A sensor that responds to oxygen partial pressure reads in proportion to absolute pressure. A true 99.5 % oxygen stream reads about 97.2 % when the air pressure falls from 1013.25 hPa to 990 hPa on an uncompensated sensor. Pressure and temperature compensation removes most of this error.
Applies to sensors that respond to partial pressure. A sensor with built-in pressure and temperature compensation, such as ACCURRA-S 3090 and 3190, corrects for this effect.
How it is calculated
Many oxygen sensors respond to the partial pressure of oxygen at the sensing surface: the oxygen fraction multiplied by the absolute pressure. If the pressure at the sensor differs from the pressure at calibration, the signal scales by the ratio of the two.
Displayed = True x P_now / P_calibration
Worked example
Calibrated at 1013.25 hPa. A weather front drops the pressure to 990 hPa. A true 99.5 % stream reads 99.5 x 990 / 1013.25 = 97.2 %, an error of minus 2.3 points.
The plant sees a quality failure that never happened. A day and night swing in barometric pressure can create a similar error twice a day.
Compensated purity analysers measure ambient pressure and temperature and correct the reading. The catalogue gives a residual error below 0.5 % for 3090 and 3190.
Limits
- Only partial-pressure sensors show this effect. A thermal conductivity or NDIR reading responds differently to pressure.
- Sample pressure at the sensor, not only the weather, matters: a sample vented to atmosphere sees atmospheric pressure.
- Temperature has its own effect on top of this.
Frequently asked questions
Why does my oxygen reading change with the weather?
Because the sensor responds to partial pressure, and barometric pressure changes the partial pressure of oxygen even when the gas is unchanged.
How much can pressure change an oxygen purity reading?
About 1 % for each 1 % change in absolute pressure on an uncompensated sensor; day and night swings can reach several percent.
Is the effect the same at altitude?
A site at altitude has lower pressure. If you calibrate on site at that pressure, only changes from it matter; otherwise the offset is large.
Which analysers compensate for pressure?
ACCURRA-S 3090 and 3190 measure ambient pressure and temperature and correct the reading.