I found some old references to this issue, but I thought I would throw it out here for newbies (I just came across it myself). When measuring coolant concentration using a refractometer, make sure to multiply the reading from the refractometer by the refractive index of the coolant. I have been happy with Tormach's Durakut, but it has a refractive index of 2.8, so when I mixed it to get the refractometer reading to 5%, I was actually at 2.8 * 5% = 14%. No harm I guess, but I used 3 times as much concentrate than I needed. I don't know why Tormach doesn't make more of a mention (or any mention) of this in their documentation. Not even in the documentation for the refractometer they sell, which is identical to the one I got on Amazon for $15. When I mixed it by hand, I was pretty close to 2% (according to the Refractometer), which is actually the right value. 2.8 * 2% = 5.6%.