Measuring it using a device like a Polar is not very accurate, it is just an estimate. I had mine measured last year using New Leaf:

http://www.newleaffitness.com/Consumer/Con_home.html

I just went to a personal trainer in the DFW area who uses the New Leaf equipment. I actually had it tested twice last year...once on a treadmill and once using my bike on a trainer. I highly recommend it if there is someone in your area who uses it.