who to believe? Salinity conundrum

[becon776]

So Ill try to summarize... no let me sum up.
built tank - used rsr450 1/19. 1st build with dry rock PITA. my refractometer was a point of sale. got new "good" one. calibrated with rodi and wemt about my business.

Had nithing but seriously struggled with corals fish always super healthy. chased all kinds of possible solutions- Swapped Lighting, PO4 (lack thereof), NO3 (lack thereof) struggled with getting fuge up and running, instituted AWC, added autofeeder 4x's/dsy to get nutrients etc.. no partucular order there.
Now i have never really trusted the apex cond probe ive read too many issues but it always read low but my refractimeter read 1.026 so i trusted the tried and true. last brs order on a whim i purchased refractometer calibration sol'n. i had to break out the little screwdriver. according to the newly calibrated refactometer my tank was running at 1.022 and has been for over a YEAR! immediately put my ato tube in awc bucket and let it go. after all this is lt turns out the apex probe was right, it now agrees with refractometer that was calibrated with 35ppt solution. Funny thing is when i put in the rodi it does NOT read zero. WHY??? In true old school fashion i started listening to my corals again and as the salinity climbed corals that had hardly opened at all are now fully opened. Zoas that never opened are now. Such an AHAH moment for my tank. Kind of embarrasing given that i have been reefing since 2001 that i was running 1.022 thinking i was @ 1.026 for over a year

 

[Webslinger]

That will do it.

 

[NS Mike D]

I never understood the instructions on RC and IO (and I presume other salt mixes too) mixed to well below 35ppt

"Recommended Specific Gravity Range: 1.020-1.026 at 77°F. NOTE: 1.4 lbs of Reef Crystals is formulated to create 5 gallons of saltwater at a specific gravity of 1.021."

 

[homer1475]

Never calibrate a refractometer with RO/DI. You always want to calibrate closest to what your going to measure.

 

[NS Mike D]

Never calibrate a refractometer with RO/DI. You always want to calibrate closest to what your going to measure.

don't you calibrate a refractometer to the zero?

 

[homer1475]

don't you calibrate a refractometer to the zero?

Absolutely not! Worst practice there is, and one that should be emitted from the instruction.

I think it was @Randy Holmes-Farley did an article a while back on RC about why one would use calibration fluid instead of RO/DI.

Basically in laymens terms, with only a 1 point calibration, if you calibrate way out of the actual usable curve, it throws then entire curve off.

 

[NS Mike D]

looked it up after I posted. So refractometers are imperfect either by manufacture or by calibration that will make the slope off. So the further along the slope from where we calibrated, say from the calibrated 1.000 to our measure of 1.026, the larger the error.


Makes sense, so if you use rodi and calibrate at zero, there can still be an error in the refractometer.

But if you buy or make a calibration solution, any error should be zero at the calibrated point.


Of course consistency is more important than the actual. number with a range of say 1.024 and 1.027 so if you consistently mix with the same error, no foul no harm.

Randy's article on the subject.

http://reefkeeping.com/issues/2006-12/rhf/index.php#13

 

[Randy Holmes-Farley]

lt turns out the apex probe was right, it now agrees with refractometer that was calibrated with 35ppt solution. Funny thing is when i put in the rodi it does NOT read zero. WHY???

don't you calibrate a refractometer to the zero?

No, that is a serious mistake, unless it is a true seawater refractometer, which is not the norm and are more expensive.

Most hobby refractometers sold are actually made for sodium chloride brine, not seawater.

If you correctly calibrate a brine refractometer to read 35 ppt seawater correctly, it MUST read RO/DI incorrectly.

 

[Doglips56]

Absolutely not! Worst practice there is, and one that should be emitted from the instruction.

I think it was @Randy Holmes-Farley did an article a while back on RC about why one would use calibration fluid instead of RO/DI.

Basically in laymens terms, with only a 1 point calibration, if you calibrate way out of the actual usable curve, it throws then entire curve off.

So now I’m wondering where my numbers are. Have always zeroed with RODI.

 

[NS Mike D]

Time to get calibration solution.

 

[becon776]

So now I’m wondering where my numbers are. Have always zeroed with RODI.

right SOOOOO cheap. to not do this. i prob spent over 1k to figure out why sticks weremt surviving. The answer costed me 5$

 

[becon776]

If you correctly calibrate a brine refractometer to read 35 ppt seawater correctly, it MUST read RO/DI incorrectly.

this!