PI - tag query

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Hello all -

We have an interesting problem. We are historizing a tag EVERY minute from PI from start of the shift (at 7:00 AM) till. current time in the shift

To explain -

at 7:01 we run a history query from 7:00 till 7:01 , store the value

at 7:02 we run a history query from 7:01 till 7:02, store the value

at 7:03 we run a history query from 7:02 till 7:03, store the value

and so forth....

To cross-verify, these values, I ran a SINGLE history query from 7:00 to 7:03 at a later time (say at 8:00).

I found that sometimes the values from both the queries do not correspond for the same date/time stamp?? This was especially true if a downtime occured at say in the middle say 7:02.

Have anyone else observed similar results when querying a tag from PI.

Any idea why this is happening? Thanks for any help. I could provide more details if needed.