1Institute for Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT), Germany
2now at: Institute of Physics, University of Sao Paulo, Brasil
Received: 05 Apr 2011 – Discussion started: 17 May 2011
Abstract. The development of an unpowered permeation device for continuous calibration of in-situ instruments measuring atmospheric volatile organic compounds (VOCs) is described. Being lightweight and compact, and containing only negligible amounts of chemicals, the device is especially suited for field use such as on board aircraft. Its speciality is to maintain the permeation process in thermal equilibrium, so that the instantaneous permeation rate can be ascribed to a simple temperature measurement. This equilibrium state is maintained by a combination of three features: (i) a thin PTFE membrane as permeation medium which guarantees short stabilization times, (ii) a water bath as heat buffer, and (iii) a vacuum-panel based insulation, in which features (ii) and (iii) minimize temperature drifts to ~30 mK h−1 per Kelvin temperature difference to the environment. The respective uncertainty of the permeation rate due to thermal non-equilibrium is kept below 1%. An extensive theory part details the major permeation processes of gases through porous polymers, being Fick's diffusion, Knudsen flow, and viscous flow. Both the measured stabilization time and the measured temperature dependence of the permeation rate independently indicate that the permeation can be described by a viscous flow model, where diffusion of the gas molecules in large pores (having a diameter of >0.05 μm) dominates.
Revised: 27 Sep 2011 – Accepted: 28 Sep 2011 – Published: 13 Oct 2011
Brito, J. and Zahn, A.: An unheated permeation device for calibrating atmospheric VOC measurements, Atmos. Meas. Tech., 4, 2143-2152, doi:10.5194/amt-4-2143-2011, 2011.