Lesson learned about using R/O for refractometer calibration

[bubbaque]

I was using R/O water to calibrate my refractometer. I wasn't having any issues with the tank but I thought it would be a good idea to buy some calibration fluid from BRS just to make sure it was fine. I thought my salinity was at 1.026, when in reality it was close to 1.022. I will never calibrate using R/O water again and if you are currently doing so, I would suggest buying some calibration fluid.

 

[Anirban]

If I am not wrong then the calibration fluid you used to calibrate your refractometer gives you a reading at 1.026 and then you test your water right. Now logically this does not mean your measurement with this fluid is right or others reading with RO/DI water is wrong. According to chemistry or physics when we use a colorimeter or something like that we do blank reading before taking a reading like we do in case of all Hanna meters. Now in a ideal condition if you going to measure a solution made up of RO/DI water you should calibrate the machine with the same RO/DI water first. If you are calibrating the refractometer with the calibration solution whats the reading of plain RO/DI water after calibration? How you know your calibration fluid is in good state? Don't believe it just because you bought it from BRS....Did you actually checked the salinity of just plain RO/DI water after calibrating the machine with the calibration solution? If so what was the reading? My assumption is it will be in negative....

 

[Downbeach]

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

 

[Sabellafella]

If I am not wrong then the calibration fluid you used to calibrate your refractometer gives you a reading at 1.026 and then you test your water right. Now logically this does not mean your measurement with this fluid is right or others reading with RO/DI water is wrong. According to chemistry or physics when we use a colorimeter or something like that we do blank reading before taking a reading like we do in case of all Hanna meters. Now in a ideal condition if you going to measure a solution made up of RO/DI water you should calibrate the machine with the same RO/DI water first. If you are calibrating the refractometer with the calibration solution whats the reading of plain RO/DI water after calibration? How you know your calibration fluid is in good state? Don't believe it just because you bought it from BRS....Did you actually checked the salinity of just plain RO/DI water after calibrating the machine with the calibration solution? If so what was the reading? My assumption is it will be in negative....

I have about 5 different calibration solution along with a few doubles and they all read differently. I also keep the tip of my refract under 78° water at all times to make sure the fluid is measured right. Honestly its better to make your own

 

[Anirban]

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

I read that article earlier. If go through the calibration part you will find that it says in an most idealistic case you should calibrate your refractomenter with water which has 0 salinity or something which has intermediate salinity or maybe multiple calibration can be done. But these refractometers are one point calibration system not like photometers where you can do a multipoint calibration.

Also this will vary with temperature...

 

[bubbaque]

Yes when I tested again with R/O water the result was negative. I did not know know calibration fluid could all show different measurements either, seems kind of pointless now to buy it if they all read different. I used the solution and waited 45 seconds according to the directions then calibrated it.

This is the first time I have ever used calibration fluid and thought it would make my salinity spot on but now I am not so sure.

 

[Anirban]

Dont worry much about salinity. Just before checking the salinity of the salt water set the refractometer with same plain RO/DI water first. You will be just fine.

 

[cmcoker]

Maybe @Randy Holmes-Farley can chime in. There are certain calibration fluids that are more reliable and there is a diy solution recipe in the linked article. Randy has helped me out with a diy recipe using KCl and it matched to the pinpoint calibration solution perfectly.

My understanding is some refractometer are not made for measuring saltwater, even though they are sold to hobbyists for that, so they can be a little off.
By calibrating to the point closer to where you are measuring it is more likely to be accurate at that salinity.
My refractometer does read negative with rodi, but I'm not testing rodi. I am measuring saltwater and would rather have it accurate at the point I am testing.

 

[reef_junkie]

I use a calibration fluid and it calibrates to 1.035. I think there's a reason that the fluid is set so high. Not sure what that reason is but nonetheless I check my meter about every other time I use it.

 

[bubbaque]

1.035sg or 35ppt?

 

[Russ265]

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

 

[reef_junkie]

On the bottle says 35ppt. Now I'm worried, thought that is 1.035

 

[jsker]

I try to run my system at or between conductivity of 33.00 to 33.8 or around 1.025 to 1.0255 or what ever is working. I really does not matter unless you are very low or very high. And with summer arriving your salinity is also going to fluctuate. As long as you are with a point relax. What I did is that my refreactor to the LFS and had them take a reading and did the math to match there's :).

 

[reef_junkie]

Nevermind, I misspoke. 35ppt is 1.025 and that's what I've been calibrating to.

 

[Russ265]

Nevermind, I misspoke. 35ppt is 1.025 and that's what I've been calibrating to.

and what does your ro read?

 

[reef_junkie]

Ive never tested my ro water. I probably will next chance I get though.

 

[Russ265]

Ive never tested my ro water. I probably will next chance I get though.

if it isnt 0. throw it away. (which it isnt)

 

[reef_junkie]

No I think ill keep it. :/

 

[Russ265]

No I think ill keep it. :/

gj. you are relying on someone else's calibration to make yours correct.

your refractometer is shot

its a great bandaid. but at the end of the day you are using faulty equipment.

 

[cmcoker]

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

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