Preprints
https://doi.org/10.5194/amt-2021-217
https://doi.org/10.5194/amt-2021-217

  10 Sep 2021

10 Sep 2021

Review status: a revised version of this preprint is currently under review for the journal AMT.

Silicone tube humidity generator

Robert Berg1, Nicola Chiodo2, and Eric Georgin3 Robert Berg et al.
  • 1NIST, 100 Bureau Drive, Gaithersburg, Maryland 20899 USA
  • 2LCM LNE-CNAM, Laboratoire Commun de Métrologie LNE- CNAM, La Plaine-Saint Denis F93210, France
  • 3CETIAT, Centre Technique des Industries Aerauliques et Thermiques, Villeurbanne, France

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

Robert Berg et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Referee comment on amt-2021-217', Anonymous Referee #1, 28 Sep 2021
    • AC1: 'Reply on RC1', Robert Berg, 09 Nov 2021
  • RC2: 'Comment on amt-2021-217', Anonymous Referee #2, 12 Oct 2021
    • AC2: 'Reply on RC2', Robert Berg, 09 Nov 2021

Robert Berg et al.

Robert Berg et al.

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Short summary
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 five meters 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.