Join Ruby Ochoa, of Trace Analytics LLC, as she discusses water vapor or dewpoint in compressed air or gas.
What is dewpoint?
Dewpoint is the temperature at which your compressed air becomes saturated. The air can’t hold any additional water vapor, so the vapor begins to condense.
As the pressure of a system increases, the dew point temperature will rise and approach saturation. Dewpoint is a common measure in compressed air systems because it is possible to reach the saturation point and introduce liquid water into the compressed air by simply increasing the pressure. As the pressure of a system is decreased, the dew point temperature becomes lower and the gas will become relatively dryer as the difference between dew point and ambient temperature increases
A pressure dew point of -37°C at 90 psi would be a -34°C at 125 psi. So, the higher the pressure the wetter the results.
For some applications, it is vital that the product is not contaminated by moisture.
What are some sources of contamination?
- The length and condition of the distribution line, piping, drops, dead ends, condensate traps, and polymer tubing can collect liquid water. This creates a breeding ground for additional contamination.
- Polymer/rubber hose
- Valves
- Filter malfunction
- Lack of maintenance
- New installation
- System alteration
- Insufficient purge after installation
- Sampling port installation
- System or fitting air leaks
- Dead end or drop
- Poorly placed sampling port
- Compressor air intake
- Improperly sized filtration
- Points-of-use filter not designed for industry
Recommendations from ISO 8573-3 C3
If possible, prepare for measurements by flushing sample lines and hygrometers with dry gas, or by evacuating to low pressure. Drive off stray residual water by baking assemblies if possible (but not instruments - unless designed for this!). The lower the moisture content to be measured, the more dramatically the drying time multiplies.
Avoid hygroscopic materials. At low humidities (anything below a dew point of 0 °C) the amounts of water given off by organic and porous materials can dramatically affect the value of air humidity. The lower the level of air moisture, the more significant the effects.
Choose impermeable materials, to avoid inward diffusion of moisture through sampling tubes and enclosures. Steel and other metals are practically impermeable. Polytetrafluoroethylene (PTFE) is only slightly permeable and will usually be satisfactory for dew points above 220 °C, and sometimes below this level. Materials such as polyvinyl chloride (PVC), nylon, and rubber are relatively permeable and so totally unsuitable at low humidities, and not really satisfactory in any humidity range.
Surface finish is important at very low humidities. Even the tiny quantities of water absorbed on the surfaces of nonhygroscopic materials can have a significant effect. Polished or electropolished stainless steel is recommended for the best results.
Clean environments are always best for humidity measurements, but this is especially critical at very low humidities. Even fingerprints harbor water. High purity cleaning agents are recommended; Analytical Reagent quality solvents for oil-based contaminants, and purified water (distilled or deionized) for salts. Cleaning should be followed by thorough drying by a clean method.
Sample tubing should be as short in length as possible. The surface area should be minimized by using the narrowest tubing that the flow conditions will permit. Avoid leaks. Minimizing the number of connections (elbows, tees, valves, etc.) helps with this.
Adequate flow of the gas sample should be ensured, to minimize the influence of sources of stray water in the flow path.
"Dead ends" in tubing should be avoided, as they cannot easily be flushed.
Back-diffusion of moisture should be minimized, e.g. by fast flowrates
How to take a sample?
Fittings, watch the test, read the results, replace tube fittings
For more information on testing compressed air or gas for water vapor contamination, please contact our AirCheck experts at 512-263-0000 ext 5.