Refractometer Calibration Clarification

[george9]

Hello!
Long story short - I noticed after my water changes that my corals would be temporarily ticked off for about a day and thought my refractometer could be mis-calibrated since I was using RODI to calibrate (bad, I know I know)

Well, I check with the calibration fluid last night and it was about 1.5 pts too high so my tank which I thought was 1.025 was actually almost 1.027. not the end of the world but still - yikes!

I have re-calibrated using calibration fluid to 35ppt salinity but now RODI water is reading as 1PPT instead of 0. I am thinking this is okay because since it is now calibrated to seawater, it will give false readings at higher ranges near 0 but just wanted to double check I am thinking correctly here and am safe to use the instrument.

thanks!

 

[Garf]

Hello!
Long story short - I noticed after my water changes that my corals would be temporarily ticked off for about a day and thought my refractometer could be mis-calibrated since I was using RODI to calibrate (bad, I know I know)

Well, I check with the calibration fluid last night and it was about 1.5 pts too high so my tank which I thought was 1.025 was actually almost 1.027. not the end of the world but still - yikes!

I have re-calibrated using calibration fluid to 35ppt salinity but now RODI water is reading as 1PPT instead of 0. I am thinking this is okay because since it is now calibrated to seawater, it will give false readings at higher ranges near 0 but just wanted to double check I am thinking correctly here and am safe to use the instrument.

thanks!

That’s correct, assuming your calibration solution is good.

 

[george9]

That’s correct, assuming your calibration solution is good.

I guess the only way is to make my own solution and double check!

 

[Garf]

I guess the only way is to make my own solution and double check!

Yep, I just make Randy’s DIY solution.

 

[Reef Diaper]

I just had the same problem. I ordered new calibration fluid and it showed that my salinity was very high. I ordered another brand of calibration fluid and it was 3 ppt higher than the first one. I decided to make my own calibration fluid using Randy's recipe and it was in-between the two. Here are the results with my refractometer calibrated to 35 ppt using the diy solution.

brand X = 37 ppt
diy = 35 ppt
brand y = 34 ppt

Randy's diy solution is very easy to make and it is what I will be using from now on. I borrowed a digital scale from my daughters chemistry class which worked very well. It is inexpensive and available on Amazon. Here is the link.

Amazon product

 

[Gtinnel]

I make my own using Randy’s recipe and it works great.
Also, I know of at least one popular vendor who keeps his salinity at 1.027, so I wouldn’t worry about that level at all as long as your tank is staying consistent.

 

[Koh23]

Ro water and salifert solution reads the same in my case.....

So, right or wrong, its the same in my case....

 

[Garf]

Ro water and salifert solution reads the same in my case.....

So, right or wrong, its the same in my case....

Excellent, some refractometers do agree. When mine is calibrated at 35ppt, RODI reads negative 4 ppt ish. It also stays calibrated for months on end then suddenly it goes off, don’t know why.

 

[92Miata]

Below is your basic refractometer. In order for it to be accurate across the whole scale, the prism needs to be perfectly cut and shaped, the flap needs to be perfectly even thickness, the bimetallic strip needs to flex according to temperature at exactly the predicted rate, and the scale needs to be at exactly the correct distance and printed at the correct scale (and flat, and perfectly perpendicular, etc).

There is an enormously high degree of precision needed - and that's pretty much impossible to consistently achieve in a $27 device.

When you have things out of tolerance, there's generally one of two ways that it presents:
1. The measurement is offset - IE, the relationship of where the light comes out of the prism, and how high the chart is isn't as expected. IE, it always reads .02 high.
2. The slope is wrong - the crystal is mispositioned, or the scale is slightly tilted, or whatever, and changes in salinity aren't accurately reflected (IE, every .01 you increase salinity, the meter goes up by .012).

If yours is just offset, you can calibrate with RODI, and be fine. But if yours has an imperfect slope (and most do) - it can only be exactly accurate at one point, and the further you get from this point, the less accurate its going to get. IE, the further from 1.026 you calibrate the meter, the less accurate it gets at 1.026.

If you calibrate with 1.026, and then check it with RODI, and it's 0 - then your slope is good, and you can validate with RODI - as long as nothing changes with the meter. You drop it, you scratch it, you let it get too hot - anything that can change the physical geometry or optical clarity - and all bets are off.

 

[Koh23]

Mine is very old (10 years or more), cheap chinese refractometer.

I always wonder is rodi calibration enough, so i try with various solutions with known salinity, and it always matched. 0=0.

Problem that i encountered is that you need to calibrate each time. So, calibrate-test.

If testing again, i need to calibrate again, if not, difference is in few points, so..... Something is off with that refractometer, for sure

 

[george9]

I find it a bit infuriating that you cannot even trust calibration fluid to be consistent. If I made my own solution I will likely question its integrity as well even If I take every precaution to be precise, so I suppose I'll never win. I plan on calibrating it each time before I use it and to take extra care to not knock it around - I didn't realize this can greatly impact its accuracy.

I am assuming my refractometers have a slightly imperfect slope since they both read slightly above 0 with RODI. The good thing is, my corals were telling me *something* was off the last couple water changes and it makes sense after calibration and testing with an additional new refractometer that it was too high rather than too low.

Thank you for the insightful comments everyone!

 

[92Miata]

I find it a bit infuriating that you cannot even trust calibration fluid to be consistent. If I made my own solution I will likely question its integrity as well even If I take every precaution to be precise, so I suppose I'll never win. I plan on calibrating it each time before I use it and to take extra care to not knock it around - I didn't realize this can greatly impact its accuracy.

One of the big issues we have in the hobby is that there's essentially no regulation - so manufacturers are free to sell pretty much whatever they want as whatever they want, as long as they don't stray into areas regulated by the EPA/FDA/etc.

You could sell a bottle of shampoo as "Reef Salinity Calibration Solution" and it's totally legal. And you could pay the folks on YouTube to tell reefers it's the bees knees, and that's legal too.

There are some instructions on making your own calibration solution here with pure water and table salt:

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

 

[Randy Holmes-Farley]

Ro water and salifert solution reads the same in my case.....

So, right or wrong, its the same in my case....

With a true seawater refractometer, both 35 ppt and 0 ppt (ro/di) should read correctly.

But many/most refractometers thst hobbyists buy are brine refractometers, and those should not read Ro/di and 35 ppt correctly at the same perfect calibration.

 

[minus9]

If the calibration fluid in question has to be shaken to use, don't use it, as it's not a solution at all. The only trusted brand that I've used is Aqua Craft (or diy). Most cheap refractometers are not meant to be used with seawater, but a brine solution, which is the first issue. But as mentioned above, quality matters here. I've had great luck with DD Ocean refractometer, which I've only had to adjust a few times since 2015. VeeGee makes great refractometers, but they're not cheap. I always back up my readings with my trusted glass hydrometer.

 

[george9]

If the calibration fluid in question has to be shaken to use, don't use it, as it's not a solution at all. The only trusted brand that I've used is Aqua Craft (or diy). Most cheap refractometers are not meant to be used with seawater, but a brine solution, which is the first issue. But as mentioned above, quality matters here. I've had great luck with DD Ocean refractometer, which I've only had to adjust a few times since 2015. VeeGee makes great refractometers, but they're not cheap. I always back up my readings with my trusted glass hydrometer.

This is the one I purchased and it does say to shake before use. AccuBrate Refractometer Salinity Calibration Fluid – 60 ml Solution to Accurately Calibrate Refractometer for Testing Natural Saltwater or Synthetic Sea Water - Made in the USA (60 ml) https://a.co/d/acKxeOi

Now I’m questioning if even that is accurate

 

[minus9]

This is the one I purchased and it does say to shake before use. AccuBrate Refractometer Salinity Calibration Fluid – 60 ml Solution to Accurately Calibrate Refractometer for Testing Natural Saltwater or Synthetic Sea Water - Made in the USA (60 ml) https://a.co/d/acKxeOi

Now I’m questioning if even that is accurate

I've tried the Brightwell, BRS, etc... solutions. All of them need to be shaken to work, which is not what you want. Do we shake our tank water when testing? It's not a solution if it has to shaken to work, in fact, I have no idea what these fluid are?

 

[george9]

I've tried the Brightwell, BRS, etc... solutions. All of them need to be shaken to work, which is not what you want. Do we shake our tank water when testing? It's not a solution if it has to shaken to work, in fact, I have no idea what these fluid are?

Not sure what their thinking is behind shaking but I’m willing to bet at times the people writing the instructions for these products don’t necessarily have the highest understanding of the product or hobby

 

[92Miata]

I've tried the Brightwell, BRS, etc... solutions. All of them need to be shaken to work, which is not what you want. Do we shake our tank water when testing? It's not a solution if it has to shaken to work, in fact, I have no idea what these fluid are?

No, we just spend thousands of dollars on pumps and wavemakers to stir it and keep it evenly mixed.

 

[Koh23]

With a true seawater refractometer, both 35 ppt and 0 ppt (ro/di) should read correctly.

But many/most refractometers thst hobbyists buy are brine refractometers, and those should not read Ro/di and 35 ppt correctly at the same perfect calibration.

Agree.

Like i said, if i calibrate with rodi at 0, calibration solution at 35ppt will be 35ppt on refractometer.

If i calibrate with solution at 35ppt, rodi reading will be 0.

Only problem is that, probably due to age of refractometer, i need to calibrate every measuring, otherwise i will get different result..

 

[minus9]

No, we just spend thousands of dollars on pumps and wavemakers to stir it and keep it evenly mixed.

True, but we only mix once. Once you mix sodium chloride with water, there's no need to mix again, so I have no idea what these people/companies are making?