Last September,I started to collect some lettuce seed and separate the
seed from the chaff by using sieves.It did not work very well after
several separations,but then remembered that I had read that one
method is to put the seed into a glass of water-the good seed falls to
the bottom,whereas the rubbish floats on the top.It seemed to
work,although the amount of good seed was probably only 20% of the
total.However I removed the rubbish from the liquid surface,and
recovered the 'good' seeds from the bottom.I put them on a plate and
dried them as quickly as possibly-probably took till the following
day.I worried of course whether the dampness over a period of a twelve
hours would affect their subsequent germination.

I have now sown two varieties of lettuce seed last month and can
report that I have achieved 100% (or as near as I can judge)
germination.So sorting seed using flotation works!
Cheers,Michael