Our experience of plate counts is that 5 micron ODS column gives up to 100,000 plates per metre when tested using isocratic conditions.
Recently an associate showed me an example of a plate count of 100,000 for a 15cm column - that is 600,000 plates per metre ( used millenium software). Hand calculation gave same result.
This was a gradient separation - 10% ACN to 90%ACN in 20 mins - is this why the plate count is so unbelievably high ?.

Yes. See previous postings on this topic.

Thanks Chris