Lesson learned about using R/O for refractometer calibration

[reef_junkie]

You must be some kind of psychic to know that all the way from wherever you are. Gj, you're part of the problem.

 

[Anirban]

According to the article by Randy "For many refractometers used by reef aquarists, the manufacturer calls for pure freshwater to be used for calibration. With a perfectly made refractometer (that hasn't changed since its manufacture), that single point calibration at the end of the range would be adequate, albeit not perfect. A better single point calibration might be performed in the middle of the range being used, and for higher accuracy, more than one calibrating solution would be used." But you should remember the thing you measure is dependent on temperature too so it will vary depending on temperature. For your reference on calibration I am copying some text from the article......

"Tips on Calibrating a Refractometer

Despite the fact that many refractometers sold to aquarists recommend calibration in pure water, such a calibration alone will not ensure accuracy for the reasons described above. So my recommendation for calibration is as follows:

1. First calibrate the refractometer in pure freshwater. This can be distilled water, RO (reverse osmosis) water, RO/DI water, bottled water and even tap water with reasonably low TDS (total dissolved solids). Calibrating with tap water that has a TDS value of 350 ppm introduces only about a 1% error in salinity, causing readings in seawater to read a bit low. So 35 ppt seawater (specific gravity = 1.0264) will read to be about 34.7 ppt, and will show a specific gravity of about 1.0261.

This calibration should ordinarily be carried out at room temperature using an ATC refractometer. The directions with some ATC refractometers insist that the calibration be carried out at a specific temperature, but I've never understood how that could matter and I would not worry about it. If the refractometer is not an ATC refractometer, then careful temperature control or correction is necessary, and such corrections are beyond the scope of this article.

Calibration is usually performed by putting the freshwater on the refractometer, letting it sit for at least 30 seconds so it comes to the same temperature as the refractometer, and adjusting the calibration screw until it reads a value appropriate for freshwater (e.g., refractive index = 1.3330, salinity = 0 ppt, specific gravity = 1.0000). Normally, this step is a quick and easy procedure, and may often be all that is required IF the refractometer has been verified to have passed the second calibration step below at least once. This is an offset calibration, as described above.

2. The second step in calibration should be performed at least once before relying on a refractometer to accurately measure the salinity of a reef aquarium. This step involves testing it in a solution matching the refractive index of 35 ppt seawater (or some similar solution near the range of measurement). Remember to let it sit for at least 30 seconds so it comes to the same temperature as the refractometer. Suitable commercial and do-it-yourself standards were described earlier in this article. Using one of them, place a drop onto the refractometer and read the value. If it reads approximately 35 ppt, or a specific gravity of 1.0264, or a refractive index of 1.33940, then the refractometer is properly calibrated and is set to go.

If it does not read correctly, and is off by an amount that is significant relative to your salinity precision requirements, then you need to recalibrate it using this second fluid. I suggest that a salinity error of ± 1 ppt or a specific gravity error of ± 0.0075 is allowable. If the refractometer is off significantly, and you used a do-it-yourself standard made with crude techniques such as Coke bottles, a good next step might be to buy a commercial standard.

To correct errors using these seawater standards, simply adjust the calibration screw on the refractometer until it reads the correct value for the standard (35 ppt, or a specific gravity of 1.0264, or a refractive index of 1.33940). This type of slope calibration makes the refractometer suitable to read solutions whose salinity is close to seawater's. After such a calibration, refractometers may not read freshwater correctly."

 

[Russ265]

You must be some kind of psychic to know that all the way from wherever you are. Gj, you're part of the problem.

im incredibly psychic.

if you used that calibration fluid to correct it's mistake and used 0 tds and got something other than 0.... your refractometer is wrong.

0 is 0.

all you did was bandaid a problem. id trust a hydrometer over that refractometer.

infact. you did exactly that. hydrometers are calibrated for specific temps. marking the temp and gravity is all you did here today.

 

[reef_junkie]

Sure, calibration fluid can be batched wrong, and a tds meter on an RO can also fall out of calibration so really we do the best we can. There's no reason to poo-poo all calibration fluid. So if i were to measure my refractometer against RO water should I also be concerned with how far out of calibration my tds meter is, beyond that should we worry that whatever method used to calibrate a tds has also been calibrated. I don't personally know anyone with lab grade testing equipment.
All I'm saying is there's no need to scare newbies.

 

[scardall]

Everything has a shelf life. If the powder or liquid shelf life hasn't expired it is good, PERIOD. Salinity cal.sol. is 1.026 or 35ppt the same level, done. Refractometer is not the same as a colorometer. RODI is not to be used to Cal. Period. There are instruction out there that are not for using a refractometer to mea. salinity in an aquarium. They say use RODI to cal. They Do NOT apply to our use here.

 

[redfishbluefish]

I believe there are some here who have already determined what works best for them, and regardless of what I say, will continue doing their own thing.

However, here's the poop on refractometer calibration (or for that fact, any instrument used to make measurements): you want to use calibration solutions that are closest to whatever you are measuring! Since most of us maintain our tanks at 1.026 SG, the 53.0 mS (or 1.026SG, or 35ppt, or 1.3394 RI) calibration fluid is what you want to use. You don't know if your refractometer is perfectly linear, and by using RO/DI (0 SG), the wanted 1.026 is miles away from where you just calibrated.

The other factors that impact your readings are temperature and light wavelenght. Most refractometers I've seen today have ATC (automatic temperature compensation), so that shouldn't be an issue. As far as impact of light wavelength, what is critical here is that whatever light you use to calibrate your refractometer is the light source you need to use when making your readings. So if you use natural sunlight to calibrate, use sunlight to make your reading....and if you use some florescent light to calibrate, use florescent to make your reading....and so forth. I hope this is clear.

 

[Russ265]

No the device wasn't made to measure seawater. Different salts have different refractive indexes, so the index for NaCl is different than KCl is different than seawater, so the scales are different not necessarily broken

NaCl's atomic weight is 58.44 g/mol
KCl's atomic weight is 74.5513 g/mol

roughly 20% difference

not sure if that would swing things one way too far or another

ill tell you what.

i can keep my reef at 1.025 to 1.028 just fine.

my refractometer is calibrated out of the tap water. lol
for yucks... my ro top off reads 0.

at the end of the day i wouldnt trust that refractometer if it was blessed by some hindu priest on the top of a mountain.

 

[Russ265]

Everything has a shelf life. If the powder or liquid shelf life hasn't expired it is good, PERIOD. Salinity cal.sol. is 1.026 or 35ppt the same level, done. Refractometer is not the same as a colorometer. RODI is not to be used to Cal. Period. There are instruction out there that are not for using a refractometer to mea. salinity in an aquarium. They say use RODI to cal. They Do NOT apply to our use here.

so their refractometer to measure 0 tds rodi water should differ from 0.

this is sounding more and more like bad practice.

0 is 0.

maybe you guys get some alibaba refractometers from somewhere i cant find on my globe, but i really question some motives when calibration fluid is used to supercede a good $30 unit.

btw. i have calibration fluid. i just thought everyone with out of whack refractometers bought from the same vendor in hong kong.

 

[scardall]

so their refractometer to measure 0 tds rodi water should differ from 0.

this is sounding more and more like bad practice.

0 is 0.

maybe you guys get some alibaba refractometers from somewhere i cant find on my globe, but i really question some motives when calibration fluid is used to supercede a good $30 unit.

btw. i have calibration fluid. i just thought everyone with out of whack refractometers bought from the same vendor in hong kong.

We are talking about a salinity refractometer Only, yes? // And yes 0 is 0. // We are not measuring 0 salinity. // we are measuring a target of 1.025-.027 . // So we cal. to 1.026.(industry std.)// As far as refractometers being out of cal. IMO maybe it could be due to the deviation from what is suppose to be std. cal. of Refractometers used for our hobby and how it is practiced, And or Cal.solution out of shelf life/contaminated. // I'm a little confused by this part of one of your points, ( i really question some motives when calibration fluid is used to supercede a good $30 unit.) // IMO What ever quality of meter you buy A good calibration solution is required as it applies. // My refractometer cost me around that $30 mark and I also use a Conductivity meter made by American Marine (pinpoint) . I found out that I had out of date cal. solution/contaminated ???? when I got my salinity meter and fresh cal. solution. PS: There are junk meters out their that I'm sure are causing alot of headaches and together we can Id these plus help others over time. PLUS I looked at my new Pinpoint cal. solution and it does not have a shelf life date on it. hmmmm One Last thing Russ265, I believe You are Right when you say "this is sounding more and more like bad practice" . My best to all

 

[bubbaque]

i use ro to calibrate

i think refractometers that are dropped or damaged use the fluid to offset their cheaply made/damaged devices.

think about it...

you just calibrated to 1.026 using 1.026 fluid and register negative numbers with ro.

the device is shot

My refractometer is brand new, I hope it's not shot.

 

[davejep]

Hey Randy,
If I use aquarium salt instead of mortons to make up my reference solution, will that work?

 

[ngvu1]

I hardly ever calibrate my ro/di. and I use RO water to calibrate...maybe not the best way I agree. When I trade frags with other reefers, I ask for their salinity and I often check that....JUST to have a feel if my refractometer is still good ; I am very gentle with my refractometer; If you drop it, replace it!

 

[alxjrz]

I have had my Refractormeter going on 16 years. I have never questioned its accuracy, resulting nothing but success. Today I'll drive down to my lfs and compare my findings with my buddy's Refractormeter to several closed systems at the shop. Cheapest Calibration at its finest. Don't fail me now Blue! (Just so happens to be Blue) Haha

 

[Kaba]

So coming from an analytical background, using RODI water isn't calibration. That would be called a blank. And if you us specific gravity and are using rodi to calibrate it to 1.000, you're 0.003 too high.
When we calibrate our pH probes, we use 7 and 10 because we will be using it in water between 7 and 10. We dont use 2 and 4 to calibrate.
Before I use my refractometer I'll use rodi water to make sure 0ppt is 0ppt, then use 35ppt calibration solution to calibrate it.
You're more than welcome to use just rodi, but then you're not calibrating.

 

[bubbaque]

So coming from an analytical background, using RODI water isn't calibration. That would be called a blank. And if you us specific gravity and are using rodi to calibrate it to 1.000, you're 0.003 too high.
When we calibrate our pH probes, we use 7 and 10 because we will be using it in water between 7 and 10. We dont use 2 and 4 to calibrate.
Before I use my refractometer I'll use rodi water to make sure 0ppt is 0ppt, then use 35ppt calibration solution to calibrate it.
You're more than welcome to use just rodi, but then you're not calibrating.

What do you mean you use RODI water then use calibration fluid? If I have to adjust it when I use the calibration fluid what was the reason to use RODI water?

 

[jason2459]

Hey Randy,
If I use aquarium salt instead of mortons to make up my reference solution, will that work?

No, it wont work using the measurements he gave in the article.


https://www.reef2reef.com/threads/does-your-calibration-solution-need-calibrated.250133/

And for those that are saying to calibrate to 0 with freshwater that will only be viable for a true seawater refractometer which most sold to hobbyist are not. Veegee, Vitalsine, D&D H2Ocean, Red Sea, and Milwaukee digital have true seawater refractometers that could be calibrated to 0 with freshwater. Of those I can not recommend the Red Sea one. I have had and have used each of them. My favorite is the veegee/vitalsine (rebranded veegee). I would still reference a 35ppt solution for accuracy and correct mechanics.

 

[reeferfoxx]

In my experience, eye piece refractometers are consistantly incorrect even when the calibration shows it to be correct.

 

[patsheridan]

If i calibrate using RoDi and it comes in at 0, then calibrate using a solution set for 35 and it comes in at 35, I am thinking my refractometer is at that moment good to go. Yes?

 

[Russ265]

My refractometer is brand new, I hope it's not shot.

No, it wont work using the measurements he gave in the article.


https://www.reef2reef.com/threads/does-your-calibration-solution-need-calibrated.250133/

And for those that are saying to calibrate to 0 with freshwater that will only be viable for a true seawater refractometer which most sold to hobbyist are not. Veegee, Vitalsine, D&D H2Ocean, Red Sea, and Milwaukee digital have true seawater refractometers that could be calibrated to 0 with freshwater. Of those I can not recommend the Red Sea one. I have had and have used each of them. My favorite is the veegee/vitalsine (rebranded veegee). I would still reference a 35ppt solution for accuracy and correct mechanics.

i knew it. you guys are buying alibaba equipment and blessing it with 1.026 fluid and wonder why it doesnt work like a proper refractometer....

my 0 stays 0 and my 1.026 is still 1.026

youre better off buying a hydrometer and marking it with nail polish at the fish store. using calibration fluid to fix your issue is defeating the entire purpose.

if you "calibrated" with 1.026 and ro is other than 0, you have junk at the end of the day.

 

[bubbaque]

So what should I do, buy a new refractometer? I calibrate with solution at 35ppt and when I do it with RODI I get a negative number.