Understanding dissolved gas units: PPM vs percentages
Accurate analysis requires a fundamental understanding of how gas concentrations are expressed in both the dissolved phase (liquid) and the gas phase (headspace).
The measurement divide
Dissolved phase units (liquid)
Gases dissolved in liquids (like O₂ in beer or water) are typically measured in parts per million (ppm) or parts per billion (ppb) by weight or volume. These units are critical for assessing quality metrics like staling potential.
Example: 5 ppb of dissolved O₂
Gas phase units (headspace)
Gases in the air or headspace of a package are measured as a percentage (%) of volume. This is used in Modified Atmosphere Packaging (MAP) for food safety and shelf-life extension.
Example: 21% volume of O₂
Henry's Law and conversion
The relationship between the two phases is governed by Henry's Law, which dictates that the amount of dissolved gas is proportional to its partial pressure in the gas phase above the liquid. The conversion factors are highly dependent on temperature and the specific liquid matrix (e.g. water vs. ethanol solution).
$$ C = k_H P_{\text{gas}} $$
- $C$: Concentration of dissolved gas.
- $P_{\text{gas}}$: Partial pressure of the gas.
- $k_H$: Henry's Law constant, which is unique for every gas-liquid combination and temperature.
Accurate instrumentation must account for these complex thermodynamic principles to provide traceable and comparable results across both laboratory and packaging environments.