I recently ordered a refractometer from Tim Crowley on ebay (moneyreapertc is his ebay user name) for $45 including shipping and ins. It arrived today. Seems to be a good one so if anyone is interested in buying one of these, you might contact this guy.
Anyway, after calibrating it, I compared the readings of my two hydrometers with the refractometer.
Coral tanks #1 and #2
Deep 6: 1.0255
Deep 6: 1.025
bucket of SW
that has been sitting in the corner for about a week.
Deep 6: 1.028
I was surprised at how close the readings were. I have seen the Seatest and the Deep 6 vary as much as .002 SG
. I also have another Seatest laying about somewhere that always reads about .001 lower than the other Seatest. I guess this is, although not very scientific, evidence that the swing type hydrometers are close enough for our purposes. I think the refrac was worth the money though...I like the precision that it offers. Perhaps the best thing to do is get your hydrometer checked against either a refractometer, salinity meter, or water with a known salt content. Then you have a correction factor if needed.