Calibrating a refractometer

[scoopsthedog]

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?

 

[Salty1962]

It's best to get the calibration fluid for your refractor. You shouldn't need to have the same temp as it should be temperature compensating. I calibrated mine every month to be on the safe side.

 

[saltyfilmfolks]

+1 on the fluid. I calibrate mine pretty much every time I use it(paranoid? yes), mine is not temp compensating.
The temp difference is for the instrument not the water.

 

[mhaun]

I test mine every time as well

 

[Frop]

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?

 

[scoopsthedog]

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?

That's what I do too. Drops of RODI and put to 0, then put the store mixed water (I get my water from there and know its at 33PPT) then test the tank.

 

[jason2459]

I'll leave this here
http://reef2reef.com/threads/does-your-calibration-solution-need-calibrated.250133/

http://www.reefedition.com/refractometers-salinity-measurement/

 

[Frop]

I'll leave this here
http://reef2reef.com/threads/does-your-calibration-solution-need-calibrated.250133/

http://www.reefedition.com/refractometers-salinity-measurement/

so... looks like I'm still using the RODI method.

 

[jason2459]

so... looks like I'm still using the RODI method.

I personally would not. And if I did I would not assume it's accurate.

 

[skim]

Well I guess it depends on what unit you have, I know the Red Sea Refractometer requires you to use RO/DI water and when wrote to them about using Salinity calibration solution they said not to use it as it would put the unit off and in the design and calculations to make the unit that RO/DI water was used. But in the end just use whatever you feel most comfortable with.

 

[spinycheek]

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)

 

[skim]

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

 

[shiftline]

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

 

[Randy Holmes-Farley]

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?

It is a big effect. Fortunately, most refractometers sold to hobbyists have ATC, automatic temperature compensation to offset the temperature effect, at least mostly.

 

[Randy Holmes-Farley]

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)

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

 

[Randy Holmes-Farley]

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

That will be true of a perfectly made seawater refractometer. It is not true of a poorly made or broken seawater refractometer or a perfectly made brine refractometer. That's why calibration solutions with 35 ppt seawater equivalent will always work and RO/DI won't always work, but sometimes will.

 

[Randy Holmes-Farley]

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

For seawater at 25 deg C, 35 ppt = sg of 1.0264 at 25 deg C and 1.0266 at 20 deg C.

I discuss it and show data here (tables 3 and 4):

http://reefkeeping.com/issues/2004-06/rhf/index.htm

 

[jason2459]

Thank you Randy.

 

[spinycheek]

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

http://reefkeeping.com/issues/2006-12/rhf/index.php[/QUOTE]

My mistake, so 36.5 grams salt in a liter of DI water...

 

[chrissreef]

Why are so many people calibrating theirs so often? Can't you calibrate once and just put loc-tite on the screw so it will stay calibrated?