An instrument for ascertaining a viable aerobic microbe count at to employing a two-point calibration curve of tthreshold to TVC for each type of sample. One point on the calibration curve is the x-intercept value (i.e., an estimated or experimental value for the logarithm of the minimum viable aerobic microbe count at commencement of testing (to) in a sample effective for causing the sample to reach tThreshold substantially instantaneously upon commencement of incubation). The other point is ascertained experimentally from a sample having a smaller known viable aerobic microbe count at to.