Wireless Optical Dissolved Oxygen Sensor
Theory of Sensor Operation
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013-16057A
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The reference is a beam of red light emitted by a LED in the probe that reflects from the dye layer back to the
photodiode. The lifetime of the luminescence from excitation by the blue light is compared to that of the reference
value (red light) and a dissolved oxygen concentration is calculated by the sensor.
Optical and electrochemical dissolved oxygen probes determine the partial pressure of oxygen dissolved in the
sample being tested. The dissolved oxygen partial pressure is converted to concentration (mg/l) or saturation
percentage. The conversion from dissolved oxygen pressure to saturation percentage is done by dividing the
reading by 160, the pressure of oxygen in air at 760 millimeters (mm) of mercury (Hg) and then multiplying by
100%. For example, a measured dissolved oxygen pressure reading of 150 mm Hg would be converted to 93.8%
saturation (150/160 x 100%).
See the PASCO web site at www.pasco.com for more information.
Factors that Affect Dissolved Oxygen Measurement
Factors that affect dissolved oxygen measurement include temperature, barometric pressure, and salinity.
Temperature: The rate of diffusion of oxygen through the Sensor Cap changes with temperature (approximately
1.5% per degree celsius). The Optical Dissolved Oxygen Probe has a built-in thermistor, and the temperature
changes are compensated for by proprietary algorithms that use readings from the thermistor.
Barometric Pressure: The partial pressure of oxygen in a sample of air or water is not affected by barometric
pressure, but dissolved oxygen concentration in milligrams per liter is affected. The Optical Dissolved Oxygen
Probe has a built-in barometer sensor, and the reading from that device is used to determine the oxygen pressure
during calibration.
Salinity: As the salinity of water increases, its ability to dissolve oxygen decreases. (Note: The user will be able to
enter a value for the “salinity constant” in the data acquisition software.) For precise measurement of salinity, the
PS-2195 Salinity Sensor is recommended (see the web site at www.pasco.com).
Water Type
Average Salinity
Fresh water
<0.5 ppt*
Brackish water
0.5 to 30 ppt
Sea water
33 to 47 ppt
Saline water
30 to 50 ppt
Brine
>50 ppt
Optical Dissolved Oxygen
Probe
Sensing Layer (dye layer)
Sensor Cap with
Diffusion Layer
Photodiode and Light
Emitting Diodes (LEDs)
DO NOT TOUVH
Rubber O-ring
NOTE: Avoid touching
the Diffusion Layer or
the Sensing Layer
