Pressure difference

Sometimes, it is necessary to calibrate gas detection instruments at a barometric pressure (atmospheric pressure) different from that of the measurement location. In such cases, a correction factor must be applied.

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:

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 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…

Elevation (ft)
Elevation (m)
Atmospheres
0
0
1.00
328
100
0.99
500
152
0.98
656
200
0.97
1000
305
0.96
1312
400
0.95
1500
457
0.94
2000
610
0.93
2500
762
0.91
3000
914
0.89
3500
1067
0.88
4000
1219
0.86
4500
1372
0.85
5000
1524
0.83
5500
1676
0.81
6000
1829
0.80
6500
1981
0.78
7000
2134
0.77
7500
2286
0.76
8000
2438
0.74
8500
2591
0.73
5.00 avg. rating (93% score) - 1 vote