Dissolved oxygen sensor
Background
Dissolved Oxygen (DO) is really a physical distribution of oxygen molecules in water. There are
two main sources of DO in water: atmosphere and photosynthesis.
Ambient air contains about 20% oxygen and is essential for breathing, fish and other aquatic
organisms need oxygen to breathe as well. Dissolved oxygen is the amount of free oxygen in
water suitable for the breathing purpose. If there is not enough oxygen, it is letal to fish: the
amount of 2 mg/l is already deadly and the amount between 2 and 5 mg/l affects fish health.
Also dissolved oxygen data or BOD (biological oxygen demand) is needed to determine effluent
water quality. It is a common environmental procedure to determine the amount of
microorganisms in a sample. This measurement is used in wastewater treatment, food
manufacturing and filtration facilities where this quantity is important for the process and final
product. “High concentrations of DO predict that oxygen uptake by microorganisms is low along
with the required break down of nutrient sources in the medium”.
http://www.eidusa.com/Theory_DO.htm
Types
https://www.fondriest.com/pdf/ysi_do_handbook.pdf
There are two main types of dissolved oxygen sensors: optical (luminescent) and Clark
electrochemical (membrane covered electrode or amperometric). These main types have
subtypes, slightly differing from each other, see figure 1.
Figure 1. Diagram of sensor types. Source: https://www.fondriest.com/pdf/ysi_do_handbook.pdf
Different sensor types suit some applications better that the others. These properties will be
discussed later on the page, meanwhile the applications can be found from Figure 2.
Figure 2. Best applications for different types of sensors. Source:
https://www.fondriest.com/pdf/ysi_do_handbook.pdf
Optical sensors
Optical sensing of oxygen is based on the measurement of the red fluorescence of a
dye/indicator illuminated with a modulated blue light as shown in Figure 3.
Figure 3. Principal of oxygen detection using fluorescent dye. Source: A comparison of
amperometric and optical dissolved
oxygen sensors in power and industrial water applications
The probe emits a blue light of the proper wavelength that causes the dye in the sensing element
to luminesce or glow red. Oxygen constantly diffuses through the paint layer, affecting the
luminescence of the sensing layer. The amount of oxygen passing through to the sensing layer is
inversely proportional to the lifetime of the luminescence in the sensing layer.
The sensor measures the lifetime of the dye’s (sensing layer’s) luminescence, caused by the
presence of oxygen, with a photodiode (light detector) in the probe. To increase the accuracy and
stability of the measurement the reading is compared to a reference. The lifetime of the
luminescence from excitation by the red light acts as the reference (“the sensor
emits a red light that is reflected by the dye layer back to the photodiode in
the sensor”), so the lifetime of luminescence of the blue light is compared to it, and the stable
oxygen concentration is calculated by the probe. (Source:
https://www.fondriest.com/pdf/ysi_do_handbook.pdf)
The oxygen concentration is determined with the Stern-Volmer equation, that sets the
relationship between luminescence lifetime (intensity) and oxygen concentration see Figure 4.
Figure 4. Stern-Volmer equation. Source: https://www.fondriest.com/pdf/ysi_do_handbook.pdf
The most significant advantage of an optical dissolved oxygen sensor is low maintenance cost
and the possibility of less frequent calibration. Other advantages and disadvantages can be found
from Figure 5.
Figure 5. Advantages and disadvantages of optical sensors. Source:
https://www.fondriest.com/pdf/ysi_do_handbook.pdf
Electrochemical sensors
The electrochemical method of measuring dissolved oxygen requires a cathode, anode,
electrolyte solution and a gas permeable membrane of a specially selected oxygen permeable
material (See Figure 6). Oxygen defuses through a membrane to the sensor at a rate proportional
to the pressure difference across the membrane. The oxygen molecules are consumed by the
cathode which creates a partial pressure across the membrane. This reduction produces an
electrical signal that travels from the cathode to the anode and then to the instrument.
“Since oxygen is rapidly reduced or consumed at the cathode, it can be assumed that the oxygen
pressure under the membrane is zero. Therefore, the amount of oxygen diffusing through
the membrane is proportional to the partial pressure of oxygen outside the
membrane”. (https://www.fondriest.com/pdf/ysi_do_handbook.pdf)
So a dissolved oxygen sensor actually measures the pressure of oxygen in the liquid. It can be
used to measure DO in any medium. (http://www.eutechinst.com/techtips/tech-tips15.htm)
Figure 6.An illustration of an electrochemical sensor. Source:
https://www.fondriest.com/pdf/ysi_do_handbook.pdf
Polarographic or Clark Cell Method
“In a polarographic sensor the system is completed by a circuit in the instrument that applies a
constant voltage of 0.8 volts to the probe, which polarizes the two electrodes, and a meter to read
the dissolved oxygen response from the sensor.
The electrolyte held under the membrane allows the electrical signal to travel from the cathode to
the anode. The signal continues down to the meter as shown by the basic circuit diagram in
Figure 7. The polarographic sensor operates by detecting a change in this current caused by the
variable pressure of oxygen while the potential is held constant at 0.8 V. The more oxygen
passing through the membrane and being reduced at the cathode, the greater the electrical signal
(current) read by the probe. As oxygen increases, the signal increases and, conversely, as oxygen
decreases, the signal decreases.” https://www.fondriest.com/pdf/ysi_do_handbook.pdf
Electrolyte used: KCl or KBr
Anode
2Ag + 2Cl- → 2AgCl + 2e-
Cathode (Platinum, gold or palladium)
Total Reaction
2e- + ½ O2 + H2O → 2 OH2e- + ½ O2 + H2O + 2Ag + 2Cl- → 2 OH- +
2AgCl + 2e-
From the above reaction, every time oxygen is reduced at the cathode, 4 electrons or current is
generated directly proportional to the oxygen consumed (reduced) at the cathode.
http://www.eutechinst.com/techtips/tech-tips15.htm
Figure 7. A simplified diagram of a polarographic sensor and circuit. Source:
https://www.fondriest.com/pdf/ysi_do_handbook.pdf
There are four major problems associated with this type of DO measurement:
Problem
Isolation of Anode
Description
Since the net result of the chemical reaction is
AgCl, over time, a build up of AgCl will coat
the anode. Once the whole anode is covered,
reaction stops and the oxygen probe stops
working. The probe can be reactivated by
cleaning the anode to remove the AgCl deposit
which can be a time-consuming procedure.
Zero shift
The result of the above reaction produced more
OH- ions which will move the pH value of the
electrolyte. The electrolyte, which is around
neutral pH value, will moves into the alkaline
range. This causes a zero shift, and over time,
the electrolyte will need to be changed.
The major disadvantage is the need for an
external power source of approximately 800
mV to be applied to the electrode. As soon as
the probe is disconnected, power supply is cut
off. On connecting the probe again, the user
must wait for the probe to be polarized, that is,
for the current loop to be stabilized. This
Warm-up Time
Depletion of Chloride
warm-up time is approximately 10 minutes.
Any measurement taken before this warm-up
time period will be normally a higher value and
will result in wrong readings.
The net reaction also consumes Cl- ions. Over
time, the chloride ions will be consumed and
the electrolyte needs to be replaced.
http://www.eutechinst.com/techtips/tech-tips15.htm
Galvanic Cell Method
“The main advantage of a galvanic probe is that is does not need an external power supply to
provide polarization as required by the Clark Cell. This is achieved by using two dissimilar
metals. In the presence of a electrolyte, there is an electromotive voltage produced between the
two metals. At approximately 800 mV, this is large enough to reduce the oxygen at the cathode.
If lead and gold or lead and silver is used, the differential voltage is approximately 800 mV.
Hence, a galvanic probe is really a self-polarizing amperometric cell. The single biggest
advantage is the fact that the cell is now always ready and there is no warm up time.”
http://www.eutechinst.com/techtips/tech-tips15.htm
Electrolyte used: KCl or KBr
Anode (Zinc or Lead)
Cathode
Total Reaction
Zn → Zn2+ + 2e2e- + ½ O2 + H2O → 2OHZn + 2e- + ½ O2 + H2O → Zn2+ + 2e- +2OHZn + ½ O2 + H2O → Zn (OH)2
→ ZnO (white precipitate) + H2O
or: Zn + ½ O2 → ZnO
http://www.eutechinst.com/techtips/tech-tips15.htm
Figure 8. A simplified diagram of a galvanic sensor and circuit. Source:
https://www.fondriest.com/pdf/ysi_do_handbook.pdf
“One molecule of oxygen produces 4 electrons and there is a direct relationship between the
oxygen consumed at the cathode and the current produced by the cell.
The net result of the chemical reaction is simply ZnO which is reasonably stable and does not
coat the anode. Water is recreated and the electrolyte is not consumed. Theoretically, the
electrolyte will go on forever without replenishment”.
http://www.eidusa.com/Theory_DO.htm
An electrochemical sensor is less costly than an optical sensor, other advantages and
disadvantages can be found from Figure 9.
Figure 9. Advantages and disadvantages of electrochemical sensors. Source:
https://www.fondriest.com/pdf/ysi_do_handbook.pdf
Measuring dissolved oxygen with either sensor type
Dissolved oxygen sensors, both electrochemical and optical, do not measure
the concentration of dissolved oxygen in mg/L or ppm (parts per million
which is equivalent to mg/L). Instead, the sensors measure the pressure of
oxygen that is dissolved in the sample. To simplify the readings displayed
by an instrument, the pressure of the dissolved oxygen is expressed as DO
% Saturation. The instrument converts the dissolved oxygen pressure value
from the sensor to % Saturation by dividing the sensor output in mmHg by
160*** (the pressure of oxygen in air at 760 mmHg) and then multiplying 38 39
by 100. Thus, a measured oxygen pressure of 150 mmHg would be
displayed by a YSI instrument as 93.8 % Saturation (150/160 * 100).
The fact that the sensor measures oxygen pressure and not dissolved oxygen
concentration is known to be true because a sample of fresh water can
dissolve more oxygen than a sample of sea water at the same temperature
and at the same altitude (or under the same barometric pressure); however,
the sensor’s output signal is identical in both samples since the oxygen
pressure is identical in both media. See figure 32 for an example of this
concept.
***The pressure of oxygen at sea level is 160 mmHg because oxygen is
about 21% of the earth’s atmosphere and 21% of 760 (average sea level
barometric pressure) is about 160 mmHg.
Figure 32. DO sensors measure % saturation.
variableS that aFFect diSSOlved
Oxygen meaSurementS
There are several factors that affect the measurement of dissolved oxygen.
These variables include temperature, salinity, flow or stirring dependence,
and barometric pressure.
Temperature and salinity are compensated for during instrument calibration
dO Sensors measure % Saturation
If two samples, one of fresh water and one of sea water, are fully
saturated with oxygen the dissolved oxygen concentration will be:
Fresh water at 25ºC = 8.26 mg/L
Sea water (36 ppt) at 25ºC = 6.27 mg/L
However, the signal output from either sensor type will be identical in the
two samples. Since both are 100% saturated, % saturation (or oxygen
pressure) is what both sensors are measuring
https://www.fondriest.com/pdf/ysi_do_handbook.pdf
Variables that affect DO measurements
There are several parameters that affect the DO measurement accuracy and reliability, they are
temperature, salinity, barometric pressure and flow (stirring).
Temperature
Temperature is the most significant variable for the measurement accuracy, therefore it should be
ensured that the temperature sensor on the prode is working correctly. Temperature can influence
the DO measurement in two ways:
 Diffusion of oxygen through the membrane (electrochemical) or sensing element
(optical) on the probe increases/decreases with higher/lower temperature due to change in
molecular activity (up to 4% difference per °C)
“Therefore, the sensor signal must be compensated for changes in temperature. This is done by
adding a thermistor to the circuit of older, analog instruments. For newer, digital instruments, the
software compensates for temperature changes with proprietary algorithms that use the
temperature readings from the probe’s thermistor”.
https://www.fondriest.com/pdf/ysi_do_handbook.pdf

Ability of water to dissolve oxygen, directly proportional to temperature. Warmer water
dissolves less oxygen than colder water.
Therefore, the mg/L concentration must be compensated reading
per the temperature of the sample.
Salinity
“As the salinity of water increases, its ability to dissolve oxygen decreases. Thus, salinity (along
with temperature) must be factored into the instrument’s calculation of mg/L.
Some dissolved oxygen instruments also measure conductivity, so the salinity value measured by
the conductivity sensor is used for the mg/L calculation. Therefore, it is important to ensure the
conductivity sensor is calibrated and reading accurately in order to obtain accurate DO mg/L
readings. For such dissolved oxygen instruments that do not have a conductivity sensor,
the salinity value of the sample must be manually entered by the end user”.
https://www.fondriest.com/pdf/ysi_do_handbook.pdf
Pressure
“Barometric pressure affects the pressure of oxygen
in a sample of air or water.
The effect of barometric pressure is overcome by proper sensor calibration.
Barometric pressure is used in the majority of dissolved oxygen sensor
calibrations as described in the calibration section since it determines
the absolute pressure of oxygen in a sample of air or water at the time of
calibration and it is this pressure which is measured by all oxygen sensors.
When calibrating oxygen sensors, the sensor’s output is set to this known
pressure of oxygen. If the sensor output changes after calibration, then
the instrument would calculate a % saturation based on a simple linear
regression calculation. Thus, as long as the system does not drift, the
sensor’s output can always be used to define the oxygen pressure in any 44 45
medium after performing a proper calibration and the use of the barometric
pressure (or altitude) at the time of calibration is the key factor in setting
the proper calibration coefficient. Therefore, it is not necessary to correct
for changes in barometric pressure after performing a proper calibration in
order to obtain accurate readings in the field.
In summary, as barometric pressure changes due to a change in altitude
or local weather front, the pressure of oxygen changes. However, there is
never any reason to compensate for this change if a proper calibration has
already been performed and the sensor has not drifted.”
https://www.fondriest.com/pdf/ysi_do_handbook.pdf
Calibration
Same link
Electrochemical sensors require more frequent calibrations than optical
sensors. Steady-state galvanic and polarographic sensors should be
calibrated every day that they are used in a spot sampling application. In
general, it is best to calibrate frequently at first and, as experience dictates,
you may reduce calibration frequency.
Optical sensors have greater stability and are less susceptible to drift than
traditional electrochemical sensors. Experience and scientific studies have
shown that optical sensors can hold their calibration for many months.
However, for the highest data accuracy, YSI recommends verifying the
optical sensor’s calibration on a regular basis. To verify the instrument’s
calibration, place the sensor in its calibration environment and check to see
that the instrument’s DO% reading matches the calibration value based on
the barometric pressure. Refer to Appendix B for the DO% calibration values
based on barometric pressure and altitude.
NOTE: It may be useful to check the DO calibration before and after an
unattended monitoring study to ensure high data quality.
When calibrating DO % saturation to barometric pressure/altitude, it is
not necessary to recalibrate the instrument due to a change altitude or
barometric pressure because the DO % saturation and mg/L readings will
remain accurate. However, if Local DO% is being reported with a 556 or
650 it may be desirable to recalibrate after a significant change in altitude
or barometric pressure in order to keep the % saturation value at 100% in the
calibration environment. This is not a requirement if only mg/L values are
being recorded since these values will remain accurate without recalibrating
Local DO %. It is also not required to recalibrate instruments that have an
on-board barometer for accurate Local DO% readings, like the Pro Plus and
ProODO, since the barometer will automatically assure that the Local DO%
value remains at 100% in a fully saturated sample. For more information
on Local DO, refer to the Local DO % Measurements section of this booklet.
calibratiOn methOdS
In general, calibration consists of exposing the sensor to a sample of known
oxygen content and calibrating the instrument to read that value. There are
three primary methods for calibrating a dissolved oxygen instrument:
• Winkler titration
• Air-saturated water
• Water-saturated air