I'm sure any measurement tool is subject to error. However, my experience is that my $45. Refrac just saved me close to $200 in livestock and I'm not exagerating, if I consider the old live stock as well as the new, probably a lot more than that.
My deep six, showed I was at 1.026. Had a grama and blenny, along with a CUC and Sally light foot.
The light foot died and I couldn't figure out why. All my parameters were perfect. SG, Ammonia, nitrates, nitrites, ph, alk, everything.
I had been pondering a refrac and decided to order. My light foot died about a week before I had new livestock coming and needed to know what was wrong fast. I got the refrac, and tested with calibration fluid and my tank twice, once before calibration and once after. The calibration was spot on out of the box. The readings were the same before and after: 1.030 - 1.031. Bordering on lethal to livestock. My guess is the high salinity killed my lightfoot.
My hydromiter still read 1.026 even after sooking in warm water over night, getting all the bubbles off, everything.
Over the course of a week I lowered the salinity slowly to 1.026 based on the refrac. I wondered why the Grama and blenny I purchased locally were in 1.019 water. I had to acclimate them to 1.030 water. Which, I am surprised they survived now.
When my new livestock came I tested their water. They were between 1.23 and 1.025.
I could acclimate very quickly because of the closeness of their water to mine. If hadn't lowered my salinity, I'm sure I would have killed something trying to acclimated them to 1.030 water.
Not to mention how much salt I was using to maintain 1.030. I use significantly less salt now!!
I'm keeping the hydrometer so I always will know if my refrac starts getting off. If it's not .005 different from my refrac readings, then I know there's something wrong.
And my hydro reads .005 LESS than my refrac. I don't know what max sg inverts and fish can tolerate, but, I've read anything above 1.030 can start to become toxic. .005 is pretty signficant to be off and risk hundreds of dollars of livestock.
Maybe the lesson is not that refracs are required. They are reccomended, but it may also be a reccomendation to have 2 or more sg measures to validate against.