http://img218.imageshack.us/img218/8186/master2nb8.jpg

OK, I have taken some new shots of the new hydrometer (on the right)
and scanned and laid the old hydrometer (on the left) as best as I can
fit it to the same markings. There are a couple of inconsistencies
with what I said before I th ink. Firstly the old hydrometer is from
1973 (the second being 1983) and second the calibration temperature on
the old hydrometer is 21C (70F) whereas on the new hydrometer it is
20C (68F).

You can see that by the time you are over 1.090 the scales differ
greatly. Also the PA interpretation is rather different. Fair
enough, as Luc reiterated there are many ways of interpreting PA.
However, interesting that one company would change its interpretation,
scale and calibration. It would make sense that it was due to
computational differences as the UK moved from Imperial to Metric.
This has interested me for a long time!

Hope that provides some food for thought either way Frederick.

Regards, Jim

p.s. I decided to scale the old hydrometer size to match the new
one. At first I scaled so that 0.090 matched and 1.20 matched. Then
decided that it was more likely that the inconsistencies between the
two hydrometers would arise at greater extremes of density/PA so I
scaled to match as many SG graduations as I could. If I have
introduced greater error in the process this should be noted.