Permeation tubes are used to dynamically blend trace concentrations of formaldehyde in air. Mixtures can range from low ppb to over 500 ppm. Obtaining accurate gas standards has been particularly difficult because formaldehyde vapour condenses to a solid polymer at temperatures below 80°C, resulting in unstable concentration in stored mixtures.
In these permeation tubes, alpha-polyoxymethylene, a high-purity formaldehyde polymer, decomposes to maintain a stable vapour pressure of formaldehyde and consequently a stable emission rate of formaldehyde. Formaldehyde emitted from the tube is mixed with a known flow of zero air to form the trace concentration gas standard.
Emission flow measurements are traceable to NIST through fundamental measurements (weight change, temperature, etc.). Mixing the emission flow with a traceable flow dilution gas creates a traceable mixture.