When you pump reactive, highly polar, or oxidizing gases like chlorine (Cl2), chlorine dioxide (ClO2), or ammonia (NH3) through standard silicone, polyurethane, or Tygon tubing, the tubing acts like a chemical absorber/scrubber. The gas is actively consumed or trapped by the hose before it ever reaches the instrument’s sensors. This creates severe process safety and life-safety hazards due to three (3) distinct failure mechanisms:
1) Polymer Diffusion and Surface Adsorption
Standard elastomers and organic plastics have a molecular structure that allows reactive gases to bind to the surface (adsorption) and physically diffuse into the polymer matrix itself (absorption). MSA’s own operating manuals explicitly warn against using standard rubber or plastic tubing for reactive gases because the vapor actively diffuses into the material, depleting the sample volume to the point where sensor detection and calibration become impossible.
2) Moisture Dissolution
Ammonia and chlorine are exceptionally water-soluble. Polyurethane and silicone tubing readily harbor microscopic surface moisture or allow condensation to form on the inner walls. As the sample is drawn through the tubing, NH3 and Cl2 instantly dissolve into this trapped humidity, effectively stripping the target gas out of the air stream before it hits the detector.
3) Chemical Degradation
Strong oxidizers like Cl2 and ClO2 chemically attack the polymer chains in silicone and polyurethane over time. This reaction not only consumes the gas sample but also degrades, embrittles, and cracks the tubing, causing micro-leaks that draw in ambient air and further dilute the sample.
The Operational Impact
- False-Zero Readings: You could drop a sampling line into a confined space or vessel containing IDLH (Immediately Dangerous to Life or Health) concentrations of ammonia or chlorine, and the instrument may sit at 0.0 ppm because the tubing is absorbing 100% of the gas.
- Severe Alarm Lag: Even if the gas concentration is high enough to eventually overwhelm and “saturate” the tubing walls, the delay time (T90) can stretch from seconds into tens of minutes before a reading registers on the monitor.
- Desorption “Ghosting”: Once saturated, standard tubing will slowly off-gas (desorb) the trapped vapors over hours or days. If you move the monitor to clean air, it may continue to alarm or show lingering background concentrations, ruining subsequent spot checks.
- Failed Bump Tests & Calibrations: You cannot calibrate or bump test a reactive gas sensor through standard tubing. The tubing will strip the calibration gas from the cylinder, leading you to falsely assume a sensor is dead—or causing an instrument to calibrate to an artificially low concentration, making it dangerously hypersensitive in the field.
The Required Solution
When sampling for Cl2, ClO2, NH3, hydrogen chloride (HCl), or hydrogen sulfide (H2S):
- Use Fluoropolymer Tubing Only: You must use inert, non-reactive fluorinated polymer tubing such as PTFE (Teflon), FEP, or PFA. These materials do not absorb reactive molecules.
- Minimize Length: Keep the tubing run as short as physically possible. Even with Teflon, surface moisture and extended surface area can cause minor sample loss over long distances.
- Check Your In-Line Filters: Remove standard cellulose or particulate filters if they are not specifically rated for reactive gases. Cellulose will react with and absorb chlorines and acid gases just as badly as the wrong tubing. Use only the manufacturer’s dedicated reactive-gas sampling kits and probes.
Sources: Me, MSA, and Gemini+