings ot different immersion Depth Water Temperature 17 9 C Depth Dissolved Temp Dissolved m Oxygen C Oxygen mg 1 1 sat 0 3 9 0 18 9 102 1 0 9 8 18 9 108 20 9 6 18 9 108 0 3 0 4 19 0 106 All Dissolved Oxygen levels were unstable 220 28 T 19 Table 6 6 Instrument readings for two different Interfe
4. 13a 6 13b 6 14 6 15 220 28 T Instrument stabilisation readings for different temperature changes Power consumption Battery Characteristics Effects of different power supply voltages on instrument readings Instrument readings for different Flows at the sensor surface Instrument readings at different Depths Instrument Readings for two Interferents Instrument readings for commonly employed Calibration techniques Instrument readings at different Dissolved Oxygen levels Test 1 Instrument readings at different Dissolved Oxygen levels Test 2 Instrument readings at different Dissolved Oxygen levels Test 3 Instrument readings at different Dissolved Oxygen levels Test 4 instrument readings at different Dissolved Oxygen levels Test 5 Summary of Accuracy Data Response time tests Oxygen Sensor Response time test Temperature Sensor Instrument Readings for different levels of Salinity Field Data Class 1A River Field Data Class 3 River Instrument Readings for daily Calibration Check Class 1A River Instrument Readings for daily calibration check Class 3 River Systematic and Random Errors for daily calibration check Systematic and Random Errors for field Data il EXECUTIVE SUMMARY This report describes the results of an evaluation of a pHOX 962 Hand held Dissolved Oxygen meter The evaluation was undertaken by the NRA Thames Region at the Evaluation and Demonstration Facilities at Fobney Mead Reading and Lea Mars
5. 215 220 28 T 39 APPENDIX LABORATORY ANALYSIS OF FIELD DATA 220 28 T 41 Table Al Date 28 01 93 29 01 93 01 02 93 02 02 93 03 02 93 05 02 93 08 02 93 09 02 93 10 02 93 11 02 93 220 28 T Time 16 15 11 45 12 40 16 25 12 30 12 30 10 50 11 30 14 15 14 05 Water Quality Loboratory Analysis 1055 1A River pH 8 0 8 0 8 1 8 1 8 0 7 9 8 1 8 0 8 1 8 1 Sulphate as SO mg r 35 36 35 34 33 44 45 26 31 31 Conductivity 5 cm 538 519 535 542 539 534 535 536 538 539 as Cu pg I lt 5 lt 5 5 5 5 5 5 Ammoniacal N as N mg lt 0 05 lt 0 05 lt 0 05 lt 0 05 lt 0 05 lt 0 05 lt 0 05 lt 0 05 lt 0 05 lt 0 05 Nitnte as N mg lt 0 05 lt 0 05 lt 0 05 0 06 lt 0 05 lt 0 05 0 05 lt 0 05 lt 0 05 lt 0 05 43 Chloride as mg 23 24 22 22 5 22 23 23 31 23 Calcium as Ca mg 117 118 120 118 114 117 115 118 Magnesium as Mg mg Sodium as Na mg Potassium as K mg p Nitrate as N mg 5 7 5 1 5 7 5 7 4 7 5 7 5 6 5 8 5 5 6 0 Table A Water Quality Laboratory Analysis Class 3 River Date Time pH Sulphate Conductivity Copper Nitrite Chloride Calcium Magnesium Sodium Potassium Nitrate as as SO as Cu as N as N as CI as Ca as Mg as Na as K
6. be allowed I minute per 2 C change before a reading 15 taken This was confirmed during the evaluation The instrument display is blanked after a warning message before the readings are affected by a decrease in the power supply Only a very low flow was required gt 0 10 m 5 1 to achieve the expected 100 saturation reading however once achieved there appeared to be some instability in the reading There is no effect on the readings by varying the immersion depth The total error quadrature sum of random and systematic errors for five accuracy test concentrations varied between 2 and 4 5 It should be noted that Winkler determinations of these solutions gave a total error of 2 0 to 4 7 The manufacturer does not state a response time It can be seen that the response for the oxygen and temperature sensors are similar This will mean that the response of the instrument is not limited by either parameter The salinity correction on this instrument produced readings that were comparable with the expected readings from published salinity tables Weiss 1974 During the field trials there was no significant 95 confidence levels drift at either of the evaluation sites The total error quadrature sum of random and systematic errors was 0 19 mg I for Lea Marston and 0 31 mg 1 1 for Fobney Mead The instrument did not require any maintenance during the four months of the evaluation 220 28 T 7 4 EVALUATION PROCEDUR
7. currently commercially available 220 28 T 3 2 DETAILS OF EQUIPMENT EVALUATED Manufacturer Supplier Tel Fax Instrument Description Serial Number Sensor Type pHOX Systems Ltd Iver Road Shefford Beds 5017 530 pHOX Systems Ltd Iver Road Shefford Beds SG17 5JU 0462 817070 0462 814191 Model 962 Hand held Dissolved Oxygen Meter 7511 Galvanic The manufacturer s specification for the instrument is described in Appendix C 220 28 T 3 MAJOR FINDINGS AND COMMENTS This section provides summary of the major findings and conclusions for the evaluation The pHOX 962 dissolved oxygen meter comprises a meter unit and separate oxygen probe assembly The meter unit is a neat and functional design based in a plastic case The meter unit is both compact and light and presented no difficulties during operation The sophistication provided by the in built software particularly the use of the security protected menus means that the user is required to read the manual thoroughly before operation Once the user has become familiar with the controls the quick guides provided with the documentation allow the user to follow the required switch sequences with little difficulty The instrument display was clear and legible and incorporates a useful backlight for use in low ambient light When the probe is transferred from different temperature regimes the manufacturer states that the probe should
8. provides information on cleaning and replacement of the sensor assembly Detailed drawings are provided to enable correct re assembly of the components A fault finding section is included in the manual however no detailed servicing instructions or list of accessories and replacement parts 1s included Other minor omissions relate to the lack of dimensions of the probe assembly in the probe specification section and the pre determined period used for automatic cut off of the backlight function is not stated The manual includes general information on Health and Safety however no specific information is given for the storage use or disposal of chemicals that may be used during calibration e g Winkler reagents sulphite solutions etc 5 2 Design ond Construction The 962 dissolved oxygen meter comprises a meter unit and separate oxygen probe assembly The meter unit is a neat and functional design based in a plastic case The various functions are provided by membrane switches on the front panel some of which have multiple functions The instrument s internal software enables the use of security protected menus accessed via special keys to prevent the unauthorised use of the calibration 220 28 T 15 mode The meter weighs approximately 1 kg including batteries The IP rating of the instrument is not stated A large LCD panel meter is fitted to the meter The display incorporates a backlight facility The backlight wil
9. true value The magnitude and direction of systematic error will depend on the properties of the sample pH temperature turbidity and interfering species Drift Change of the indicators of an instrument for a given level of concentration over a stated period of time under reference conditions which remain constant 220 28 T 37 REFERENCES Baldwin I G 1992 Instrument Performance Assessment Standard Test Protocols Revision A NRA Project Report 220 9 T Baldwin I G Harman M I and van Dijk P 1992 Evaluation of Multiple Parameter Hand held Meters NRA Project Record 63 9 ST British Standards Institute 1986 Glossary of terms used in Metroiogy BS5233 British Standards Institute 1980 1987 Methods of measurement of fluid flow estimation of uncertainty of a flow rate measurement BS 5844 Harman M M I 1992 Protocol for Investigation of Hand held Dissolved Oxygen Meters NRA Project Record 220 7 T Hitchman M L 1978 Measurement of dissolved oxygen Chemical Analysis Vol 49 HMSO 1981 Methods for the examination of waters and associated materials in waters International Standards Organisation 1985 Evaluation of the performance characteristics of gas analysers ISO 8158 1985 E WAA Process Systems Committee 1992 Water Industry Specifications Information and Guidance notes WSA FWR 7 00 00 Weiss R F 1974 Carbon Dioxide in Water and Sea water The solubility of a non ideal Gas Marine Chemistry Vol 2 p203
10. 491 571531 Fax 049 579094 WRc Reference NR 3432 4245 NRA Proiect Leader The NRA s Project Leader for R amp D Contract 220 Tim Reeder Thames Region Additional Copies Further copies of this document may be obtained from Regional R amp D Co ordinators or the R amp D Section of NRA Head Office CONTENTS EXECUTIVE SUMMARY KEYWORDS 1 INTRODUCTION 2 DETAILS OF EQUIPMENT EVALUATED 3 MAJOR FINDINGS AND COMMENTS 4 EVALUATION PROCEDURES 4 1 Sensor stabilisation 4 2 Battery life 4 3 Effects of low battery power 4 4 Effect of flow at the sensor surface 4 5 Effect of immersion depth 4 6 Effects of Interferents 47 Calibration 4 8 Accuracy tests 49 Response time tests 4 10 Salinity correction compensation 4 11 Field assessments 5 OBSERVATIONS 5 1 Documentation 5 2 Design and Construction 5 3 Installation 5 4 Commissioning 5 5 Maintenance and Downtime 5 6 Ease of Use 6 RESULTS 2 INSTRUMENT BEHAVIOUR 8 MANUFACTURER S COMMENTS COST OF OWNERSHIP ACKNOWLEDGEMENTS DEFINITIONS AND ABBREVIATIONS 220 28 T 1 Page 15 15 15 16 16 16 16 17 29 31 33 35 37 CONTENTS continued REFERENCES APPENDIX A LABORATORY ANALYSIS OF FIELD DATA APPENDIX FIGURES APPENDIX C MANUFACTURER S SPECIFICATION 220 28 T ii Page 39 41 45 49 LIST OF TABLES 6 I 6 2a 6 2b 6 3 6 4 6 5 6 6 6 7 6 8a 6 8b 6 8c 6 8d 6 8e 6 8f 6 0 6 10 6 11 6 12a 6 12b 6
11. 6 15 8 0 7 9 7 5 7 7 Instrument Dissolved Oxygen sat 68 73 71 68 70 65 67 Instrument Temp C 10 9 9 3 92 8 3 10 1 8 6 8 9 Table 6 130 Instrument readings for Class 1A Calibration Check Date Time Atmospheric Dissolved Temp Dissolved Pressure Oxygen C Oxygen kPa mg 171 sat 28 01 93 15 46 108 8 10 3 14 5 102 29 01 93 11 43 101 7 11 1 11 7 102 01 02 93 12 34 103 4 11 0 11 9 102 02 02 93 10 45 103 6 12 1 8 5 104 03 02 93 11 45 103 9 11 3 11 5 104 04 02 93 12 20 103 4 11 2 11 8 104 05 02 93 11 59 103 5 11 5 10 8 104 08 02 93 11 29 103 4 11 3 11 7 104 09 02 93 10 40 103 1 10 0 16 2 102 10 02 93 12 49 102 9 11 4 11 1 104 11 02 93 12 00 103 0 11 5 10 4 104 Table 6 136 Instrument reading for Class 3 river Calibration Check 220 28 T Date 16 02 93 17 02 93 18 02 93 19 02 93 22 02 93 23 02 93 24 02 93 25 02 93 26 02 93 01 03 93 13 47 13 47 15 17 12 26 15 00 11 11 14 55 9 50 12 30 13 25 kPa 102 8 102 6 101 7 102 0 102 1 102 7 102 5 101 5 100 4 101 4 Time Atmospheric Pressure 27 Oxygen mg 12 0 10 9 11 3 11 0 12 2 11 8 8 7 11 9 11 4 14 3 7 6 12 9 11 5 13 8 74 9 3 8 7 8 2 9 7 3 8 Dissolved Temp Dissolved Oxygen sat 100 104 104 108 102 102 102 102 100 110 Table 6 14 Random and Systematic Errors for calibration data Test Class 1A River Mean 103 3 Random error 3 3 System
12. ES The evaluation and demonstration facility at Fobney Mead Reading and Lea Marston Birmingham have been previously described Baldwin 1991 as have the test procedures Harman 1992 A brief description of each test is provided for information 4 Sensor stabilisation The instrument was calibrated according to the manufacturer s instructions Following calibration the instrument was switched off and the sensor assembly stored in its transit container for at least 1 hour prior to the test The sensor was then placed in a 10046 air saturated solution under different temperature regimes Readings were taken after 10 30 60 120 180 300 600 and 1200 seconds immersion Three different temperature change regimes were tested e Sensor stored at room temperature test solution at 20 C e Sensor stored at room temperature test solution at 5 C e Sensor stored at 5 C test solution at 5 C 4 2 Battery life The power consumption was recorded whilst the instrument measured a 100 air saturated sample Measurements were also taken using the display backlight In addition note was also made of the make and type of battery fitted for comparison with the manufacturer s documentation and the nominal battery voltage and capacity 4 3 Effects of low battery power The battery or batteries were replaced by an adjustable stabilised power supply and oxygen and temperature readings were taken at a range of reduced voltages The po
13. N mg l mg p em ug mg mg mg mp mg mg 15 02 93 1600 7 1 128 835 30 4 45 0 27 100 74 18 72 15 15 5 16 02 93 1500 70 135 911 45 7 1 51 039 123 85 21 90 16 15 0 17 02 93 1445 72 148 908 40 5 1 63 0 36 124 81 20 89 15 12 4 18 02 93 1410 73 148 930 40 6 1 40 0 37 130 81 19 87 14 12 7 23 02 93 10 30 76 154 936 40 3 90 0 33 114 84 19 95 16 14 1 24 02 93 1550 70 140 956 42 3 1 70 0 29 127 74 17 98 16 13 6 25 02 93 10 00 7 1 148 979 43 0 2 60 0 34 129 85 19 93 15 11 7 26 02 93 11 57 72 144 993 66 0 3 70 0 27 142 89 20 96 14 10 5 01 03 93 1420 72 135 971 47 0 3 90 0 25 141 80 18 102 15 14 3 220 28 T APPENDIX B 220 28 T FIGURES 45 Pressure kPa Dissolved Oxygen mgl Conductivity Skam Temperature 0 Turbidity FT Figure 81 220 24 T 29193 3115 2718 42 Water Quality Parameters Class 1A River 47 Dissolved Oxygen Ammonium mg 55 pH Temperature 0 ew 1802 53 18 02 93 200293 20293 24 02 93 26 02 93 28 02 93 02 0393 Date Figure B2 Water Quality Parameters Class 3 River 220 24 T 48 APPENDIX MANUFACTURER S SPECIFICATION Oxygen Measurement RANGES 0 200 sat 0 20 0 mg 1 1 ACCURACY 1 of FSD Temperature Measurement RANGE 0 45 0 C ACCURACY 1 of FSD Instrument Environment Temperature range Not stated Water Resistance Not stated Power Supply Internal 6 x 1 2 volts re chargeable batterie
14. aced into each test solutions and once stabilised the concentration saturation and temperature readings were noted Readings were then made after adjusting the salinity compensation control to the appropriate setting 41 ess At the beginning of the test the sensor was calibrated in accordance with the manufacturer s instructions Once the instrument had been calibrated no further adjustment of the calibration took place until the end of the field test The sensor was immersed into the continuous sample stream of a Class 1A river three times 220 28 T 12 each day for a period of 2 weeks Percentage saturation dissolved oxygen concentration and temperature readings were recorded manually from the meter The sensor was returned to the transit container and the instrument switched off between readings Trplicate Winkler determinations were taken to coincide with the daily readings The time at which the Winkler samples were taken were noted to enable comparison of the results from the standard water quality monitors installed at the particular site Fach day the sensor was immersed in 100 saturated water and the displayed result noted Independent temperature and atmospheric pressure readings were also taken The instrument battery condition was checked daily and replaced if necessary Note was kept of any necessary battery changes This procedure was repeated on a Class 3 river During the test the water was monitored for the
15. acturer 20 hours The instrument display 1s blanked after warning message before the readings are affected by decreasing the power supply Table 6 3 The manual states a shut down will occur at 5 5 volts it was found to occur at 5 71 volts The effect of flow on the sensor performance is given in table 6 4 It shows that only a very low flow is required gt 0 10 m 8 1 to achieve the expected reading however once achieved there appeared to be some instability in the reading The manufacturer does not state a minimum flow rate Table 6 7 shows the effect of immersion depth on the instrument reading It can be seen that there is no effect on the readings although the readings are higher than the expected 100 Saturation Table 6 6 demonstrates the effect of the presence of two possible interferents on the meter readings At a temperature of 10 C 100 air saturation corrected for pressure would be achieved at a dissolved oxygen level of 11 30 mg 1 1 whilst at 30 C there would be 7 58 mg 1 1 dissolved oxygen present At the higher temperature levels the meter reading is correct whereas the lower temperature is incorrect This supports the findings in the stabilisation test that there may be some temperature effect on the readings The addition of sodium hypochlorite to achieve a concentration of 30 mg 1 1 of residual chlorine had no effect on the displayed values Only minor disparities were noted between the different calibratio
16. aken for each measurand provided by the instrument e g mg 1 1 sat and C and the temperature of the various test solutions recorded using a graduated mercury thermometer or type E thermocouple 220 28 T 11 4 9 Response time tests 49 1 Oxygen sensor The instrument was calibrated prior to the test using solutions prepared according to the standard method The temperature of the test solutions was 20 0 1 C The sensor was placed in each solution in turn and the time taken for the instrument to register a measurement within 90 of the step change recorded i e when the sensor was removed from the 0 solution the time required for the reading to reach 90 saturation and following stabilisation at 100 and when the sensor was placed back into the 0 solution the time required for the reading to reach 10 saturation The test cycle was repeated 3 times 4 9 2 Temperoture sensor The instrument was calibrated prior to the test in accordance with the manufacturer s instructions The sensor was placed in two test solutions 25 0 2 and 5 0 2 C in tum and the time taken for the instrument to register a measurement within 90 of the step change recorded The test cycle was repeated 3 times 4 10 Salinity correction compensation Test solutions were prepared by the addition of 2 5 10 20 and 40 g 1 1 NaCl in distilled water The solutions were maintained at 100 saturation throughout the tests The sensor was pl
17. atic error Bias 1 0 Total Error 3 5 Sample size 11 Table 6 15 Random and Systematic Errors for field data Test Class 1A River Random error 0 18 Systematic error Bias 0 40 Total Error 0 44 Sample size 29 220 28 T 28 Class 3 River 103 4 3 4 3 1 4 6 10 Class 3 River 0 68 0 36 0 77 27 7 INSTRUMENT BEHAVIOUR This following section describes the general performance of the instrument during the various test procedures During the evaluation it was noted that all percentage saturation values above 100 were reported to a resolution of 2 Table 6 1 shows the stabilisation of the percentage saturation readings for different temperature regimes It can be seen that in two of the test regimes a stable reading is not achieved until after 5 minutes The manufacturer states that the probe should be allowed I minute per 2 C change before a reading is taken A temperature change of 15 C would therefore require 7 5 minutes to stabilise Where there was no temperature change the readings still takes up to 5 minutes to stabilise for the lower temperature regime At the higher temperature the meter reading is stable after five seconds although the level is higher than would be expected This suggests that there may be a temperature effect Given the stated battery capacity Table 6 2b and the power consumption Table 6 2a the expected battery life can be calculated This value 60 hours 15 well above that stated by the manuf
18. before readings were taken Continuous aeration maintained a flow of 0 to 0 03 m 571 past the sensors Two sets of dissolved oxygen concentration 96 saturation and temperature readings were taken at 0 3 1 0 and 2 0 metres depth 4 6 Effects of Interferents The instrument was calibrated using the manufacturer s instructions The sensor was placed in twenty litres of 100 air saturated de ionised water A reading was taken once it had stabilised To produce a solution with a residual chlorine level of 30 mg 1 1 7 5 ml of 8 available chlorine sodium hypochlorite solution was added A second reading was then taken For the temperature interference test the required temperatures were maintained by the control system at Fobney The actual temperatures were recorded using type E thermocouples After calibration of the sensor according to the manufacturer s instructions readings were taken in 100 air saturated water held at 10 C 0 1 C The meter was switched off until the control system raised the test temperature to 30 C The heated water was subsequently aerated to 100 saturation and the reading recorded 220 28 T 10 47 Calibration The instrument was calibrated in air according to the manufacturers instructions Readings were then taken in 100 air saturated tap water and 100 air saturated river water The instrument was then calibrated in 100 air saturated tap water and the measurements repeated 4 8 tests
19. er are show in figures and B2 respectively Other water quality parameters were monitored by daily sampling and laboratory analysis These results are provided in tables Al and A2 220 28 T 30 8 MANUFACTURER S COMMENTS The instrument loaned for assessment was supplied with the incorrect carrying case the Model 962 dissolved oxygen meter is currently supplied with a water proof holster for use in the field The user manual will be amended to incorporate more specific details of the instrument s power supplies e g battery packs etc In the section on instrument behaviour reference is made to the instrument resolution for dissolved oxygen values above 100 saturation being only 2 This feature was in fact a requirement in the specification for the NRA multiparameter hand held meter which unfortunately had been carried over into the design of the Model 962 The software is currently being changed to allow the instrument to display results to a resolution of 1 throughout its measurement range We are already aware of the adverse temperature effect stated within the report and are currently re designing the head of the oxygen electrode to relocate the temperature sensors thereby eliminating the effect of the mass of the electrode body on the temperature stabilisation Furthermore since the loan of the model 962 for assessment we have undertaken further development of the instrument which is now incorporated into the new pHOX c
20. following parameters temperature dissolved oxygen pH conductivity turbidity and ammonium Class 3 river only Daily samples were also taken for laboratory analysis 220 28 T 13 5 OBSERVATIONS 5 1 Documentation A 51 page 5 size instruction manual was supplied with the instrument The manual is very comprehensive and provides detailed information on the instrument and its controls and instructions on the use of the security protected menus calibration of temperature and oxygen sensors maintenance procedures use of the in built data logging facilities and a general specification for the meter unit and the probe assembly The manual is well written and unambiguous and provides a very detailed description of the operational use of the instrument The need for an index in such a large manual has been offset by the use of contents pages for each section A full description of the possible calibration procedures is provided for the temperature sensor and a 2 point oxygen calibration covering air calibration and air saturated water methods including diagrams to illustrate the apparatus recommended Although an altitude correction table has been included no correction table has been given for atmospheric pressure correction No data is provided for the effects of commonly known interferents and no discussion is given of instrument component probe related and calibration related measurement errors The maintenance section of the manual
21. l automatically switch off after a pre determined period No facility is provided for housing the probe or lead during transit The probe is a membrane covered galvanic type sensor incorporating a thermistor for temperature measurement and compensation housed within the upper body of the probe assembly 5 3 Installation None Required 5 4 Commissioning None Required 5 5 Maintenance ond Downtime No maintenance was required during the four months of the evaluation 5 6 Ense of Use The meter unit is both compact and light and presented no difficulties during operation The sophistication provided by the in built software particularly the use of the security protected menus means that the user is required to read the manual thoroughly before operation Once the user has become familiar with the controls the quick guides provided with the documentation allow the user to follow the required switch sequences with little difficulty The instrument display was clear and legible and incorporates a useful backlight for use in low ambient light 220 28 T 16 6 RESULTS Table 6 1 Instrument stabilisation readings for different temperature chonges Temperature Change C Time Dissolved secs Oxygen Sensor Room Temperature gt 5 C 15 75 30 90 60 94 120 97 180 97 300 99 600 100 1200 102 Room Temperature 21 C 5 104 30 104 60 104 120 104 180 104 300 104 600 104 1200 102 5 C 5 C 15 88 30 95 60 97
22. libration check data for the Class 3 river A correlation coefficient calculated for this data against time shows that there is no significant 95 confidence limits drift with time Table 6 14 shows the systematic and random errors for the calibration check data for the Class 1A and 3 river This test should show if there is any drift in the calibration of the instrument This shows that the total error was 4 6 in the Class 3 river and 3 4 in the Class 1A river If this is compared to the Winkler determinations for the 100 saturation solution it can be seen that their total error is 4 5 This would indicate that there is more variability in the Winkler determinations than the instrument readings The same statistical test was applied to the river water results Table 6 12a and 6 12b In this case the readings were made mg 1 1 The mean of the readings is not stated since there will be naturally occurring variation tn dissolved oxygen concentration over the test period The results describe the variation of the readings given by the test instrument as compared to that made by the Winkler determinations The total error was 0 77 mg 1 for the Class 3 river and 0 44 mg T for the Class river It can be seen that the variations are small particularly if the vanability in the Winkler measurements are assumed to be similar to those seen in the accuracy tests Data from automatic water quality instrumentation for the Class 1A and Class 3 riv
23. lved oxygen levels Test 3 Atmospheric Pressure 101 3 kPa Water Temperature 20 1 C Dissolved Water Dissolved Temp Dissolved Oxygen Temp Oxygen CC Oxygen sat C mg 1 1 sat 0 18 4 0 3 18 7 3 42 0 4 0 19 6 46 73 1 19 2 6 9 19 4 77 100 9 2 19 0 102 137 5 19 8 12 1 20 0 138 137 5 20 5 12 1 20 7 140 100 DO 20 2 102 73 1 21 0 6 7 21 2 78 42 0 4 0 22 3 47 0 21 8 0 1 22 1 1 see text for details section 4 8 Table 6 8d Instrument readings at dissolved oxygen levels Test 4 Atmospheric Pressure 101 2 kPa Water Temperature 23 9 C Dissolved Dissolved Temp Dissolved Oxygen Oxygen C Oxygen sat mg 1 1 sat 0 0 2 23 0 2 42 0 3 7 24 0 46 73 1 6 4 23 3 78 100 8 7 22 5 104 137 5 11 5 24 2 142 137 5 11 3 24 4 140 100 8 7 23 1 104 73 1 6 4 24 4 79 42 0 3 6 25 8 47 O 0 0 25 2 A see text for details section 4 8 220 28 T 22 Table 6 8e Instrument readings at dissolved oxygen levels Test 5 Atmospheric Pressure 101 7 kPa Water Temperature 26 0 C Dissolved Water Dissolved Temp Dissolved Oxygen Temp Oxygen C Oxygen sat mg 1 1 sat 0 25 3 0 1 25 5 2 42 0 3 7 26 5 48 73 1 25 2 6 3 25 5 80 100 0 8 4 24 4 104 137 5 25 6 11 1 25 8 140 137 5 26 1 11 0 26 3 142 100 0 8 3 25 3 106 73 1 26 4 6 1 26 6 79 42 0 3 7 27 1 49 0 26 9 0 1 27 1 2 see text for details section 4 8 Table 6 8 Winkler Accuracy for the Test Solutions Actual Instrumen
24. n techniques used Table 6 7 220 28 T 29 The instrument accuracy was tested on 5 separate occasions and compared with a range of oxygen nitrogen gas mixtures These results are presented in tables 6 8a 6 8e The random and systematic errors for the instrument and the Winkler determinations are provided in Table 6 8f The total error quadrature sum of random and systematic errors for five test concentrations varied between 2 and 4 5 It should be noted that Winkler determinations of these solutions gave a total error of 2 0 to 4 7 The variation in the Winkler titration for the nominal zero dissolved oxygen concentration means that it is not possible to establish if hysterisis is an important factor with this instrument The manufacturer does not state a response time It can be seen that the response for the oxygen and temperature sensors are similar This will mean that the response of the instrument is not limited by either parameter The salinity correction on this instrument can be seen to produce readings that are within the tolerance limits of the instrument Table 6 11 and Appendix C The accuracy of these readings can be confirmed by referring to previously published salinity tables Weiss 1974 Table 6 13a shows the calibration check data for the Class 1A river A correlation coefficient calculated for this data against time shows that there 1s no significant 95 confidence limits drift with time Table 6 13b shows the ca
25. nkler Instrument instrument Instrument Winkler Instrument Instrument Instrument Time Winkler Instrument Instrument Instrument Temp Pressure mg r y Dissolved Dissolved Ternp mg l Dissolved Dissolved Temp img I d Dissolved Dissolved Temp C kPa Oxygen Oxygen C Oxygen Oxygen CC Oxygen Oxygen C mgl 9b sat mg I sat mgl 9 sat 28 01 93 8 6 100 8 29 01 93 8 8 101 7 16 52 11 41 11 5 99 9 0 01 02 93 8 3 103 4 16 49 12 02 11 8 100 8 2 02 02 93 7 8 103 7 16 28 12 35 12 1 102 8 1 03 02 23 83 103 9 18 09 11 76 11 9 102 83 04 02 93 7 8 103 4 18 13 11 80 12 1 102 7 8 05 02 93 7 1 103 5 16 48 12 60 2 6 104 7 1 08 02 93 90 103 4 17 03 11 35 11 5 100 92 09 02 93 8 7 103 1 1 20 11 88 11 7 100 8 7 11 02 93 7 8 1029 10 02 93 1 1 103 0 220 28 T 25 Table 6 12b Field Data Closs 3 River Date Water Atmospheric Winkler Instrument Instrument Instrument Winkler Instrument Instrument Instrument Temp Pressure Dissolved Dissolved Temp mg rb Dissolved Dissolved Temp C kPa Oxygen C Oxygen Oxygen C 96 sat 15 02 93 16 02 93 102 8 17 02 93 10 6 102 6 18 02 93 10 9 101 7 19 02 93 8 9 101 9 22 02 93 9 0 102 23 02 93 8 0 102 6 24 02 93 10 0 102 5 25 02 93 8 7 101 5 26 02 93 8 2 01 03 93 6 8 101 4 220 28 T 26 Time 16 06 14 21 16 30 11 39 16 35 11 15 13 25 Winkler mgr 7 74 7 76 7 37 7 22 Instrument Dissolved Oxygen 7
26. orporate enclosure to IP67 standard 220 28 T 3 9 COSTOF OWNERSHIP The pHOX Model 962 Hand Held Dissolved Oxygen Meter 695 00 including carrying case all accessories and an alternative shoulder holster The electrode cartridge 32 00 One supplied expected life typically one year Battery pack 35 00 One supplied expected life dependant on usage typically one year Membrane kit 11 00 One supplied expected life typically 2 years 220 28 T 33 ACKNOWLEDGEMENTS The authors would like to thank the NRA staff at Fobney Mead and Lea Marston for their help in providing laboratory and water quality data e the analysis staff at WRc for providing calibration solutions and pHOX systems for the loan of the instrument 220 28 T 35 DEFINITIONS AND ABBREVIATIONS Error of indication of a measunng instrument BS 5233 The indication of a measuring instrument minus the true value of the measurement Response time WSA FWR 7 00 02 The time interval from the instant a step change occurs in the value of the property to be measured to the instant when the change in the indicated value passes and remains beyond 90 of its steady state amplitude difference Random Error describes the way in which repeated measurements are scattered around a central value It therefore defines the precision of the instrument Systematic Error Bias is present when results are consistently greater or smaller than the
27. rbonate to eliminate interference from carbon dioxide acts as an electrolyte and fills the space between the cathode and the anode The electrolyte is confined by a thin polythene or silicone membrane which is supported by the cathode Oxygen which diffuses through the membrane is reduced at the cathode to give a current proportional to the partial pressure of oxygen A detailed description of the theory of membrane covered oxygen electrodes 15 given in Hitchman 1978 Generally the current output from the cell is converted to either a reading equivalent to the percentage saturation of oxygen in water or to the actual concentration in terms of mg I gd The evaluation was undertaken by the NRA Thames Region at the Evaluation and Demonstration Facilities at Fobney Mead Reading and Lea Marston Birmingham in accordance with an evaluation protocol jointly devised by WRc and the NRA The protocol allows the instrument to be assessed in a manner commensurate with typical use in the field The objectives of the assessment were as follows e to assess the performance characteristics of hand held dissolved oxygen meters currently in use within the NRA 3 e to provide information on the appropriate application of the instruments the correct method of use and calibration and maintenance procedures and to establish methods of use which optimise the performance and the quality of the data obtained for the instruments presently in use and those
28. rents Interferent Level Dissolved Temp Dissolved Oxygen C Oxygen mg 1 1 sat Temperature 10 C 10 3 10 2 95 30 C 7 4 30 2 102 Chlorine Omgr 8 8 21 3 102 30mg I 8 8 21 8 104 1 Toble 6 7 Instrument readings for commonly employed Calibration techniques Sample Dissolved Temp Dissolved 100 saturation Oxygen C Oxygen mg 171 sat River Water 8 6 21 0 100 River Water 8 4 21 0 98 1 Dechlorinated Tap Water 8 3 20 7 96 calibrated in dechlorinated tap water calibrated in air 220 28 T 20 Table 6 80 Instrument readings at dissolved oxygen levels Test I Atmospheric Pressure 102 0 kPa Water Temperature 19 C Pissolved Dissolved Temp Dissolved Oxygen Oxygen C Oxygen sat mg 1 1 sat 0 0 0 17 4 0 42 0 4 1 18 4 45 73 1 6 9 18 3 76 100 9 1 18 4 100 137 5 12 1 19 3 136 137 5 11 9 19 7 136 100 9 1 19 3 102 73 1 6 9 19 7 78 42 0 4 1 20 9 47 0 0 0 20 8 0 see text for details Section 4 8 Table 6 8b Instrument readings at dissolved oxygen levels Test 2 Atmospheric Pressure 101 7 kPa Water Temperature 22 1 C Dissolved Dissolved Temp Dissolved Oxygen Oxygen C Oxygen sat mg 1 1 sat 0 0 0 21 5 0 42 0 4 2 22 5 50 73 1 6 7 21 6 79 100 8 9 21 2 104 137 5 11 9 22 4 142 137 5 12 0 22 8 144 100 9 0 21 6 106 73 1 6 8 22 5 82 42 0 4 0 23 9 49 0 0 1 22 8 1 see text for details section 4 8 220 28 T 21 Table 6 8 Instrument readings 01 disso
29. s Nominal 20 hours between re charges Probe Response Time 90 change lt 35 seconds 220 28 T 49
30. t 220 28 T 220 28 T 1 INTRODUCTION This report describes the evaluation of a pHOX Model 962 Hand held Dissolved Oxygen Meter A discussion of the chemistry of oxygen in natural waters may be found in the protocol document Harman 1993 However a resume is given here to assist in the understanding of the evaluation methods applied Following the principle of Henry s Law the concentration of dissolved oxygen in a sample of water is directly proportional to the partial pressure of oxygen in equilibrium with that water sample at a constant temperature assuming that air has an oxygen content of 20 94 v v and IS Saturated with water vapour In addition the solubility of oxygen in water or air is dependent on the concentration of other dissolved species within the water and atmospheric pressure An instrumental procedure for the measurement of dissolved oxygen in water involves the use of an electrochemical cell often called an oxygen electrode or sensor the response of which 15 proportional to the thermodynamic activity of oxygen in solution Electrochemical sensors with membranes can be of two types galvanic and polarographic The pHOX Model 962 is fitted with the Mackereth electrode the most commonly used galvanic electrochemical cell The cell consists of a perforated silver cathode in the form of a cylinder which surrounds a lead anode An aqueous or gel potassium hydroxide solution often saturated with potassium hydrogen ca
31. t Readings Winkler Readings Dissolved Oxygen sat Random Systematic Random Systematic Error Error Error Error 0 1 1 1 10 5 74 2 01 42 0 5 4 1 58 8 42 5 98 73 1 5 5 1 65 3 98 4 06 100 3 4 1 90 1 88 4 14 137 5 2 5 2 67 13 42 21 43 see section 4 8 for details 220 28 T 23 Table 6 9 Response time tests Oxygen Sensor Time Time 2 Time 3 seconds seconds seconds Step change low to high Dissolved Oxygen 22 68 21 12 19 78 Step change high to low Dissolved Oxygen 36 59 29 84 30 94 see text for details section 4 9 Table 6 10 Response time Test Temperature Sensor Time 1 Time 2 Time 3 seconds seconds seconds Step change low to high Temperature C 23 37 22 75 22 63 Step change high to low Temperature C 2450 23 91 24 28 see text for details section 4 9 Table 6 11 Instrument readings for different levels of Salinity No Saline Adjustment Saline Adjustment Chlorine Water Dissolved Temp Dissolved Dissolved Dissolved setting mg I 1 Temp Oxygen C Oxygen Oxygen Oxygen mg 1 1 sat mg 1 1 sat 0 18 1 9 4 18 3 100 5 17 9 9 5 18 1 100 9 0 100 7 10 18 2 9 4 18 4 100 8 6 100 13 20 19 0 9 4 19 1 102 8 0 102 23 40 19 4 9 2 19 6 100 7 2 100 33 20 19 9 9 3 20 1 102 7 8 102 23 10 19 9 9 3 20 1 102 8 5 102 13 5 19 9 9 4 20 1 102 8 9 102 7 0 20 7 9 2 20 9 104 220 28 T 24 Table 6 120 Field Dato Class 1A River Date Water Atmospheric Wi
32. ton Birmingham according to an evaluation protocol jointly devised by WRc and the NRA The pHOX 962 dissolved oxygen meter comprises a meter unit and separate oxygen probe assembly The meter unit is a neat and functional design based in a plastic case The meter unit 1s both compact and light and presented no difficulties during operation The sophistication provided by the in built software particularly the use of the security protected menus means that the user is required to read the manual thoroughly before operation Once the user has become familiar with the controls the quick guides provided with the documentation allow the user to follow the required switch sequences with little difficulty The instrument display was clear and legible and incorporates a useful backlight for use in low ambient light In two of the test procedures there appeared to some temperature affect on the instrument readings The total error quadrature sum of random and systematic errors for five test concentrations varied between 1 6 and 5 7 It should be noted that Winkler determinations of these solutions gave a total error of 6 to 25 For the field evaluation the total error calculated against Winkler determinations was 0 77 mg 1 1 for Lea Marston and 0 44 mg 1 1 for Fobney Mead The instrument cost 695 00 No maintenance or repairs were required during the four month evaluation KEY WORDS Dissolved Oxygen Evaluation NRA Evaluation Repor
33. wer supply voltage was adjusted downwards whilst observing the dissolved oxygen and temperature readings and a note made of the supply voltage at which the readings changed or became unstable The readings were taken with the instrument probe immersed in a 100 saturated sample The instrument was allowed adequate time to discharge any capacitance before the readings were taken The voltage at which the low battery indicator if fitted operates was noted 220 28 T 9 44 Effect of flow ot the sensor surface The effect of flow on the sensor was investigated by taking measurements from the sensor in test solution at 100 air saturation at a range of flow rates The work was carried out in the outside flow tank at Fobney Flow was measured by a water current meter accurate to 0 03 m 8 1 Two sets of measurements were taken at the following range of flow rates 0 05 m 5 1 0 13 5 1 0 19 m 5 1 0 27 m 571 0 35 m s 1 and 0 37 m s 4 5 Effect of immersion depth The effects of depth on the instrument sensor were measured using a specially constructed 2 metre long 0 2 m diameter PVC tube The construction details have been described previously Harman 1992 The test column was filled with tap water and aerated to achieve a 100 air saturated solution at room temperature Th instrument was calibrated using the manufacturer s standard procedure and the sensor immersed to the specified depth and allowed five minutes to reach equilibrium
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