I kept high end SPS coral reef tanks for many years and had lots of problems with Hanna meters. Their handheld Ph meter was horribly off even when calibrated numerous times (I had two other rock solid Milwaukee meters and the Hanna was always high by 0.5+). I got a replacement from Hanna and it was off too. I even took it with me to other reefer's tanks to compare and make sure it wasn't the Milwaukees....it wasn't. I finally just put the Hanna away to collect dust rather than sell it online. I won't sell something that I know doesn't work.
Then Hanna came out with their highly anticipated "Hanna Checkers" for alkalinity, calcium, and especially, the phosphate checker. Phosphates are deadly to stony corals. All of them were completely whacky and I could never rely on them. I even joined a group on a major reefing forum that was working with the rep from Hanna. A chemist sent me lab samples to test my meters with and I posted the results to Hanna. The rep even ended up admitting that they were unreliable. They have checkers for many elements on my last check. After all of this, I decided to put Hanna in my rear view once and for all.
I also tried the Nutri-Dip Tri-Meter on our commercial grow and I couldn't keep it stable. I sent it back to the company (their rep was very helpful) and he personally re-programmed the unit and calibrated it in their lab. It was very nice of them and they covered shipping. However, I never felt like I could rely on it completely and I always double checked with my Milwaukees and Oakton. The latter two were more reliable and I really only looked at the Tri-Meter for water temp.
PH: I am now, after 10 years of hydro growing, confident in my Milwaukee and Oakton Ph2 meters. They are easily calibrated and if they differ (which is only by 0.1 - 0.2 on occasion), I just split the difference and call it good.
PPM: Milwaukee, rock solid and still going strong after 8 yrs (knock on wood). I also have two of the meters that come included with the Zero Water filter. They are always on point with the Milwaukee. They very rarely require calibration, and if they do, they are only off slightly, if at all.
I always rinse my meters with RO water after testing and calibrating. I also store my Ph meters in 7.0 calibration fluid.
There are more expensive units than the ones I use, and I'm not one to cut corners or to save a few bucks when I know something is better quality and more reliable, but the meters I use have proven themselves to me and are reliable. A classic case of if it isn't broken, don't fix it. My $0.02
