Optical Dissolved-Oxygen - Measurement Specialties, Inc.

Eureka 2 Optical Dissolved-Oxygen Sensor (HDO) 

Range: 0 to 20 mg/l 

0 to 200% saturation 

Linearity: ±0.2 mg/l and ±2% saturation 

Resolution: 0.01 mg/l and 0.1% saturation 

Units: mg/l (ppm), % saturation 

Calibration: one (air-saturated water) or 

two (air-saturated water and zero) points 

Maintenance: occasional cleaning; replace sensor 

tip approximately two - three years 

Sensor Life: 5+ years 

Sensor Type: optical sensor, lifetime method 

Pressure Limit: 200 meters of water 

Note: The specifications above apply to Eureka’s lifetime-based sensor; the range for 

the intensity-based sensor (see picture below) is 0 – 50 mg/l. 

What is dissolved oxygen? 

Liquid water is loose matrix of water molecules. If water without oxygen comes into contact with a source of oxygen, such 

as the atmosphere, then oxygen will slowly move into the gaps in the water-molecule matrix. That oxygen is called 

dissolved oxygen (DO); it’s the same thing as dissolving table salt in water. Oxygen dissolved in water comes from the 

atmosphere and/or photosynthetic organisms (algae) in the water. 

Why would I want to measure dissolved oxygen in my lake, river, wetland, estuary, or aquifer? 

Like humans, fish and other aquatic animals must breathe oxygen to live, and they are adapted to breathe oxygen 

dissolved in the water instead of oxygen in the atmosphere. Game fish, such as trout, require high DO levels; rough fish, 

such as carp, can exist at lower DO levels. Fish kills are often caused by low DO levels, which are often triggered, 

ironically, by decomposition of the algae that had been adding oxygen to the water before death. 

DO also influences basic water chemistry. Highly oxygenated waters favor the oxidized form of chemicals; poorly 

oxygenated waters favor the reduced form of chemicals. For instance, oxygenated waters favor sulfur in the form of 

sulfate, SO4, while deoxygenated waters favor sulfur in the form of hydrogen sulfide (H2S). Sulfate is a benign 

substance, but hydrogen sulfide is a poison. 

Near-zero DO levels are often found in sediment accumulated at the bottom of a water body. This chemically reducing 

realm helps lock nutrients and metals into the sediment. 

And, changes in long-term DO trends can signal the need for more detailed chemical study of the water and its 

contamination sources. 

Eureka 2 HDO Sensor www.meas-spec.com September 2012 

2113 Wells Branch, Suite 4400, Austin TX 78728 512-302-4333 WQ.sales@meas-spec.com


How is DO measured? 

There are two types of sensors commonly used to measure DO. The traditional Clark Cell consists of two electrodes 

surrounded by a water-based electrolyte solution and covered with an oxygen-permeable membrane. As oxygen crosses 

the membrane to dissolve in the electrolyte, it is consumed in a chemical reaction which generates a small electrical 

current between the two electrodes. That current is directly proportional to the amount of oxygen in the water sample. 

This method is further described in Standard Methods 4500-O G. 

Eureka’s Clark Cell DO Sensor 

A weakness of the Clark Cell is that it consumes oxygen from the water surrounding the sensor; this means that the water 

must be replaced either by natural flow or the use of a circulator to prevent the readings from drifting downward. The 

Clark Cell has performed will in the field for decades, but is slowly being replaced by optical DO sensors. The latter have 

little calibration drift in the field, are not flow-sensitive (no circulator needed), and do not require the occasional membrane 

changes that annoy Clark Cell users. 

Clark 

Cell’s 

circulator 

Clark Cell 

DO sensor 

The second type of DO sensor is the optical DO sensor, in which a blue light is directed to an oxygen-active compound 

that has been stabilized in an oxygen-permeable polymer. The blue light causes the oxygen-active compound to 

fluoresce – i.e. it absorbs energy in the form of blue light and then emits energy as red light. The fluorescence is 

quenched by oxygen – that is, the emission of red light is reduced if oxygen molecules are present to interfere with the 

oxygen-active compound. The more oxygen present, the smaller the amount of red light produced. 

When the polymer sensing surface is exposed to water, oxygen diffuses into the sensing surface according to the amount 

(“partial pressure”) of oxygen in the water. Thus, the amount of red light received by the sensor is directly relatable to the 

amount of oxygen in the water. The red light signal is calibrated to the proper DO units. 

Despite its considerably higher cost, the optical sensor is often favored over the Clark Cell because the optical sensors 

have little calibration drift in the field, are not flow-sensitive (no circulator needed), and do not require the occasional 

membrane changes that annoy Clark sensor users. 




The amount of oxygen dissolved in, for instance, a lake or river, depends on several variables. The higher the barometric 

pressure, the more oxygen can dissolve in water. And the higher the water temperature, the less oxygen can dissolve in 

water. 

Eureka’s Intensity-Based 

Optical DO Sensor 

If the water has absorbed as much oxygen as it can for a particular combination of temperature and barometric pressure, 

the water is said to be saturated with oxygen. If, on average, no oxygen is moving into the water or out of the water, the 

oxygen in the water is said to be in equilibrium with the oxygen in the atmosphere. 

Dissolved oxygen (DO) is commonly reported in two units. DO concentration is the weight of oxygen dissolved in water, 

and is reported in mg/l or ppm. DO percent saturation is the ratio of oxygen actually in the water to the maximum amount 

of oxygen that can dissolve in a water sample under the same conditions, and is reported in % saturation. 

Clark Cells were traditionally calibrated in water-saturated air, but calibration in air-saturated water is growing in favor. 

The latter is done by shaking a half-liter of water in a one-liter container for one minute, and then waiting one minute for 

the bubbles to rise to the surface and disappear. The Clark Cell is immersed in that water and given time to stabilize. 

With knowledge of the water temperature and the barometric pressure, the instrument can figure out the DO level in the 

water because it knows that the water is saturated with oxygen. The instrument sets the Clark Cell reading accordingly. 

The Clark Cell can also be calibrated in water with a DO determined by, for instance, a Winkler titration. 

Optical DO sensors can also be calibrated in air-saturated water, with the same process. Some optical DO sensor 

manufacturers recommend that the user also calibrate at zero DO, and some say that zero calibrations are rarely 

necessary. 

What should I know about DO measurement in the field? 

Flow - Clark Cells are sensitive to water flow, but optical sensors are not. Low water flow results in depleted DO near the 

active surface of a Clark Cell; this produces falsely low sensor readings. Traditionally, a sample flow rate of one ft/sec is 

required for proper Clark Cell operation. Eureka’s sample circulator provides adequate flow for accurate DO 

measurement, and also somewhat reduces biological fouling and insures the measurement is being taken on a 

representative sample. 

Temperature - Both Clark Cell and optical DO sensors are intrinsically sensitive to changes in temperature, but the 

correction for this sensitivity is done automatically by the instrument. The behavior of oxygen in water is also intrinsically 

sensitive to changes in temperature. The saturation level of oxygen in water increases with decreasing temperature. If 

you have oxygen-saturated water at 25°C and slowly raise the temperature of the water to 30°C, oxygen will leave the 

water because hot water won’t hold as much dissolved oxygen as cold water. This phenomenon doesn’t matter to DO 

concentration (mg/l) measurements. But it matters to DO saturation (% sat) measurements because % sat relies on 

knowledge of a water’s DO saturation point. Fortunately, Eureka instruments automatically compensate for this effect. 

Barometric pressure (BP) - If you have oxygen-saturated water at one standard atmosphere (760 mmHg) and slowly 

lower the BP to 740 mmHg, oxygen will leave the water because there’s just not enough air pressure to force the oxygen 

to stay dissolved. When you calibrate DO, you must tell the instrument the local barometric pressure so that it can 

calculate the proper relationship between concentration and saturation. But suppose the barometric pressure changes 

after calibration. The concentration readings will be correct, but the saturation readings will not be correct – you must tell 

the instrument the new BP so that it can make the proper calculations. 




This brings up the Austin-to-Denver example. If you calibrate DO in Austin and then take the instrument to Denver, the 

concentration will be correct because a mg/l of oxygen is the same in both cities. However, the lower barometric pressure 

in Denver means that saturated water there has less oxygen than does saturated water in Austin. So the instrument will 

continue to calculate percent saturation based on the saturation level in Austin, not Denver. The percent saturation 

reading will be wrong until you tell the instrument the barometric pressure of Denver. Then the instrument would lower its 

expectations of oxygen pressure at saturation and provide the correct data. 

Salinity - The saturation level of oxygen in water decreases with increasing salinity. If you have oxygen-saturated water at 

a salinity of 5 (on the PSS) and slowly raise the salinity of the water to 30, oxygen will leave the water because salty water 

can’t hold as much dissolved oxygen as can pure water because salt ions are clogging up the spaces in the watermolecule 

matrix normally available for oxygen molecules. This phenomenon doesn’t matter to DO concentration (mg/l) 

measurements. But it matters to DO saturation (% sat) measurements because % sat relies on knowledge of a water’s 

DO saturation point, and that saturation point falls as salinity rises. Fortunately, Eureka instruments with conductivity 

sensors calculate salinity, and compensate the DO readings accordingly. 

The length of time that DO sensors operate well in the field is influenced by the drift characteristics of the sensor, and by 

sensor fouling. Optical sensors are said to have much lower drift than Clark Cells, but there is little empirical evidence in 

field deployments to quantify any difference in deployment times. 

Fouling - Clark Cells are sensitive to passive foulants, i.e. material in the water that neither consumes nor produces 

oxygen. When, for instance, silt accumulates on the Clark Cell membrane, the rate at which oxygen passes through the 

membrane is reduced. Not knowing any better, the instrument reports an errantly low DO reading. Optical DO sensors, 

however, are not so sensitive to passive foulants because they do not consume oxygen and so do not rely so much on 

unimpeded oxygen movement. 

However, optical DO sensors are more sensitive to active foulants, i.e. material in the water that produces and/or 

consumes oxygen. When, for instance, photosynthetic algae accumulate on the optical sensor, the sensor tends to report 

the DO associated with the algae colony, not the DO of the bulk water sample. This causes the DO reading to be errantly 

high during the sunlight hours when the algae produces oxygen, and errantly low during the night hours when the algae 

consumes oxygen. 

Response time - When lowering a Manta 2 from the air into low-DO waters, the Clark Cell especially takes time to reach a 

stable reading, particularly in cold waters. In most situations, the DO reading will be within specifications within 2 minutes, 

though more accurate readings can be achieved by waiting at least 5 minutes. In monitoring mode, this is less of an 

issue, since the sensors reach equilibration with the sample, and natural DO levels are unlikely to go through large, rapid 

step changes. 

Maintenance - Optical dissolved-oxygen sensor maintenance is nothing more than occasionally cleaning the sensing 

surface (the red material; about a centimeter diameter) with a soft cloth and soapy water. 

Clark Cell maintenance is little more than refilling its electrolyte and replacing the membrane. There should be no bubbles 

in the electrolyte, and the membrane should be taut with no wrinkles or holes. 

How does Eureka’s DO measurement compare with the competition? 

Most Clark Cells are similar in performance, although Eureka seems to spend more time checking for flow sensitivity and 

response time. YSI has a clever adaptation, called the Rapid-Pulse sensor, which largely eliminates the need for a 

circulator – however, that sensor is reported to require considerable maintenance, and may present an error of up to -5% 

or reading in still waters. 

There is considerable discussion about the difference between “lifetime” and “intensity” types of optical Do sensors, 

mostly driven by Hach (Hydrolab) because they were the first to market a lifetime sensor. There are some interesting 

technical differences between the lifetime and intensity methods, but no one has presented a convincing case that one is 

better than the other in field measurements. Eureka is the only manufacturer to offer both the lifetime sensor (we call ours 

the “Hamilton”) and the intensity sensor (we call ours the “Insite”). Both appear to work quite well in the field, but the 

Insite has the advantage of never needing its membrane cap replaced. All other optical DO sensors – Hydrolab, YSI, In- 

Situ, and Eureka’s Hamilton – require that the cap be replaced annually, at a cost of between $200 and $300. 




Eureka’s Insite sensor will thus save you $1000 over a five-year instrument life. The Insite sensor is pretty much 

bulletproof (it was designed for work in waste-water sludge pits), and has a typical lifetime of over seven years. Its single 

disadvantage is that it is somewhat larger tha 

n other optical DO sensors. 

TEXAS OFFICE 

NORTH AMERICA 

HEADQUARTERS 

Measurement Specialties, Inc. 

2113 Wells Branch Parkway 

Suite 4400 

Austin, TX 78728 

USA 

Tel: 1-512-302-4333 

Fax: 1-512-251-6842 

Sales: WQ.sales@meas-spec.com 

Measurement Specialties, Inc. 

1000 Lucas Way 

Hampton, VA 23666 

USA 

Tel: 1-757-766-1500 

Fax: 1-757-766-4297 

Toll Free: 1-800-745-8008 

Sales: WL.sales@meas-spec.com 

The information in this sheet has been carefully reviewed and is believed to be accurate; however, no responsibility is assumed for inaccuracies. Furthermore, this 

information does not convey to the purchaser of such devices any license under the patent rights to the manufacturer. Measurement Specialties, Inc. reserves the right 

to make changes without further notice to any product herein. Measurement Specialties, Inc. makes no warranty, representation or guarantee regarding the suitability of 

its product for any particular purpose, nor does Measurement Specialties, Inc. assume any liability arising out of the application or use of any product or circuit and 

specifically disclaims any and all liability, including without limitation consequential or incidental damages. Typical parameters can and do vary in different applications. 

All operating parameters must be validated for each customer application by customer’s technical experts. Measurement Specialties, Inc. does not convey any license 

under its patent rights nor the rights of others.

Optical Dissolved-Oxygen - Measurement Specialties, Inc.

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