The diffusion of gases through a silicone rubber membrane, and its application to an in-line carbonation meter.

Kesson, J.

Abstract
Compared with natural rubber, polystyrene, polyethylene and cellulose, silicone rubber is much more permeable to gases and carbon dioxide permeates this material almost 5 times faster than oxygen and 20 times faster than nitrogen. Based on this a device has been developed for continuous in-line measurement of carbon dioxide in beer. The meter consists of a small chamber, containing a pressure transducer, which may be mounted on a beer line, the chamber being separated from the beer by the gas permeable silicone rubber membrane. Carbon dioxide in the beer diffuses across the membrane until its pressure in the chamber is equal to the partial pressure of carbon dioxide in the beer. The carbon dioxide content of the beer can be computed from the pressure provided the beer temperature is known. Despite the lower permeability of the membrane to oxygen dissolved oxygen in beer, because of its lower solubility, exerts a high pressure relative to dissolved carbon dioxide and can lead to significant errors in carbon dioxide measurement. This may be obviated by periodic evacuation of the chamber. Indeed, the difference in equilibrium pressure before and after such a refresh cycle may be translated in to a concentration of air so that the meter may serve as an oxygen alarm also. Although equilibrium pressures are reached more quickly with thinner membranes a membrane of 0.008 in thickness is required in order to provide a degree of robustness sufficient to withstand fluctuating beer line pressures in excess of 100 psi. Using a membrane of that thickness and 2 in in diameter equilibrium pressure is reached in approximately 5 min. In practice this is too slow but it is possible to achieve an apparent response time equivalent to a membrane of 0.002 in thickness by using a microcomputer to dynamically predict the equilibrium pressure.
Keywords: analysis beer carbonation carbon dioxide gas permeability rubber silicone