Hi Manilycan,
Unless you're using a REALLY old version of DIAdem, I'd suggest instead the ANALYSIS function
Call ChnSub(ChannelOne, ChannelTwo, ChannelDiff)
If you really need to use the Channel Calculator for some reason, you should try explicitly setting all the channel lengths the same before running this calcualtion:
ChnLength(L16) = 6000
ChnLength(L17) = 6000
ChnLength(L15) = 6000
Call FormulaCalc("ch(L16):= ch(L17) - ch(L15)")
Brad Turpin
DIAdem Product Support Engineer
National Instrumentss