SALINITY shocker

[OoooDRAGONoooo]

Help.i bought a cheap salinity checker (deep six),brought up the salinity in tank to 1023. I then read that for intended reef use ,I should buy a more expensive refractometers (the one u hold up to the light and look through like a telescope).so I did ,and it read 1034.(that's after recalibrating it with rodi water).i then borrowed a hydrometer ,which read,1028.....so what is right??????

[ChimolaFish]

well, they're supposed to be calibrated with a calibration fluid, which is a little different than rodi water. That being said, it shouldn't be too far off. Salinity is about consistancy. A lot of animals can be kept anywhere from 1.017-1.028. The ideal range is definitely a lot safer on the animals though, so shoot for 1.023-.1025

[disc1]

Depending on the type of refrac you got you may not be able to calibrate it with pure water and expect it to read saltwater accurately. Very few will do that. Check it against a known 35ppt solution like Pinpoint before you get too carried away.

[c.rob]

I am inclined to say the refactometer is correct. The deep six can be off from micro bubbles. The hydrometer can be read wrong example of not reading the miniscus line properly.

[c.rob]

How did the refactometer manufacturer say to calibrate thier unit ?

[whosurcaddie]

Take it to a local fish store and they will calibrate it for you. No more guessing.

[thegrun]

Originally Posted by c.rob

How did the refactometer manufacturer say to calibrate thier unit ?

It doesn't matter how the manufacturer recommends you to calibrate the refractometer, if you use anything other than a 35ppt you are not going to get the most accurate saltwater reading.

[1987]

Call around to the lfs around you and see who does this service, otherwise just drop the 10$ for the calibration fluid

[OoooDRAGONoooo]

Hi,ty for replies,instructions on refractometers said to calibrate with distilled water.is 1034 much too high? Shall I do water change using just rodi water to bring it down?

[kurt_n]

Don't do anything until you calibrate the refractometer using 35ppt solution. Using DI water (regardless of the instructions) may or may not give you accurate results - it totally depends on the individual refractometer and not even the brand.

And yes... 1.034 is too high. You're looking for 1.025-ish. But don't change anything until you know what your current salinity is... for sure.

[disc1]

Originally Posted by OoooDRAGONoooo

Hi,ty for replies,instructions on refractometers said to calibrate with distilled water.is 1034 much too high? Shall I do water change using just rodi water to bring it down?

Post the instruction sheet and I'll show you why this is a bad idea.

Most of the refracs that most of us have are saltwater refracs, not seawater refracs. The relationship between salinity and refractive index is different for saltwater and seawater. If you calibrate a saltwater refrac with RODI and then try to measure 35ppt seawater you'll get something around 36.5 and it looks like it is too high.

The manufacturer tells you in the instruction sheet to use RODI because that's the best the manufacturer can recommend. The manufacturer does not know what you intend to measure with their instrument, so they give you the closest thing they can to a calibration. But in ALL cases it is sub-optimal. No matter what you use a refractometer for, you should always try to calibrate it as closely to the thing you want to measure as possible. This is the nature of single point calibrations.

A calibration does two things, it sets the zero point and it also tells you the slope of the response. It would be much easier to think about a bathroom scale. We all know how to set the scale to zero with nothing on it, and that sets the zero point. But the slope determines how much the scale moves for each pound you add. Imagine we had a scale where the dial only moved one half a pound for every pound you put on it. The reading would always be exactly half the real weight even though you have zero set at the right place.

To really use a scale, you'd have to calibrate at two weights. You use one to set the zero, and a second weight to make sure that the divisions on the scale are right. The second weight sets the slope of the response. It makes sure that for every pound you add you actually move the scale by one pound.

Unfortunately with a refractometer we only get one point to calibrate. We can't adjust the scale. So there's never any guarantee that for every 1ppt the salinity increases that the reading on the refractometer actually increases by 1ppt. In many cases it doesn't and that can lead you to a false reading if the only point you know to be accurate is the 0 mark.

So with single point calibration the best we can do is to set the thing to read right at one point. We can only guarantee one point to be accurate.

Would you rather know that the 0 mark is accurate and 35ppt may be off? Or would you rather know that 35ppt was accurate and 0 might be a little off?

Now if the manufacturer knew you were going to measure seawater with this instrument, they would recommend you to calibrate it to something close to seawater. But lots of people buy refractometers for lots of different purposes from salt brine to beer. The manufacturer doesn't know which one of these uses you are going for, so they just say to zero it with water because that is as good as any other point they tell you to calibrate to if it isn't the point you're measuring to and because RO water is easy to come by while a calibrant may not be.

[Turbo4toy]

I would say try to go by your LFS and have them measure it or if there is a fellow reefer maybe they can do it for you. I was in a similar bind and almost killed all my corals. I was using a hydrometer that was off and I used a refractometer that had been calibrated and my tank was at 1.031. I slowly lowered it and everything recovered. I think one good thing was that I was using the same device to measure the salinity do I didn't have a huge swing in the levels but it was consistently wrong.

[Pigpen17]

I just went through the same thing. I had no idea what to trust. Even my LFS was wrong. I bought a Milwaukee Refractometer. It is super easy to calibrate. It comes with a bottle of steam distilled water to zero out your meter, and than a 1.025 test solution to check that you are calibrated correctly.

I was at 1.028. I brought it down over a few weeks with no ill effects.

BTW, my LFS said I was at 1.022 and needed to bring up my salinity.

[Big_Boss_77]

Originally Posted by Pigpen17

I just went through the same thing. I had no idea what to trust. Even my LFS was wrong. I bought a Milwaukee Refractometer. It is super easy to calibrate. It comes with a bottle of steam distilled water to zero out your meter, and than a 1.025 test solution to check that you are calibrated correctly.

I was at 1.028. I brought it down over a few weeks with no ill effects.

BTW, my LFS said I was at 1.022 and needed to bring up my salinity.

I just bought one of these this week. I had a junky swing arm and it said I was at 1.026... zero and check digital refractometer and sure enough I'm at 1.022... score one for calibration