Q: I do gas detection for our company, and travel to several facilities. Two of them are located at fairly high elevation. How does atmospheric pressure affect these measurements? How do I correct for this?

A:When a gas concentration is measured or reported, we may talk in terms of ppm, but in reality, most instruments measure the partial pressure of the gas. Of course, based on the ideal gas law, these parameters are related. Fortunately, most of the common toxic gases can be treated as “ideal” at their typical low concentrations.

Dalton’s Law of Partial Pressure states that the pressure of a gas mixture is the sum of the partial pressures of the individual components of the gas mixture.

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 correction factor shown in the chart below.

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

Atmospheric pressure where calibrated ÷ Atmospheric pressure where measurement is performed

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 hectopascals, it does not matter, as 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. FSM

Michael D. Shaw is Mr. Gas Detection. He is a regular contributor to Facility Safety Management and executive VP and director of marketing for Interscan Corp. For more on his company, go to www.gasdetection.com. To ask him a gas detection or industrial hygiene related question, email him at mike@fsmmag.com.