Proper Compensation for DO Measurementin Gas with Hamilton’s Optical Sensors
Technical Note

Dissolved oxygen measurement effects due to changing environmental
conditions from humidity and pressure
Applications: Exhaust gas in laboratories, nitrogen blanketing

The measurement of oxygen concentration (in % volume or% air saturation) with optical dissolved oxygen (DO) sensors
is affected by environmental pressure and humidity. Hamilton optical DO sensors refer to a concentration in water
saturated air. The factory settings of atmospheric pressure
are set to 1013 mbar at 25°C and 100% air humidity. For an
accurate measurement Hamilton recommends to correct
the atmospheric pressure and compensate the air humidity.

70Measured %-vol.

60
504030Compensatedlevel201001013 mbar

2000 mbar

3000 mbar

Effect of environmental pressure
The figure 1 shows the influence of the environmentalpressure on the oxygen concentration measured by an
optical DO sensor. The measured oxygen concentration(in % volume) is directly proportional to the environmentalpressure, i.e. the atmospheric pressure or the pressure
applied in the process vessel.

Environmental pressure
OxygenFigure 1: The measurement of oxygen concentration with optical DO sensor
is influenced by the environmental pressure.

Set the pressure in ArcAir:
Configuration / Advanced / Measurement / Air pressure

Therefore, before installing an optical DO sensor, verify
that the air pressure of the process environment and thepressure setting in the sensor are equal. Compensate
the atmospheric pressure in the sensor with the following
parameter settings:

Set the pressure in HDM:
Measurement / Parameter / Air pressureOptical DO sensor in air with different humidity
100

Oxygen reading in %-vol.

90Effect of humidity

8070Humidity may only impact oxygen concentration measurement in gas but not in liquids. All Hamilton optical DO sensors refer to a concentration in water saturated air (100%humidity). Figure 2 shows the effect of humidity on themeasured oxygen concentration at different temperature.
This effect is greater at higher temperatures.

60504030201000255075

Measured Temperature °C

0% Humidity
25% humidity
50% humidity
75% humidity
100% humidity

100As example
If the optical DO sensor is exposed to air at 40°C in 50%
humidity the correct oxygen concentration is 21.8% and not
20.95%, resulting in an error of almost 4%.
To correct the oxygen concentration measured in gas youneed to measure humidity and compensate the measured
oxygen concentration using the following formula:

Figure 2: Humidity influence on the oxygen reading at different temperature.

[% −.] =[% −.]((−−2*,2),)ExplanationrH : relative humidity in air (e.g. 0.5 for 50%)
Patm : Atmospheric air pressure
PH2O,Tmeas : Theoretical water vapor partial pressure at the measurement temperature© 2015 Hamilton Bonaduz AG. All rights reserved.
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10/2015

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