chosen such that aðyÞ is lowest (equal to 1) when the temperature is equal to a reference temperature of 20 1C, it equals 20 when the temperature is 0 1C and it is highest (equal to 80) when the temperature equals the critical temperature of 100 1C. The expo- nential functions are chosen in order to enable this very high dependence with the temperature. Roughly speaking, the units fail 80 times more often when the temperate is 100 1C than when it is 20 1C.