I ran a little experiment last night and thought I would pass along the results.

I typically use both a SeaTest (Aquarium Systems) and Deep Six (coralife) hydrometer. I have noticed a difference of .001-.002 between these two hydrometers. This concerned me somewhat since I run my salinity at near the suggested range, 1.024-1.025 and evaporation, my MH’s can cause a daily increase of at least .001.

Although S.G. varies with temperature, both Deep Six and Sea Test claim that their hydrometers compensate for temperature. Here is a pretty good article I found that discusses salinity measurement with swing arm hydrometer. The article emphasizes the need for proper cleaning of hydrometers. Deposits on the swing arm do affect accuracy. I clean mine with vinegar every couple of weeks.

Here is the article:

http://www.advancedaquarist.com/issu.../chemistry.htm


I used a borrowed refractometer to act as a calibration standard for my hydrometers.
Here are the results of what I found.

Refractometer: 1.0265

Deep Six: 1.025

SeaTest: 1.027

Well, I guess I’ll go do a FW water change. Its obvious that my salinity is a bit high.

Gary