I think the specified aging of +/- 5 x 10^-6 per year is quite fine and to cite a specific case, it may not affect my rpm read out ( 005 to 200 ) in any way. In this instance with a 60 segment pickup, my maximum recordable frequency is 200 Hz and the drift is +/- 0.000005 Hz per year ! I will tell the customer not to touch the instrument for any calibration ( and incidentally there is nothing that HE can adjust even assuming he wants to ).

Thanks.

raghu