An embodiment of the invention includes receiving a request to purchase a product from a vending machine. The device includes a pair of cleaning members or rollers disposed on an underside of the device which can roll with respect to the device and with respect to one another. The novel method The membrane exhibits good resistance to plasticization by hydrocarbon components in the gas mixture under practical industrial process conditions. Absence of the user is inferred upon the presence detector not detecting the presence of the user.