Refractometer Calibration Fluid

[bqq100]

Anyone have their calibration fluid shift over time?  I finally decided to calibrate my refractometer after a few months, and it read 2ppt too high.  I adjusted it accordingly, but I did think it was a little strange, because it seemed to be reading correctly with freshly mixed water (using same amount of water/salt I have been for a while).  Decided to try DI water, and now it was reading DI water at -2ppt.  I thought the calibration fluid was closed up tight, but it seems like there must have been some evaporation.  Anyone else have that happen before?  I'll probably have to pick up some new calibration fluid just to be sure.

[Coral Hind]

I have seen the same thing years ago. I just use regular RODI and don't buy calibration fluid any longer. Make the meter and fluid are not far off in temp or it can throw off the test some.

[bqq100]

Ya I always give the sample a couple of minutes for the temps to equalize before reading the value.  Eventually I'll probably switch over to using just DI water for calibration. 

[gmerek2]

http://blog.captive-aquatics.com/captive_aquatics/2011/01/refractometer-calibration-the-myth-and-risks-of-using-rodi.html

[lnevo]

http://blog.captive-aquatics.com/captive_aquatics/2011/01/refractometer-calibration-the-myth-and-risks-of-using-rodi.html

+1

 

The point of calibration fluid is to meter the device to what your testing. The further away you get from the calibrated number the further the drift.

[jaddc]

A refractometer is linear measurement tool. An analog thermometer is also a linear measurement tool.

 

The liquid in a thermometer travels the same distance from 20-30 degrees as it does from 60 to 70 degrees or 100 to 110 degrees (measure it with a ruler). To calibrate a thermometer at the factory you pick two points (used to be water freezing and boiling in the old days) and then divide the distance between those points into even parts, or degrees. That scale is printed on the shaft and cannot be adjusted. And so once calibrated, the thermometer does not need to be calibrated again. If you take 10 liquid thermometers of the same make and model, you will see that the scales are slightly different on each one.

 

It is similar for a refractometer. At the factory, the device is calibrated and they print the scale onto the view finder. That is why analog refractometers are technically never calibrated, only zeroed, by the consumer. 

 

If you zero out a refractometer (which you should do every time you use it because the prism can move) and then measure a standard solution, it should be dead on with no further adjustments needed.

 

Often it is not the case with hobby grade devices because it may not have been calibrated properly, constructed properly, handled properly, etc.

 

If a properly zeroed refractometer does not read a standard solution properly after the temperature has matched, then the refractometer is defective.

 

One option is to buy a new refractometer.

 

Another option is to zero the device, not at zero with RODI, but at the reading you are interested in. That is where the standard solutions come in (they are standard solutions, not calibration solutions). If you move the scale to read 35ppt when a 35ppt standard solution is in the refractometer, then you minimize the error in that local region. Obviously, this is a cheaper method than buying a good refractometer.

 

Here's a home recipe from Randy: http://www.reefkeeping.com/issues/2004-06/rhf/

[bqq100]

Agreed, that I would rather zero the device (jaddc has a good point that we zero, not calibrate refractometers) with 35ppt standard solution.  However my tank, freshly mixed saltwater, and RODI water all point to my standard solution drifting over time.  If the standard solution can drift 2ppt over the course of a few months, and there are no expiration dates on it, how can you trust the standard solution?

 

In any case I have a new bottle of standard solution, a precision scale, and a calibration weight that should be here on Wednesday.  That way I can test my old standard solution, new standard solution, and Randy's DIY standard solution and find out what I can really trust.  With the precision scale/calibration weight I should be able to make Randy's DIY solution in small batches with an accuracy of +/- .3ppt.

[jaddc]

Agreed, that I would rather zero the device (jaddc has a good point that we zero, not calibrate refractometers) with 35ppt standard solution.  However my tank, freshly mixed saltwater, and RODI water all point to my standard solution drifting over time.  If the standard solution can drift 2ppt over the course of a few months, and there are no expiration dates on it, how can you trust the standard solution?

 

In any case I have a new bottle of standard solution, a precision scale, and a calibration weight that should be here on Wednesday.  That way I can test my old standard solution, new standard solution, and Randy's DIY standard solution and find out what I can really trust.  With the precision scale/calibration weight I should be able to make Randy's DIY solution in small batches with an accuracy of +/- .3ppt.

 

Oh, I can believe that your standard is drifting. Evaporation and precipitation would be your main culprits.

 

I would proceed just as you are doing.

[Coral Hind]

Nice post above jaddc, makes sense on the difference between zero and calibrate.

[Origami]

Another option is to zero the device, not at zero with RODI, but at the reading you are interested in. That is where the standard solutions come in (they are standard solutions, not calibration solutions). If you move the scale to read 35ppt when a 35ppt standard solution is in the refractometer, then you minimize the error in that local region. Obviously, this is a cheaper method than buying a good refractometer.

 

I zero all of my refractometers against a refractometer standard simply because I consider them all cheap, hobby-grade instruments. Be sure that you're using an appropriate (refractometer) standard and not a type specifically used (only) for conductivity or specific gravity. Some commercial standards can be used for multiple calibrations, but not all.

[bqq100]

I zero all of my refractometers against a refractometer standard simply because I consider them all cheap, hobby-grade instruments. Be sure that you're using an appropriate (refractometer) standard and not a type specifically used (only) for conductivity or specific gravity. Some commercial standards can be used for multiple calibrations, but not all.

 

How do you store your standard solutions and how often do you replace them?  My old standard solution is Aqua Craft from BRS, and the new one on order is the Pinpoint solution.  Both of them have the appropriate refractive index according to BRS and Drs F&S.

[Origami]

Stored in the original bottle, capped tightly, and - if possible - with the air expelled to the extent possible. I use Pinpoint solution. I'll sometimes make a refractometer from Kosher salt and RO/DI water and tweak it to match the Pinpoint. If the solution is every cloudy or has residue on the bottom, toss it. Sometimes you'll see - in the DIY standard - that you can get a growth in the bottle that probably came from some bacteria or something in the bottle at the time of mixing it up. Thus, I won't keep a DIY solution much more than a month anymore. If you do, you may want to sterilize the container, boil the water in advance of using it when you mix up your standard, or keep it in the dark. There may be other ways of keeping this risk down, too (maybe using a little bleach in the solution), but I've not tried it. Stock solution is cheap enough to just get and have on the shelf.

[jaddc]

Stored in the original bottle, capped tightly, and - if possible - with the air expelled to the extent possible. I use Pinpoint solution. I'll sometimes make a refractometer from Kosher salt and RO/DI water and tweak it to match the Pinpoint. If the solution is every cloudy or has residue on the bottom, toss it. Sometimes you'll see - in the DIY standard - that you can get a growth in the bottle that probably came from some bacteria or something in the bottle at the time of mixing it up. Thus, I won't keep a DIY solution much more than a month anymore. If you do, you may want to sterilize the container, boil the water in advance of using it when you mix up your standard, or keep it in the dark. There may be other ways of keeping this risk down, too (maybe using a little bleach in the solution), but I've not tried it. Stock solution is cheap enough to just get and have on the shelf.

+1 FWIW

[zygote2k]

I use distilled water to zero mine. Never saw a need to use the solution.

[GraffitiSpotCorals]

I always use calibration solution, seen too many problems with people usin ro water. I even calibrate every time I check my water.

And I use the 35ppt calibrations solution so it's ranging around what I am testing.

[bqq100]

Well the new standard solution matches the old standard solution almost exactly.  The small batch of DIY i made came out ~33.5 so I'll have to work on my weighing techniques to get that up to par.  I wonder how I mishandled the refractometer to throw the calibration so far out of whack.  At least now I have a high level of confidence in my standard solutions 

 

Also I switched out my ATO bucket with a bucket of freshly mixed SW to bring the salinity back up to target.

[jaddc]

With the hobby grade devices you don't have to do much. All it takes is for the prism to shift slightly for the scale to be off. 

[Origami]

Keep an eye on it and check it frequently. If something's loose and shifting, then it's going to be unreliable and you'll probably want to replace it.

[zygote2k]

I always use calibration solution, seen too many problems with people usin ro water..

I bought one of the Marine Depot refractometers 6 years ago and the instructions specifically said to use steam distilled water or 35ppt solution. It also said not to use R/O water because of fluctuation issues.

I still use the same one and have calibrated it once a year and it has been reliable. I use it as a reference against the swing arms that I use frequently.

Edited January 16, 2014 by zygote2k

[GraffitiSpotCorals]

For some reason I find that if I don't calibrate with the 35ppt solution within a month or so the scale slides off a little. Sometimes it happens sooner. It's nothing major but i have seen it shift by not addressing it over time. And have seen a lot of people's tanks end up with salinity issues from not calibrating correctly or often. A 5 dollar bottle of solution gives me peace of mind and lasts forever.