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Hamilton dissolved oxygen and dissolved carbon dioxide sensors use measurement technology based on partial pressure. This article looks at atmospheric pressure as a potential source of error that could occur during calibration.

Why is partial pressure important?

All gas molecules are constantly in motion. They exert kinetic energy as they collide with each other or even the walls of this container shown below. Each type of gas molecule exerts its own kinetic energy. Some gas molecules are more active than others. The force exerted by each type of gas is referred to as the partial pressure of the single gas. For example, if the gas mixture is air, you can add up the partial pressure for each gas to get the atmospheric pressure of air. This equation is referred to as Daltons Law.

Let’s consider how atmospheric pressure relates to the partial pressure of oxygen (pO2). Differences in elevation directly impact the atmospheric pressure. At sea level, atmospheric pressure hovers around 1013 millibar. In Bonaduz, Switzerland, where Hamilton Process Analytics is based, the elevation is 658 meters which equates to 939 millibar. In both locations oxygen is the same 20.95% of air but the measurement of oxygen will be less in Bonaduz due to the difference in partial pressure.

How does this relate to calibration?

Since Hamilton cannot predict atmospheric pressure where the sensor will be used, Arc sensors come pre-programmed for sea level: 1013 millibar. The atmospheric pressure should be entered into the sensor using ArcAir prior to calibration to avoid any potential error. For sensors with newer firmware such as the VisiFerm mA or CO2NTROL, atmospheric pressure is part of the calibration wizard as shown below. With other sensors, this value can be found in ArcAir under SETTINGS > MEASUREMENT SETTINGS.

Determining Atmospheric Pressure?

Please avoid using weather websites. These websites normalize the atmospheric pressure value back to sea level. This removes error due to elevation, which is helpful for understanding weather; but the pressure value will be incorrect for sensor calibration.

Hamilton recommends using a certified atmospheric pressure meter (Example vendor – Testo) to get the correct value for calibration. If a meter is not available then an estimated atmospheric pressure can be determined based on the elevation where the sensor will be installed. Look for a website or chart the uses the barometric formula to model elevation with atmospheric pressure. Wikipedia is a good example.

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