Funky refractometer readings

Myka · Dec 29, 2017

Every once in awhile when I go to check the salinity with a refractometer I will get a funky number, usually much lower than expected, about 5 PPT. If I wipe that off and try again I will get the number I expect, 35 PPT. This doesn't happen very , maybe once every 100 or so checks, but I have had it happen with different refractometers - from cheap Chinese ones to VeeGee. Visually, everything looks exactly the same both in the faulty reading and the correct reading. Any idea what might be happening?

Fudsey · Dec 29, 2017

Do you clean it with rodi after every use?

Randy Holmes-Farley · Dec 29, 2017

No idea, sorry.

skiergd011013 · Dec 29, 2017

I had the same issues when i used handheld refracts. I now have a digital milwaukee one and that problem is solved.

Cory · Jan 3, 2018

If there are bubbles in thr lense or you dont cover the slide completely with water it will read wrong.

Myka · Jan 6, 2018

Randy Holmes-Farley said:
No idea, sorry.

Exactly, right?!

Myka · Jan 6, 2018

Fudsey said:
Do you clean it with rodi after every use?

No, that's not necessary. It is wiped off with a cotton towel, and polished clean with a microfiber cloth. If there was residual salt, that would cause a faulty reading on the high side.

skiergd011013 said:
I had the same issues when i used handheld refracts. I now have a digital milwaukee one and that problem is solved.

The Milwaukee isn't as accurate as a handheld refractometer, and I travel with the instrument a lot, so a digital one is not the best option.

Cory said:
If there are bubbles in thr lense or you dont cover the slide completely with water it will read wrong.

Yes. Refractometer use 101.

I've been using a refractometer for 24 years, and this weird misreading that shows up once in a blue moon has always stumped me. I have not been able to make it repeatable. I do find it happens more often on the VeeGee than the Link I have.

skiergd011013 · Jan 6, 2018

"The Milwaukee isn't as accurate as a handheld refractometer, and I travel with the instrument a lot, so a digital one is not the best option" maybe....but mine is dead on everytime when i measure my 1.026 calibration fluid. Guess im just lucky.

Fudsey · Jan 6, 2018

Myka said:
No, that's not necessary. It is wiped off with a cotton towel, and polished clean with a microfiber cloth. If there was residual salt, that would cause a faulty reading on the high side.

That was my point, using RODI would cause low reading. I do the same as you mentioned above.

chipmunkofdoom2 · Jan 6, 2018

I used to have something similar happen. I would test saltwater with a clean, dry refractometer. The result would be off, usually low. I would clean with freshwater, wipe clean, then try again. The reading would then be what I expected. I used to begin by calibrating my refractometer, which is how I discovered this in the first place. First test, salinity was always off. Clean, perform second test, salinity was always spot on (or at least very close). Every measurement after that the salinity was steady, but not until after that first measurement. End procedure was always the same, clean with freshwater, dry with lint-free towel microfiber cloth.

I since got a conductivity meter to measure salinity. Wish I would have done it much sooner.

Myka · Jan 6, 2018

Fudsey said:
That was my point, using RODI would cause low reading. I do the same as you mentioned above.

Haha ok good!

chipmunkofdoom2 said:
I used to have something similar happen. I would test saltwater with a clean, dry refractometer. The result would be off, usually low. I would clean with freshwater, wipe clean, then try again. The reading would then be what I expected. I used to begin by calibrating my refractometer, which is how I discovered this in the first place. First test, salinity was always off. Clean, perform second test, salinity was always spot on (or at least very close). Every measurement after that the salinity was steady, but not until after that first measurement. End procedure was always the same, clean with freshwater, dry with lint-free towel microfiber cloth.

I since got a conductivity meter to measure salinity. Wish I would have done it much sooner.

Interesting, I definitely notice this on the first test of the day now that you mention it.