A method of controlling the flow of a heat activated expandable sealant is provided in which a heat activated sealant is coated with a flow control agent having a melt flow rate which is lower than that of the sealant. The problem presents itself of making available a sensor and a method for determining soot concentrations in flowing gases, with which even small amounts of soot can be reliably recorded. This efficiently prevents the resultant wafers from sticking together. A film containing luminescent silicon nanoparticles is optically coupled to the solar cell. If the comparison result is inconsistent, a restoring call information is sent out.