Great product I have been using it for years.

Great product I have been using it for years.  Read Less

Does its job but there is no color calibration between 8 and 11.

Does its job but there is no color calibration between 8 and 11.  Read Less

I had an opportunity to test these strips against a pro-grade refractometer. I had an HVAC tech over the other day to help me r...  Read More

I had an opportunity to test these strips against a pro-grade refractometer. I had an HVAC tech over the other day to help me raise the polyethelene glycol concentration in my radiant heat system (it had become slightly diluted after 10+ years).Over the course of an hour we compared readings about 7 times as we brought up the concentration. Since he had the more accurate device, I'd call out my reading first and then he'd share his. I was within 2-3% of his concentration readings each time ... and I had a ph reading as well.Now to be fair, I got better over the series of trials because I could calibrate my readings off of his. But even so, you should be confident that you can be within 5% in a concentration estimate without the benefit of a refractometer (as long as you are not color blind).But I did notice that the bottle has an expiration date ... as it should, since the chems on each strip won't last forever. I think the exp date gives you about three years, and with 50 strips, you'd need to test a lot of glycol to go through a bottle in that time. So buy a bottle and share it with a friend, and keep your eyes on that expiration date !  Read Less