I use IsoPrime to run dD of water by Cr reduction method. The raw values of the working standards sometimes fluctuate day by day. The values of  -360 and -79 for the first day can be -354 and -65 the next day. There is no big change of the machine condition except a slight shift of peak center. However, the calculated dD values of the repeated samples are perfectly match from day to day. It seems not a real big problem. I will feel better if more people telling me they have the similar experiences.
Dachun Zhang