Salinity...what to believe?!?!

[cincyreef]

Hey all. I've been having some minor issues with my tank so I figured I'd start with the basics. I have a refractometer that I calibrate with pinpoint calibration fluid. In my home tank my refractometer shows 1.026. When I used my old swing arm hydrometer to compare it showed at almost 1.030.

Well I decided to take my water to my lfs, and they laughed when I asked them to check for salinity. Any way long story short I watch them calibrate their refractometer with what looks like pinpoint solution. They say holy cow your salinity is 1.030. I went home and recalibrated my refractometer and it says 1.026. I've had my calibration fluid for about 8 months I'd say, but I always put the lid back on.

I did however check last night just for the heck of it. I calibrated my refractometer with RO water, and then I do get a 1.030 reading from my tank.

So what do I believe? I'm kind of at a loss with something that should be so easy lol

Any help is appreciated.

 

[twilliard]

I say your calibration fluid needs replacing.

 

[ngvu1]

Get a new refractometer or check with your reef buddy for a second opinion. I have 2 refractometers just in case ; And do not drop the hydrometers. Treat them like eggs.

 

[TheEngineer]

Have the LFS calibrate your refractometer and see how it compares. Then I'd say make your own calibration fluid and see how it compares.

 

[cincyreef]

Thanks for the responses. I just purchased the Milwaukee digital refractometer from Premium Aquatics. Luckily I'm only 1 day shipping from there, so I should know tomorrow for sure.

 

[The Coral Dudes]

That Milwaukee digital one is amazing!!!! We have a few of them and love them

 

[cincyreef]

That Milwaukee digital one is amazing!!!! We have a few of them and love them

That's the one I got the digital. That's good to hear!

 

[Randy Holmes-Farley Verified Community Expert]

If the calibration fluid was allowed to evaporate enough, the readings will be lower than the salinity reality. So that might be the case here.

 

[Crna]

Don't have a digital yet, but they are awesome! Having a concise answer should make a big difference!

 

[Berlibee]

That Milwaukee digital one is amazing!!!! We have a few of them and love them

I want this one too ...

 

[Randy Holmes-Farley Verified Community Expert]

Don't have a digital yet, but they are awesome! Having a concise answer should make a big difference!

Conductivity probes are also digital, and have other uses that refractometers do not, such as limewater potency.

 

[w.lohrman]

Just calibrate with ro water

 

[steiger33]

How do you make your own calibration fluid?

 

[atoll]

Just use RO and DI water with a zero TDS.

 

[Randy Holmes-Farley Verified Community Expert]

Just use RO and DI water with a zero TDS.

RO/DI is is appropriate for the Milwaukee digital, but not for most refractometers sold to hobbyists. If they are not labeled as true seawater refractometers, they need to be calibrated with 35 ppt seawater (or something like tank water, anyway) and not RO/DI water, regardless of what the manufacturer may state.

There are many commercial fluids now available, but here's a recipe for a DIY 35 ppt calibration fluid. Using the balance to make it is by far the best bet:

Refractometers And Salinity Measurement
http://www.reefedition.com/refractometers-salinity-measurement/

from it:

Do-it-yourself Refractometer Standards

In a previous article I described how to make a do-it-yourself refractometer standard matching 35 ppt seawater, and I will just summarize that recipe here.

To provide a standard for refractometers requires a solution whose refractive index is similar to normal seawater. Seawater with a salinity of 35 ppt has a refractive index of 1.3394. Likewise, the refractive index of different sodium chloride solutions can be found in the scientific literature. My CRC Handbook of Chemistry and Physics (57th Edition, Page D-252) has such a table. That table has entries for 3.6 and 3.7 weight percent solutions of sodium chloride that span the value for normal seawater. Interpolating between these data points suggests that a solution of 3.65 weight percent sodium chloride has the same refractive index as 35 ppt seawater, and therefore can be used as an appropriate standard (Table 5).

This 3.65 weight percent sodium chloride solution can be made by dissolving 3.65 grams of sodium chloride in 96.35 grams (mL) of purified freshwater. This recipe can be scaled to any appropriate size if suitable instruments are available (36.5 grams in 963.5 grams (mL) of water, 0.365 grams in 9.635 g (mL) of water, etc.).

This concentration roughly corresponds to ¼ cup (73.1 g) of Morton’s Iodized Salt dissolved into two liters (2000 g) of water (giving very slightly more than 2 L of total volume).

For a rougher measurement in the absence of an accurate water volume or weight measurement:

1. 
   
   
   • Measure ¼ cup of Morton’s Iodized Salt (about 73.1 g).
   • Add one teaspoon of salt (making about 79.3 g total salt).
   • Measure the full volume of a plastic 2 L Coke or Diet Coke bottle filled with purified freshwater (about 2104.4 g).
   • Dissolve the total salt (79.3 g) in the total water volume (2104 g) to make an approximately 3.65 weight percent solution of NaCl. The volume of this solution will be slightly larger than the Coke bottle, so dissolve it in another container.

[Note: the standard described here using soft drink bottles is subject to variation in the volume of the bottle. It turns out that such bottles can vary in total volume, and this can lead to at least a one ppt error in the salinity of standards matched to seawater of 35 ppt salinity. Standards made with accurate measurements of salt and water, however, will accurately match 35 ppt.]

 

[TheEngineer]

I followed Randy's recipe, but made a smaller batch. I have a scale so I could weigh everything.

 

[waucedah_joe]

Just use RO and DI water with a zero TDS.

I understand that this is not a good idea. Hobby grade refractometers are not linear and should be calibrated with a solution that is close to the desired measurement range. I know it makes a difference with mine.

 

[reefwiser]

I use the Milwaukee digital and still use a calibration fluid to make sure it is reading right. I have had it get off calibration and my salinity got off had to bring it down slowly.

 

[Cory]

Id believe the reading after calibration with ro/di water. Id also make Randy's recepie to varify. You can take some salt to a pharmacist and ask them to weigh it out for you, ive done that no problem. Although they looked at me weird witha zip lock bag full of white powder lol. Id bring the salt in the box.

 

[cincyreef]

Well the Milwaukee showed up. I calibrated it, and unfortunately my LFS was correct. My salinity has been high the entire time my tank has been setup, I'm assuming. It was 1.029. I'll slowly bring it down over a few days.