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SRM Power/Watt Meter 7950 50 X 34 Compact Double
The SRM Power Meter 7950 compact double uses the robust DuraAce 7950 chainrings, so you don’t lose any of the power you’re trying to measure with flexible ultralight chainrings.
Power measurement is transmitted to the Power Control 7 wirelessly from the strain gauges in the SRM Power Meter crankset. Other head units can be used also, such as the Garmin Edge 500 or 800 models.
See the discussion of how SRM measures power.
I weighed the SRM 7950 crankset before it was installed on the bike.
Right crank and chainrings: 587g
Left crank: 193g
TOTAL: 788g (May 2011 unit)
+4g for cadence magnet
TOTAL: 778g (2nd unit Nov 2011)
A plain (non-SRM) 7950 crankset weighs 661 grams, so the SRM setup extracts a weight penalty of 117-127 grams.
If you’re getting SRM, I recommend the Power Control 7 with it, though various Garmin units work with it, and probably other options too.