Is it possible to measure expired gasses as a partial pressure (mmHg) rather than a percentage fraction using the ML206 gas analyser?

Expired gasses are usually expressed in two forms.  Fraction of Expired gas (FEO2and FECO2) is expressed as a % while Partial Pressure (PO2 & PCO2) is expressed as a pressure (mmHg or KPa).

The ML206 records gas concentrations as a %, but this can be converted to a pressure using the Arithmetic function if the atmospheric pressure is known.  

Standard atmospheric, sea-level, pressure is approximately 760 mmHg and the inspired % of Oxygen is 20.95%.  The partial pressure of Oxygen is, therefore, (760 x 0.2095) ~159.22 mmHg.  This pressure will rise/fall depending on the altitude of the laboratory and the prevailing weather conditions so the expired PO2 can be tracked accordingly.

The formula:

P x (Ch”x”/100)

Where: P = Pressure (externally recorded)

                x = Channel number of particular gas (O2/CO2)

The units of measure will be denoted by whatever unit the pressure is recorded in.


The pressure should be recorded at the start of each experiment or recording session and adjusted in the formula.  If users find a way to record the atmospheric pressure directly into LabChart, the same formula can be used but “P” will be replaced by the relevant channel number.


To make this more accurate you can also take into account the water vapor pressure associated with expired air.  This will differ with changes in temperature but at standard body temperature water vapor pressure is approximately 47 mmHg.  The formula will therefore need to be changed to:


(P-47) x (Ch”x”/100)



Note: The atmospheric pressure should be recorded, and the formula amended, as often as the researcher feels necessary as there are unlikely to be significant changes in this over the course of an experiment.


There may be a degree of inaccuracy behind this method depending on the accuracy of the barometer used and the response time of the sensors.

For further technical assistance with this or any other issue, please contact ADInstruments Technical Support by clicking HERE.