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Should I be calibrating every time I test with distilled water and what if my distilled water is at a different temperature than my tank water?
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All I know is the fish store said they calibrate theirs with RODI water. So I test mine with RO/DI and then check my saltwater.
How much does temp effect the reading anyways? Are we talking about shrink on plastic parts that tiny? Or something like the gage not moving as easily?
I personally would not. And if I did I would not assume it's accurate.so... looks like I'm still using the RODI method.
All I know is the fish store said they calibrate theirs with RODI water. So I test mine with RO/DI and then check my saltwater.
How much does temp effect the reading anyways? Are we talking about shrink on plastic parts that tiny? Or something like the gage not moving as easily?
For most functioning, non-damaged refractometers, RO/DI water is a sufficient calibration fluid. Now if you dropped it or suspect something isn't right, then invest in a calibration fluid. Or make your own (35 grams NaCl into 1 liter DI water = 35 ppt saltwater ~ 1.026)
I have two refractometer and if I check with RODI it's 0. If I check with calibration solutions it's right on the money. So I would assume either would. Ideally you calibrate it to 1.026 but if I didn't have it I wouldn't hesitate to use RODI to get me by
Its funny, I see this quite often 35 ppt = 1.026 and as far as I know at 77 ( 25 C ) 35.2 = 1.025 and 36.5 ppt = 1.026
Salinity in ppt
F (C) 1.020 1.021 1.022 1.023 1.024 1.025 1.026 1.027
74.0 (23.3) 28.0 29.3 30.6 31.9 33.3 34.6 35.9 37.2
75.0 (23.9) 28.2 29.5 30.8 32.1 33.5 34.8 36.1 37.4
76.0 (24.4) 28.4 29.7 31.0 32.3 33.7 35.0 36.3 37.6
77.0 (25.0) 28.6 29.9 31.2 32.5 33.9 35.2 36.5 37.8
78.0 (25.6) 28.8 30.1 31.4 32.7 34.1 35.4 36.7 38.0
79.0 (26.1) 29.0 30.3 31.6 32.9 34.3 35.6 36.9 38.2
80.0 (26.7) 29.2 30.5 31.8 33.2 34.5 35.8 37.1 38.5
81.0 (27.2) 29.4 30.7 32.0 33.4 34.7 36.0 37.4 38.7
82.0 (27.8) 29.6 30.9 32.3 33.6 34.9 36.3 37.6 38.9
83.0 (28.3) 29.8 31.2 32.5 33.8 35.2 36.5 37.8 39.2
84.0 (28.9) 30.1 31.4 32.7 34.1 35.4 36.7 38.1 39.4
85.0 (29.4) 30.3 31.6 33.0 34.3 35.6 36.9 38.3 39.6
86.0 (30.0) 30.5 31.8 33.2 34.5 35.8 37.2 38.5 39.8
87.0 (30.6) 30.8 32.1 33.4 34.8 36.1 37.4 38.8 40.1
http://reefkeeping.com/issues/2006-12/rhf/index.php[/QUOTE]The claim is not correct and that calibration fluid is not going to give proper results for seawater.
Seawater is different than sodium chloride. That is the crux of the problem because many refractometers sold to hobbyists are made for brines (sodium chloride solutions) but seawater has a different relationship between refractive index (that they measure) and salinity or specific gravity (which they report).
That is why distilled and/or RO/DI may be inherently inaccurate for calibrating such a refractometer for measuring seawater, and a proper 35 ppt seawater equivalent is always going to work unless it is an electronic device whose software simply cannot use such a fluid for calibration.
I discuss these issues here in detail:
Refractometers and Salinity Measurement by Randy Holmes-Farley - Reefkeeping.com
http://reefkeeping.com/issues/2006-12/rhf/index.php