Refractometer wonky??

[hunggi74]

I wanted to test my water parameters this morning. Started off testing salinity with a Vertex refractometer. Put my Rodi water in and noticed unit was out if calibration. Had to adjust unit because plain rodi was reading at 1.013 salinity. So recalibrated, and put in my tank water. Sure enough, water read 1.01 salinity. I know my tank water was at 1.025 two days ago after water change and I've got a tunze osmolator to keep it there. So what's up with my Vertex unit?? Thanks!!

[Proteus]

Chance your pipette had water in it

[hunggi74]

No chance. The pipette was empty. I also tested my container of saltwater that I always have on hand and same results (1.01 reading). This water has a heater and powerhead on constantly and tested the same as my DT two days ago. Ill use my old Instant Ocean hydrometer tonight and see what it reads. Just can't figure a reason why the refractometer is acting up...

[Myka]

Vertex refractometers like most in the hobby are "saltwater" refractometers, as opposed to "seawater" refractometers. "Saltwater" refractometers measure the amount of Sodium chloride content, as opposed to "seawater" refractometers that measure entire mineral content of seawater. The scale is different for each refractometer.

Why does this matter? If a "saltwater" refractometer is calibrated using distilled or RO/DI water it will read 36.5-38 ppt when you measure 35 ppt seawater. A "saltwater" refractometer needs to be calibrated using calibration fluid at 35 ppt.

When using RO/DI water you are assuming your TDS meter is also calibrated properly. If your TDS meter is off then your refractometer will also be off.I suppose if you're in a pinch you could just set your refractometer 1.5 ppt higher than 0 when measuring distilled or RO/DI water, but calibration fluid costs about $5 and removes one more point of possible error.

Also, the temperature of the refractometer when calibrating and measuring is important. The temperature of the one or two drops of water you add to the crystal doesn't matter so much as such a small amount of water will quickly become the temperature of the refactometer. The refractometer should be close to 20C when measuring or calibrating for highest accuracy.

I think your RO/DI water is off because you adjusted your refractometer by 0.013 and it just so happens your reading was also off by roughly 0.013. Make sure temperature is correct, recalibrate properly, and re-test.

EDIT: Here's a good article. http://www.theaquariumsolution.com/specific-gravity-salinity-and-its-measurement

[hunggi74]

I will get some calibration fluid to retest the water. And I'll make sure the unit is close to 20c when I'm testing. You mentioned that my Rodi unit might be off? As in the amount if TDS in the water is not zero and that the TDS amount is affecting my reading?

[Myka]

I will get some calibration fluid to retest the water. And I'll make sure the unit is close to 20c when I'm testing. You mentioned that my Rodi unit might be off? As in the amount if TDS in the water is not zero and that the TDS amount is affecting my reading?

I am guessing that maybe your TDS meter is off which implies that maybe it's time to change some filters if you're getting 10 or 13 ppm out of it. What are you using to test TDS? Has that instrument been calibrated recently? :D

[hunggi74]

I bought a BRS 5 stage unit with built in TDS meter about two months ago. Ill get a handheld TDS unit too I guess... The amount if swing I'm seeing in calibration almost makes the unit is busted. Ill take it to J&L and see if they can't calibrate it. Ill also get some calibration fluid from them. Funny, it was them that told me just to use bottled water to calibrate instead of the calibration fluid...

[Myka]

I bought a BRS 5 stage unit with built in TDS meter about two months ago. Ill get a handheld TDS unit too I guess... The amount if swing I'm seeing in calibration almost makes the unit is busted.

The built in TDS meter can be calibrated as well. I'm not sure if J&L carries TDS calibration fluid or not - it's not listed on their website. You can use 442 ppm solution or standard 342 ppm solution, both are NaCl (Sodium chloride). Again, using distilled water or RO/DI isn't the best option as you can't tell very easily if your meter is adjusted too low in which case you would get a false "0" reading.

Calibrating inline TDS meters sucks. The meters are designed to be calibrated using moving fluid. They will not calibrate accurately otherwise. Another tidbit many people don't know. That's why I stick to my trusty handheld TDS meter.

I will check my inline meters by collecting fluid just after the inline meter then use my calibrated handheld meter in a cup to measure. Then I adjust the inline meter while it is running to match what I measured on the handheld. It's not a perfect technique, but it's good for "eye-balling" when on the lookout for problems in the system. I still rely on the handheld meter as my "final say".

Straight from HM Digital, DM-1 and DM-2 calibration technique: http://www.filterwater.com/docs/hmdigital/hmdigital-dm1-calibration.pdf

Ill take it to J&L and see if they can't calibrate it. Ill also get some calibration fluid from them. Funny, it was them that told me just to use bottled water to calibrate instead of the calibration fluid...I'm glad to see they finally carry refractometer calibration fluid. I don't know if they use it themselves, but they used to just use distilled water. It's a terrible habit, and most people who say it's ok don't understand why it's not ok.

[hunggi74]

Thank you for the great advice and insight