Silicone tube humidity generator

Abstract. We describe the model and construction of a two-flow (or divided-flow) humidity generator, developed at LNE-Cnam, that uses mass flow controllers to mix stream dry gas with humid saturated 28 ∘C. It can generate wide range humidity, mole fractions in 0.7×10-6<x<9000×10-6, without using low temperature or high pressure. This is suitable for calibrating balloon-borne instruments measure stratosphere, where x∼5×10-6. The generator's novel feature saturator comprises 5 m silicone tubing immersed water. Water enters by diffusing through wall until flowing saturated. design provides simple, low-cost generator an accuracy acceptable many applications. key requirement be long enough ensure saturation so saturator's output independent dimensions permeability tube. A length only few meters was sufficient because tube made silicone; other common polymers have permeabilities are 1000 times smaller. verified transition from unsaturated measuring while three lengths, two which were too short saturation. As more complete test, we used as primary device after correcting calibrations determined mixing ratio. At 50×10-6<x<5000×10-6, fraction xgen agreed within 1 % value xcm measured calibrated chilled-mirror hygrometer; words, their ratio fell xgen/xcm=1.00±0.01. smaller fractions, differences xgen-xcm=±1×10-6.