Altitude makes a difference when you are measuring CO2 levels. CO2 sensor modules are calibrated at sea level, and are not designed to automatically compensate for higher altitudes.
When you think about altitude, it helps to imagine a vertical column of air rising from the earth’s surface to the outer limits of the atmosphere. The gas molecules in air have weight. The weight is heaviest at the earth’s surface, but becomes lighter as you measure it higher up the vertical column. At the top of the column, the weight becomes zero.
When talking about the “weight of air” scientists use the term atmospheric pressure. AP is typically measured in hectopascals (hPa), inches of mercury (inHg), or millimeters of mercury (mmHg). At sea level these are as follows:
- 1,013.25 hPa
- 29.921 inHg
- 760 mmHg
As you go higher in the atmosphere, each of these numbers get smaller. So to get the correct pressure temperature compensation when measuring CO2 levels, you simply divide the atmospheric pressure at sea level by the atmospheric pressure where you take the CO2 measurement. It doesn’t matter which scale you use; the answer will always be the same ratio. Multiply this fraction by the CO2 measurement at sea level, and you have the new CO2 level at any altitude.
Here’s an example.
The city of Denver, Colorado is approximately 5,000 feet above sea level. At this altitude, the atmospheric pressure is 632 mmHg (barometric pressure table). The pressure temperature compensation for a CO2 sensor calibrated at 400 ppm at sea level at normal temperature would read 400(623/760) or 332 ppm.
A rule of thumb is that the CO2 level at altitude will change about 3% for every 1,000 feet (300 m). At 5,000 feet, that would be 400 ppm less 15%, or about 340 ppm.
This type of correction can have real-world consequences. For example, suppose a grower in Denver wanted to maintain exactly 1,000 ppm CO2 in their greenhouse. In order to achieve this, they would have to set the sensor to 15% above 1,000 ppm, or about 1,150 ppm.
Note that while CO2 sensor modules do not account for changes in altitude from sea level, some CO2 meters have built-in barometers that can compensate in real time. So if you are at high altitudes and need extremely accurate CO2 level readings, you should look for this feature before making a purchase.
Like CO2 sensors, oxygen sensors also require pressure temperature compensation. The 21.9% oxygen level commonly stated assumes STP, or standard temperature and pressure. Go to Colorado and the oxygen level is 17.3%. Here's a handy chart to show you the differences. Before purchasing an oxygen sensor check whether it is automatically compensated for pressure with a barometric pressure sensor on-board, or if you will need to adjust the oxygen level either by a lookup table or with a separate pressure sensor.