On Fri, 30 Aug 2013 16:57:21 +0100 (BST), "Dave Liquorice"
wrote:

On Fri, 30 Aug 2013 13:49:35 +0100, Bill Wright wrote:

Put a thin white line on one blade. Photograph the fan whilst it rotates
at various shutter speeds. Calculate the RPM from the angular movement
of the line at each shutter speed. Average the result.

From the motion blur, I like that. Don't use flash though. B-)


It works to a point but the beginning and the end of the blur are hard
to determine. I get around 3,400 rpm at low speed which could well be
3,000 rpm. I like the idea of having a laser tacho (not sure why, I
just like the idea) as suggested by F Murtz so I've ordered one from
amazon.uk at less than a tenner.



--
AnthonyL