Here is the manual for the meter that I use, Hanna Instruments Pure Water Tester, which reads microsiemens/cm.
From what I've read, conductivity in microsiemens is a pretty standard purity benchmark for water, but I agree that it can be anywhere from questionably useful to completely useless on other fluids.
I calibrate every month or two with a certified calibration solution that you can purchase from Hanna, which is why I feel comfortable making the claim that my coolant is of guaranteed purity.
Generally, "good" distilled should measure less than 0.5µS/cm, typically it is around 0.2-0.3µS.
The funny thing is, Grade II ASTM only states that the water is to be purer than 1µS/cm (along with a couple other specs), so in reality, Grade II ASTM water may actually be
less pure than distilled from the supermarket. :p:
The difference is that when I buy 5 gallons of ASTM Type II water, I have a guarantee of what I'm getting. When I buy supermarket distilled, it can go anywhere from superb to absurd. Usually though, from my experience, they're of equal quality.
I know this sounds funny, but IMO a superior option to buying my coolant (for people that change their loops regularly and use distilled), is to just fork out the $70 + $15 for meter and calibration solution, then they can be absolutely sure of what they're putting in their loop.
An average of 0.002-0.003µm, up to 0.005µm. I haven't had my solution lab tested, but that is based upon a lab test done by someone using identical production methods, and lack of agglomeration vs. measured conductivity. From what I've gathered, as long as all production protocols are followed, the solution is extremely uniform from batch to batch.