  In this work the linearity in the response of an ionization-chamber as a
function of $^{99m}$Tc activity was checked as part of a calibration procedure
for $^{192}$Ir activity measurements. The non-linearity was found to be caused
mainly by the electrometer range switching. A model based on the hypothesis
that the non-linear effect did not influence the decay constant was tested.
This method allowed to correct the non-linear effect and obtain the 99mTc
activity with good acuracy.
