We made a humidity generator that adds water vapor to a flowing gas. Its range of humidity is useful for calibrating balloon-borne probes to the Earth's stratosphere. The generator’s novel feature is a saturator that comprises 5 m of silicone tubing immersed in water. The length was enough to ensure that the saturator’s output was independent of the dimensions and permeability of the tube. This simple, low-cost design provides an accuracy that is acceptable for many applications.