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

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.

An example of this can be seen here.

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.