point is to take the average of the two values e g t start u2212 t stop 2 in an alternative embodiment the same u201c triggering u201d of the time measurement can be used by u201c triggering u201d we mean for the purposes of this invention the determination of a starting point for the time measurement or put in other words the determination of t 0 for the purpose of integration in this alternative embodiment only the increasing part of the sensor signal indicated with b u2032 will be used for this purpose the mentioned part b u2032 of the sensor signal curve will entirely or partly be fitted to a mathematical function e g e u2212 t u03c4 which is an exponential function the simplest way of doing this is to take the logarithm of the measurement data at this part and to plot this against time from the slope of the linear portion of that plot the time constant u03c4 of the exponential function can be determined the point on said curve portion b u2032 of fig2 b corresponding to the point on the time axis at t min sensor u03c4 will be center of mass of the exponential curve which is the point up to which t mn will be calculated from t 0 strictly mathematically 0 7 u03c4 should be used for the identification of the center of mass but for the purpose of this application the approximation to u03c4 is adequate u03c4 can be calculated by fitting the sensor element signal from the point p s in fig2 c.