  We study the effect of humidity on the charge accumulation of polymer
granulates shaken vertically in a stainless steel container. The setup allows
to control the humidity level from 5 % to 100 %RH while performing automated
charge measurements in a Faraday cup directly connected to the shaking
container. We find that samples of approximately 2000 polymer spheres become
highly charged at low humidity levels (<30 %RH), but acquire almost no charge
for humidity levels above 80 %RH. The transition between these two regimes does
depend on the material, as does the sign of the charge. For the latter we find
a correlation with the contact angle of the polymer with only very hydrophilic
particles attaining positive charges. We show that this humidity dependence of
tribo-charging can be used to control segregation in shaken binary mixtures.
