Refractometer calibration

[steveoutlaw]

I have a refractometer that I have been using for a long time. Well, it hadn't been calibrated for a long time so I got a hydrometer just to see if they were close.......well they were way off. The refractometer is reading 1.026 and the hydrometer is reading off the charts. Does anybody know if the LFS like Petco carry calibration fluid? Can you just calibrate it with RODI to read at zero?

 

Thanks.

[FearTheTerps]

You can use rodi water to calibrate it to zero, but most people say its best to cailbrate it to 35ppt.

 

If you just calibrate it to zero, it should still be pretty close, and not "way off" like the hydrometer seems to be. Im guessing the hydrometer is just way off, lots of things can cause them to very inaccurate, even a tiny air bubble can throw the reading way off.

[GraffitiSpotCorals]

I tried to calibrate mine with Rodi water and it didn't work, it read 1.026 when it was really 1.021. I would say definatly get the solution, BRK has it.

[hypertech]

It can be several points off if you calibrate it to 0 with RODI. Don't do that.

 

Get the solution.

[Reefoholic]

Solution is the answer. Also, if you mix your own water you can temporarily calibrate it to that as well. I mix my own water and use very exact measurements every time I mix it and I get 35 ppt exactly.

[hypertech]

nooooooooooo!

 

Don't do that. Humidity in the air, differences in lots, all sorts of things can mess with your salt mix.

 

Refractometers need to be calibrated, but once they are set, they pretty much stay set. Just get the solution. I think it costs all of $1.

[Reefoholic]

Mine is already calibrated tho.

[zygote2k]

When I purchased my refractometer, the instructions said to calibrate it with distilled water at room temperature. I have also used r/o and it still comes out the same. Why spend $10 on "calibration solution" when distilled is what is recommended?

[Origami]

When I purchased my refractometer, the instructions said to calibrate it with distilled water at room temperature. I have also used r/o and it still comes out the same. Why spend $10 on "calibration solution" when distilled is what is recommended?

 

Many of the refractometers that we hobbyists have are what are for brine - or straight salt water (sodium chloride plus water) - not for the complex mixture of ions that we call artificial salt water (ASW). The refractive index for brine does not track exactly with the refractive index for ASW at various concentrations. This means that they will not give you a correct reading from 0 ppt all the way through our range of interest up to 35 ppt (and beyond). If you're not absolutely sure what kind of refractometer you have, it's imperative to calibrate your refractometer with a standard whose concentration is in the area of where you want to keep your tank. In this case, 35 ppt.

 

Don't use RO/DI to calibrate unless you're sure or have solid evidence that your refractometer is one specifically built for ASW and not for brine.

[BowieReefer84]

Best place online to get solution? I have always use distilled, and am curious to see if there is a difference...

[Origami]

References:

http://www.reefkeeping.com/issues/2006-12/rhf/index.php

(Scroll down to "Imperfect Refractometer Use: Scale Misunderstanding and Salt Refractometers" for a quickie on how the refractive index of saline differs from ASW).

 

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

(More technical. But shows how saline standards need to very depending upon the parameter being measured (e.g. specific gravity, refractive index, or conductivity. Also gives a nice DIY formula for a salinity calibration standard.)

 

Here's a writeup on the d-d refractometer, one of the few ASW refractometers out there:

http://www.glassreef.com/review_d-d_refractometer.php

[Steve G]

Ok, here's a question. What if I have a solution packet that's five years old. Still good? It's sealed in a pouchy looking container.

[Origami]

Ok, here's a question. What if I have a solution packet that's five years old. Still good? It's sealed in a pouchy looking container.

Sounds like it's a hermetically sealed pouch. It's probably still good. But, really, new solution is so cheap and salinity so critical, I'd part with a couple of bucks and get some fresh reference solution.