Diffusion Chamber

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Chamber The diffusion chamber considered here is to measure gas fluxes through a flat layer. At either side of the layer, a compartment can be closed or flushed with a gas mixture. Measurements are by oxygen electrodes in each chamber. Later, a microelectrode was added to measure inside the layer. Click the figure to see it in full detail.

The standard procedure is, flushing both chambers until a steady profile in the layer can be expected, then closing at least one chamber and recording the oxygen partial pressure PO2 in the chamber(s). From the in- or decrease, conclusions about the fluxes into or out of the chambers can be drawn, but that is not as simple as would be expected. The reason is, that there is a gas mixture, mostly, nitrogen (N2) and oxygen (O2), and that both gases diffuse at a different rate causing volume changes in the gas chambers. For an inert flat layer, the oxygen pressure change in an adjacent closed chamber is (1).

dP = ART ℘ (1 − c P)ΔP
dt V L
where P is oxygen partial pressure, A exposed surface area of the layer, R gas constant, T absolute temperature, ℘ oxygen permeability of the layer, ΔP oxygen pressure difference across the layer, and c a correction term. The correction term depends on stiffness of the layer and permeabilities of the co-diffusing gases. For an inflexible layer, c would be zero. Else, for a nitrogen-oxyen gas mixture:
c = 1 − ℘N2/℘O2
(1 + η)PTot
where η depends on layer stiffness and PTot is the total gas pressure in the chamber.

So, for inert flat layer also a measurement of permeability ratio is possible. This needs a correct analysis of both chamber recordings but results can be quite accurate (1),(2).

(1) Hoofd L de Koning J Kreuzer F Lamboo A: Determination of permeabilities for two gases from recording the partial pressure of one gas. Pflügers Arch - Eur J Physiol 407(3): 320-326 (1986).
(2) Hoofd L Lamboo A: Oxygen permeability of methemoglobin solutions soaked in Millipore filters. In: Oxygen Transport to Tissue VII, Adv Exper Med Biol 191: 565 570 (1985).

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