When a gas concentration is measured or reported, we may talk in terms of ppm, but in reality we are measuring the partial pressure of the gas. (Refer to Dalton's Law of Partial Pressures)

Thus, any instrument reading is going to be affected by changes in ambient pressure. It is necessary to correct the readings, if the instrument will be used in an environment that is at a different atmospheric pressure from where the instrument was calibrated.

To correct for the influence of pressure, the value read on the instrument must be multiplied by the following correction factor:

It is the ratio that is important, not the units. Whether you are measuring atmospheric pressure in millibars, millimeters of mercury, inches of mercury, or kilopascals, it does not matter, so long as you are consistent.

For maximum accuracy, be aware that most barometers must also be corrected for temperature and gravity effects.

Finally, if aboard an aircraft, it is the cabin pressure that is of importance, rather than the exterior pressure.

The following table will give you an idea of the magnitude of pressure effects...