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Hey John,

Thanks for the input.  I'm well aware that the cache needs to be refreshed to achieve a proper histogram reading.

When I have the full image I use the lasso tool to select the boundary of water and sediment, then I use an action sequence that I've created which copies the selection to a new layer and renames it "Pen".  There shouldn't be any issue with transparent pixels. 

You were right on the money regarding 16-bit/channel halving the histogram pixel count; I switched back to 8-bit/channel mode and it gave me an accurate reading.  It's just strange to me that the histogram is the only readout that is effected.  I've been using the ruler tool heavily as well, which doesn't change at all in different modes.  Also, I work with a pure black layer created by copying a selection created through the color range tool, this selection is consistent between 8 and 16 bit modes.  Incidentally, the "Pen" layer gives different readings even when filled with pure black.  I'm just stumped as why this is the case.

Thanks for the help everyone.  I've been able to get around this problem fairly easily and was mostly just curious as to what exactly was going on here.  

If anyone want's to see the color range selected layer I've added it to the Google Drive docs.

-Steve