Field Comparison of Optical and Clark Cell Dissolved 

Oxygen Sensors in the Tualatin River, Oregon, 2005 

By Matthew W. Johnston and John S. Williams 

Open-File Report 2006–1047 

U.S. Department of the Interior 

U.S. Geological Survey
U.S. Department of the Interior 

Gale A. Norton, Secretary 

U.S. Geological Survey 

P. Patrick Leahy, Acting Director 

U.S. Geological Survey, Reston, Virginia 2006 

For product and ordering information: 

World Wide Web: http://www.usgs.gov/pubprod 

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its natural and living resources, natural hazards, and the environment: 

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Telephone: 1-888-ASK-USGS 

Suggested citation: 

Johnston, M.W., and Williams, J.S.,2006, Field Comparison of Optical and Clark Cell Dissolved Oxygen 

Sensors in the Tualatin River, Oregon, 2005: U.S. Geological Survey Open-File Report 2006-1047, 11 p. 

Any use of trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government. 

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report. 

ii 
Contents 

Abstract ................................................................................................................................................................. 1

Introduction .......................................................................................................................................................... 1

Methods................................................................................................................................................................. 3

Results ................................................................................................................................................................... 4

Discussion........................................................................................................................................................... 10

References Cited ............................................................................................................................................... 11

Figures 

Figure 1. Map of the Tualatin River Basin, showing location of dissolved oxygen sensor 

comparison study. ............................................................................................................................................... 2

Figure 2. Graph showing uncorrected and corrected dissolved oxygen concentration data 

from the YSI 6600EDS Clark cell dissolved oxygen sensor. ......................................................................... 5

Figure 3. Graph showing uncorrected and corrected dissolved oxygen concentration data 

from the YSI 600XLM Clark cell dissolved oxygen sensor. .......................................................................... 6

Figure 4. Graph showing uncorrected and corrected dissolved oxygen concentration data 

from the Hydrolab MS5 LDO optical dissolved oxygen sensor. .................................................................. 7

Figure 5. Graph showing uncorrected and corrected dissolved oxygen concentration data 

from the Troll 9000 RDO optical dissolved oxygen sensor. .......................................................................... 8

Figure 6. Graph showing uncorrected and corrected dissolved oxygen concentration data 

from the YSI 6600 ODO optical dissolved oxygen sensor............................................................................. 9

Figure 7. Graph showing dissolved oxygen concentration data from all five sensors in the 

comparison study and results of Winkler analyses. ................................................................................... 10

Tables 

Table 1. Cleaning- and calibration-correction data for the YSI 6600EDS Clark cell 

dissolved oxygen sensor .....................................................................................................................................5

Table 2. Cleaning- and calibration-correction data for the YSI 600XLM Clark cell 

dissolved oxygen sensor .....................................................................................................................................6

Table 3. Cleaning- and calibration-correction data for the Hydrolab MS5 LDO optical 

dissolved oxygen sensor .....................................................................................................................................7

Table 4. Cleaning- and calibration-correction data for the Troll 9000 RDO optical 

dissolved oxygen sensor .....................................................................................................................................8

Table 5. Cleaning- and calibration-correction data for the YSI 6600 ODO optical 

dissolved oxygen sensor .....................................................................................................................................9

iii
Conversion Factors 

Multiply By To obtain 

foot (ft) 0.3048 meter (m) 

mile (mi) 1.609 kilometer (km) 

Acknowledgments 

The authors thank Joseph Rinella, Stewart Rounds, and Micelis Doyle, all USGS, for their 

technical advice and other contributions to this report.

iv
Field Comparison of Optical and Clark Cell Dissolved 

Oxygen Sensors in the Tualatin River, Oregon, 2005 

By Matthew W. Johnston and John S. Williams 

Abstract 

Comparison of two Clark cell type dissolved oxygen sensors with three optical sensors in the 

Tualatin River, Oregon, indicated that the optical sensors were less prone to fouling drift and calibration 

drift. In two cleanings and calibrations over the 3-week study, the Clark cells exhibited fouling drifts 

ranging from 0.17 to 0.37 mg/L (milligrams per liter) and calibration drifts ranging from –0.22 to 0.03 

mg/L. The optical sensors had fouling drifts ranging from 0 to 0.02 mg/L and calibration drifts ranging 

from –0.09 to 0.02 mg/L after 2–3 weeks of deployment. Measurements by the Clark cell and optical 

sensors compared well to each other and to point measurements of oxygen concentration using the 

Winkler method, indicating that the optical sensors were as accurate as the Clark cell sensors under the 

study conditions. 

Introduction 

The U.S. Geological Survey monitors dissolved oxygen (DO) concentration in the Tualatin River, 

west of Portland, Oregon, as part of a long-term assessment of the water quality of the river. (A 

description of the study can be found at http://or.water.usgs.gov/tualatin/) The Tualatin River study 

deploys polarographic, Clark cell type DO sensors as components of multiparameter datasondes that 

contain sensors for DO, temperature, pH, specific conductance, and turbidity. 

Clark cell sensors measure DO indirectly through an electrochemical reaction. The tip of the sensor 

contains a positive electrode (cathode) and a negative electrode (anode) connected electrically by a 

saturated electrolyte solution, all covered by a permeable Teflon membrane. Oxygen molecules dissolved 

in the water pass through the membrane and are chemically reduced within the sensor, generating an 

electrical current that is proportional to the oxygen concentration in the water. The current is converted to 

a DO concentration that is either displayed on a meter or stored as data for later retrieval. 

Clark cell DO sensors have been used for decades and provide accurate and precise DO data; 

however, they have drawbacks: (1) Clark cell units can require frequent calibration and maintenance. (2) 

Because oxygen is consumed at the surface of the membrane, water in that vicinity must be moving to 

obtain good measurements; in slow-moving water or in protective housings, mechanical stirring is 

necessary for most models, although YSI, Inc., has developed a technology that reportedly eliminates the 

need for stirring (http://www.act-us.info/Download/Do_Evaluations/ACT_VS04-04_YSI_DO.pdf, 

accessed March 3, 2006). (3) The Teflon membrane can be punctured by aquatic insects, improper 

handling, and waterborne debris. (4) Fouling of the membrane by algae and fine waterborne materials can 

significantly affect measurement quality; some units employ an automatic wiper to alleviate this problem. 

Optical DO sensors are a relatively new technology that can potentially reduce operating costs as a 

result of less-frequent required calibration and maintenance. Optical DO sensors have a tip coated on the 

inside with a thin layer of oxygen-sensitive fluorescent dye. A light-emitting diode (LED) shines blue 

light on the dye layer, causing the dye to emit red fluorescent light that travels to a photodetector. Oxygen 

diffusing into the sensor tip interferes with the light emitted by the dye, reducing the intensity of the light 

emitted, the amount of time the dye fluoresces, and the amount of time between blue light emission and 

1
red light response. This phenomenon is known as “quenching.” The degree of quenching is directly 

related to oxygen concentration, and different brands of oxygen sensors use different physical aspects of 

quenching to calculate DO concentration in water. 

From September 14, 2005, through October 7, 2005, the USGS conducted a comparison of three 

models of optical DO sensors and two Clark cell sensors at an existing continuous water-quality 

monitoring site in the Tualatin River near Oswego Dam at river mile 3.4 (fig. 1). The purpose of the study 

was to compare calibration drift—the tendency of instrument readings to diverge from the true value over 

time—and fouling drift—deviation in readings due to an accumulation on the sensor of algae and/or other 

organic and inorganic material in the water column—in the two types of sensors. The site was chosen 

because of the potential for sensor fouling in the Tualatin River at that time of the year and because the 

site already had two Clark cell sensors deployed there. 

Figure 1. Tualatin River Basin, showing location of dissolved oxygen sensor comparison study. 

2
Methods 

The oxygen sensors assessed in this study were components of multiparameter datasondes. The 

datasonde models and their DO sensor type were: 

• YSI 6600EDS (Rapid PulseTM Clark cell sensor with automatic wiper), already deployed as part of 

monitoring program 

(http://www.ysi.com/extranet/EPGKL.nsf/447554deba0f52f2852569f500696b21/5adb2bc006585c2c8

5256bb50067bf19!OpenDocument, accessed March 3, 2006) 

• YSI 600XLM (Rapid Pulse Clark cell sensor without wiper), already deployed as part of monitoring 

program 

(http://www.ysi.com/extranet/EPGKL.nsf/447554deba0f52f2852569f500696b21/8db42369ec1b6e3a8

5256cef00562ec6/$FILE/600XL%20600XLM%200103%20E55.pdf, accessed March 3, 2006) 

• Hach® Hydrolab® MS5 (LDOTM optical sensor), deployed September 14, 2005 

(http://www.hydrolab.com/products/ldo_sensor.asp, accessed March 3, 2006) 

• In-Situ TrollTM 9000 (RDOTM optical sensor), deployed September 16, 2005 (http://www.insitu.com/In-Situ/Products/TROLL9500/TROLL9500_RDO.html, accessed March 3, 2006) 

• YSI 6600 (ODO optical sensor with wiper—still in development at the time of this study), deployed 

September 22, 2005 

The sondes were housed in perforated PVC pipe at a depth of 2 feet. The three sondes containing the 

optical DO sensors logged data internally once every hour, on the hour, while the two sondes containing 

the Clark cell sensors collected data through a Campbell CR-10 data logger at the same time interval. 

Data collected on the CR-10 is telemetered via cell phone and displayed at 

http://or.water.usgs.gov/tualatin/monitors/. Data from the optical sensors were manually downloaded to a 

laptop computer at the end of the deployment period. 

The two Clark cell sensors were cleaned and calibrated on August 31, prior to the study, and on 

September 16 and 28, during the study. The optical DO sensors were calibrated at deployment, cleaned on 

October 6, and recalibrated on October 7, at the end of the study period. The two Clark cell sensors were 

calibrated in a water-saturated-air (100% relative humidity) chamber (Radtke and others, 1998), whereas 

the optical sensors were calibrated in a chamber filled with air-saturated water per the manufacturers’ 

protocols. The YSI ODO sensor could not be calibrated to the reference value for air-saturated water at 

ambient temperature and barometric pressure; however, the probe reading differed from the reference 

value by only 0.08 mg/L (milligrams per liter), probably less than the inherent error of the calibration 

method. The final data set was corrected for the 0.08 mg/L difference. At the time of this test, the YSI 

ODO sensor was still in development, and we do not presume that the calibration problem experienced 

during this test extends to the production version.

Two types of shifts (corrections) were applied to the data, a cleaning shift and a calibration shift. The 

cleaning shift is applied as a result of decreased probe performance as the probe becomes coated with 

algae and fine suspended material from the river. This shift is determined by observing the DO 

concentration, as measured by the DO sensor, before and after cleaning the probe. This difference is 

applied to the data set and prorated back to a difference of 0 mg/L at the time of the previous cleaning. 

The calibration shift is applied to the data set in the same way, but the shift is determined by observing the 

DO reading in a water-saturated-air or air-saturated-water chamber (depending on the sensor type) and 

noting the difference between that value and the 100% saturation value. 

Because the DO concentration in a stream can vary over a relatively short time, a calibrated backup 

sensor typically is deployed while the primary sensor is being cleaned to measure DO changes that might 

3
occur between when the primary sensor is withdrawn from the stream for cleaning and when it is placed 

back in the stream after cleaning, a process that takes only a few minutes. The difference in the reading 

before and after cleaning is corrected by the amount of change detected by the backup sensor as follows: 

( ) DO DO DO DO pa pb −−− ( ba bb ) (1) 

where is the reading of the primary DO sensor after cleaning, DOpa

DOpb is the reading of the primary DO sensor before cleaning, 

DOba is the reading of the backup DO sensor after cleaning, and 

DObb is the reading of the backup DO sensor before cleaning. 

A backup sensor was deployed during cleaning only for the Clark cells sensors, not for the optical 

sensors; however, the site’s permanent monitor showed no significant change in the DO concentration in 

the river during the cleaning operation for the optical sensors, so the general result of the comparison was 

not affected. 

Winkler DO analyses (Radtke and others, 1998) were performed on September 30 and October 6 to 

provide a standard against which to measure DO sensor performance. Samples for the Winkler analysis 

were collected at the same depth as the test probes were deployed. 

Results 

Data completeness was generally good; however, the Hydrolab MS5 LDO optical sensor lost data 

between 9-27-2005 and 9-30-2005 due to battery failure caused by an overly long warmup period. The 

YSI 6600EDS and YSI 600XLM Clark cells had a few missing values on 9-16-2005 and 9-28-2005 due 

to a calibration visit. The Troll 9000 RDO and YSI 6600 ODO optical sensors had no missing data. 

Results of the comparison are shown in figures 2–7 and tables 1–5. The Clark Cell sensors required 

larger shifts to the data than the optical sensors because of both calibration drift (–0.22 to 0.03 mg/L) and 

fouling drift (0.17 to 0.37 mg/L). The ranges of calibration and fouling drift for the optical sensors at the 

conclusion of the test were –0.09 to 0.02 mg/L and 0.00 to 0.02 mg/L, respectively. Fouling due to algal 

growth was substantial on all sensors. 

4
Cleaning and calibration

Cleaning and calibration

14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 1 2 3 4 5 6 3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

Corrected data set

Uncorrected data set

YSI 6600EDS

DISSOLVED OXYGEN, IN MILLIGRAMS PER LITER

SEPTEMBER

2005

OCTOBER

Figure 2. Uncorrected and corrected dissolved oxygen concentration data from the YSI 6600EDS Clark cell 

dissolved oxygen sensor. 

Table 1. Cleaning- and calibration-correction data for the YSI 6600EDS Clark cell dissolved oxygen sensor 

[Dissolved oxygen (DO) concentrations are in milligrams per liter. Value for saturated air, DO concentration in watersaturated air at ambient water temperature and barometric pressure; --, not applicable] 

Cleaning Calibration 

Before cleaning After cleaning 

Date 

Primary 

sensor 

Backup 

sensor 

Primary 

sensor 

Backup 

sensor 

Fouling drift 

correction

 Value for 

saturated 

 air 

 Meter 

reading 

Calibration 

drift 

correction 

08-31-2005 -- -- -- -- -- 8.95 8.95 -- 

09-16-2005 5.95 6.18 6.14 6.04 0.33 9.98 10.20 –0.22 

09-16-2005 -- -- -- -- -- 9.98 9.98 -- 

09-28-2005 6.59 6.98 6.88 6.93 0.34 9.29 9.26 0.03 

09-28-2005 -- -- -- -- -- 9.29 9.29 -- 

10-12-2005 6.60 7.49 6.88 7.40 0.37 10.11 10.20 –0.09 

5
14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 1 2 3 4 5 6 3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

DISSOLVED OXYGEN, IN MILLIGRAMS PER LITER

Corrected data set

Uncorrected data set

YSI 600XLM Cleaning and calibration

Cleaning and calibration

2005

SEPTEMBER OCTOBER

Figure 3. Uncorrected and corrected dissolved oxygen concentration data from the YSI 600XLM Clark cell dissolved 

oxygen sensor. 

Table 2. Cleaning- and calibration-correction data for the YSI 600XLM Clark cell dissolved oxygen sensor 

[Dissolved oxygen (DO) concentrations are in milligrams per liter. Value for saturated air, DO concentration in 

water-saturated air at ambient water temperature and barometric pressure; --, not applicable] 

Cleaning Calibration 

Before cleaning After cleaning 

Date 

Primary 

sensor 

Backup 

sensor 

Primary 

sensor 

Backup 

sensor 

Fouling drift 

correction

 Value for 

saturated

 air 

 Meter 

reading 

Calibration 

drift 

correction 

08-31-2005 -- -- -- -- -- 8.82 8.82 -- 

09-16-2005 6.01 6.16 6.19 6.04 0.30 9.90 9.97 –0.07 

09-16-2005 -- -- -- -- -- 9.93 9.93 -- 

09-28-2005 6.82 6.97 7.13 6.93 0.35 9.49 9.63 –0.14 

09-28-2005 -- -- -- -- -- 9.52 9.52 -- 

10-12-2005 6.73 7.47 6.83 7.40 0.17 10.06 10.15 –0.09 

6
Test probes cleaned

Battery

failure

14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 1 2 3 4 5 6 3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

Corrected data set

Uncorrected data set

Hydrolab MS5 LDO

DISSOLVED OXYGEN, IN MILLIGRAMS PER LITER

SEPTEMBER OCTOBER

2005

Figure 4. Uncorrected and corrected dissolved oxygen concentration data from the Hydrolab MS5 LDO optical 

dissolved oxygen sensor. 

Table 3. Cleaning- and calibration-correction data for the Hydrolab MS5 LDO optical dissolved oxygen 

sensor 

[Dissolved oxygen (DO) values are in milligrams per liter. Value for saturated water, DO concentration in airsaturated water at ambient water temperature and barometric pressure; --, not applicable] 

Cleaning Calibration 

Date 

Before 

cleaning 

After 

cleaning 

Fouling drift 

correction 

Value for 

saturated 

water 

Meter 

reading 

Calibration 

drift 

correction 

09-14-2005 -- -- 8.74 8.74 -- 

10-06-2005 4.76 4.78 0.02 -- -- -- 

10-07-2005 -- -- -- 8.56 8.65 -0.09 

7
16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 1 2 3 4 5 6 3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

Corrected data set

Uncorrected data set

Troll RDO

DISSOLVED OXYGEN, IN MILLIGRAMS PER LITER

Sensor cleaned

2005

SEPTEMBER OCTOBER

9000

Figure 5. Uncorrected and corrected dissolved oxygen concentration data from the Troll 9000 RDO optical 

dissolved oxygen sensor. 

Table 4. Cleaning- and calibration-correction data for the Troll 9000 RDO optical dissolved oxygen 

sensor 

[Dissolved oxygen (DO) values are in milligrams per liter. Value for saturated water, DO concentration in airsaturated water at ambient water temperature and barometric pressure; --, not applicable] 

Cleaning Calibration 

Date 

Before 

cleaning 

After 

cleaning 

Fouling drift 

correction 

Value for 

saturated 

water 

Meter 

reading 

Calibration 

drift 

correction 

09-16-2005 -- -- 8.81 8.81 --

10-06-2005 4.07 4.09 0.02 -- -- --

10-07-2005 -- -- -- 8.73 8.71 0.02

8
Sensor cleaned

Data begins with a shift of 0.08 milligram per

liter because the unit could not be calibrated

.

22 23 24 25 26 27 28 29 30 1 2 3 4 5 6

2 0 05

3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

Corrected data set

Uncorrected data set

YSI 6600 ODO

DISSOLVED OXYGEN, IN MILLIGRAMS PER LITER

to the value for -saturated air at the

ambient temperature and barometric pressure.

water

SEPTEMBER OCTOBER

Figure 6. Uncorrected and corrected dissolved oxygen concentration data from the YSI 6600 ODO optical dissolved 

oxygen sensor. 

Table 5. Cleaning- and calibration-correction data for the YSI 6600 ODO optical dissolved oxygen 

sensor 

[Dissolved oxygen (DO) values are in milligrams per liter. Value for saturated water, DO concentration in airsaturated water at ambient water temperature and barometric pressure; --, not applicable] 

Cleaning Calibration 

Date 

Before 

cleaning 

After 

cleaning 

Fouling drift 

correction 

Value for 

saturated 

water 

Meter 

reading 

Calibration 

drift 

correction 

09-22-2005 -- -- -- 8.68 8.60 0.08a

10-06-2005 4.71 4.71 0.00 -- -- --

10-07-2005 -- -- -- 8.60 8.50 0.10b

a

Sensor could not be calibrated to the reference value for air-saturated water at ambient temperature and barometric pressure. b

 Actual correction is 0.02 mg/L when initial calibration discrepancy is subtracted. 

All of the sensors recorded fine- and large-scale hourly and daily variations in the DO concentration 

at the river test site (fig. 7), as well as two large concentration swings caused by low-DO plumes on

October 1–2 and October 5–7 that resulted from storm runoff. Values from the Troll 9000 RDO were

9
generally about one-half milligram per liter less than those from the other optical units. The values 

recorded by the Clark cell sensors were generally slightly higher than those from the optical sensors. 

Concentrations from the Winkler analyses fell midway between the concentrations recorded by the Troll 

9000 RDO and those from the other sensors; thus, all of the units had similar accuracy relative to the 

reference standard of the Winkler analysis. 

14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 1 2 3 4 5 6 3.5

4.0

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

Corrected data for all sensors

DISSOLVED OXYGEN, IN MILLIGRAMS PER LITER

Optical sensors cleaned

YSI 600XLM (Clark cell)

YSI 6600EDS (Clark cell)

Hydrolab MS5 LDO (optical)

YSI-6600 ODO (optical)

Troll 9000 RDO (optical)

Winkler analysis

SEPTEMBER OCTOBER

2005

Figure 7. Dissolved oxygen concentration data from the five sensors in the comparison study and results of Winkler 

analyses. 

Discussion 

The results of this comparison study indicate that the optical sensors had accuracies similar to those 

of the Clark cells but were less prone to decreasing accuracy over time caused by fouling and calibration 

drift. Note, however, that this comparison used only single optical-sensor units from each manufacturer 

and consequently could not measure variation within brands. Therefore, the performance of each 

instrument in the comparison might not be indicative of the performance that can be expected from offthe-shelf units from each manufacturer. Further testing will be required to confirm that optical sensors 

routinely outperform Clark cells with regard to drift and thus require less-frequent servicing, a cost 

savings. 

Another USGS study (Fulford and others, 2005) found that drifts due to fouling were similar between 

the two types of sensors in a study conducted in a brackish-water, estuarine environment for a longer time 

period, 30 days; the sensors were not serviced during the test. Other comparisons (Alliance for Coastal 

Technologies, 2005) have found, similar to this one, that the optical sensors were less prone to fouling 

drift and required less-frequent calibration. 

10
References Cited 

Alliance for Coastal Technologies, 2005, Technology Evaluations Reports: http://www.actus.info/evaluation_reports.php, accessed March 3, 2006. 

Fulford, J.M., Davies, W.J., and Garcia, L., 2005, Field comparison of optical and Clark cell dissolvedoxygen sensors, in Walton, R., ed., World water and environmental resources congress 2005—Impacts 

of global climate change, May 15–19, 2005, Anchorage, Alaska, USA, Proceedings: American Society 

of Civil Engineers, p. 312. 

(http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=ASCECP00017304079200031

2000001&idtype=cvips&gifs=yes, accessed March 3, 2006) 

Radtke, D.B., White, A.F., Davies, J.V., and Wilde, F.D., 1998, Dissolved oxygen—Chapter 6.2, in

Wilde, F.D., ed., Field measurements: U.S. Geological Survey Techniques of Water-Resources 

Investigations Reports Book 9, Chapter A6, 

http://water.usgs.gov/owq/FieldManual/Chapter6/6.2_contents.html, accessed March 3, 2006.

11