>I built one of these and find that it works well.  It was originally from beehacker.
http://www.beehacker.com/wp/?page_id=55

I built something similar to this and found that the friction in the pulley added a significant variable element, leading to weight dependent errors (heavier hive, greater friction in the pulley, etc).  I suggest either use a high quality pulley (with ball bearings) or reengineer the design to place the scale between the pulley and the hive lift point; makes it a bit clunkier, admittedly.

It also helps to have a well defined pivot point on the hive front; I implemented this by using a piece of 1/2" angle aluminum (apex upward) to support the front of the hive in a groove that I cut in the bottom of the bottom board.  If you think about where the center of mass of the hive is, the lift point (for the scale) should be equidistant from the CM to the front pivot point in order for the 2x weight calculation to work accurately.  If trends are all you care about, this detail is not relevant other than to have a stable and repeatable front pivot point.

Mike Garvey
Swampscott, MA

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