Refractometer Calibration Inconsistencies

[JHawlz9989]

Hey guys,

For some reason I'm really struggling to get back to the basics on refractometer calibration. I bought the portable refractometer from Drs Foster & Smith a long time ago, which was rock solid in its calibration. However, it was dropped over the years and seems to fall out of calibration really easy now. Every now and then I can turn the screw which feels really loose for 1/4 - 1/2 of a turn, and nothing happens to the calibration before I feel it catch again and start recalibrating. I recently bought the BRS refractometer as an alternative.

Now I seem to get inconsistent readings on both refractometers, and one of the variables seems to be how angled the instrument is relative to the light. Sometimes if I'm looking at the light directly, I can calibrate it to 35ppt, but if I hold the refractometer parallel to the ground, it seems to fall to 33 or 32 ppt. In fact, sometimes the reading seems to change on a continuum based on how angled the instrument is. Sometimes the reading is consistent, but I have my hard cutoff on the blue line, and then another softer blue line 1 or 2 ppt below it. I've tried multiple light sources to no avail. I also look to make sure there are no air bubbles, and I don't see anything.

Can anyone provide some guidance as to what I'm doing wrong?

Josh

[kinnadian]

The glass face of the refractometer should be parallel to the ground, requiring you to hold the body of the refractometer at an angle.

The clear plastic panel that pushes the water on to the glass surface should be perfectly parallel to the glass surface as well. You may need to tweak the little black bit that is connected to the clear plastic panel.

You are aiming to achieve a consistent thin gap between the clear plastic panel and the glass surface. With water on the glass surface and the glass surface parallel to the ground (don't need to worry about looking through the lens at this point), make sure that when you bring down the clear plastic panel it forms an even and consistent water film on the glass (without you holding down the clear plastic panel). If there are patches or it pools to one end of the glass surface, you won't get a good reading.

Also I find using the blue lights of your fish tank much easier to see the measurement than with typical white or yellow household lights.

Also it won't hurt to clean the glass surface and the clear plastic panel with some methylated spirits or ethanol.

Hope that helps.

[Ron Reefman]

I've quit using my refractometer!

I have one and I use regular calibration fluid to keep it accurate.

But then I compared the refractometer reading to my cheap hydrometer. The hydrometer read 0.004 too low, so I put a label on it stating that I should add 0.004 to the reading. That was about 5 years ago. After calibrating my refractometer every so often, I would again compare it to my hydrometer and it was always 0.004 too low. ALWAYS!

Refractometers can be calibrated so they are very accurate. But they need to be re-calibrated because they aren't reliable or consistent. Hydrometers can't be calibrated, so they are much less accurate. But once you know the error you can adjust for it and they are crazy reliable and consistent. At least in my experience and I've turned several people in my local club onto this idea and they have all had similar results.

You don't need a refractometer.

Just take your hydrometer to a friend who has one, or your LFS and ask to compare their freshly calibrated refractometer with your hydrometer and put a label on it!

[snorvich]

I've quit using my refractometer!

I have one and I use regular calibration fluid to keep it accurate.

But then I compared the refractometer reading to my cheap hydrometer. The hydrometer read 0.004 too low, so I put a label on it stating that I should add 0.004 to the reading. That was about 5 years ago. After calibrating my refractometer every so often, I would again compare it to my hydrometer and it was always 0.004 too low. ALWAYS!

Refractometers can be calibrated so they are very accurate. But they need to be re-calibrated because they aren't reliable or consistent. Hydrometers can't be calibrated, so they are much less accurate. But once you know the error you can adjust for it and they are crazy reliable and consistent. At least in my experience and I've turned several people in my local club onto this idea and they have all had similar results.

You don't need a refractometer.

Just take your hydrometer to a friend who has one, or your LFS and ask to compare their freshly calibrated refractometer with your hydrometer and put a label on it!

A very interesting and informative post!!!!

[Docdiggy]

I agree.

[outssider]

here are some hints...

1..whatever light you use to calibrate, use the same light at the same angel when you test your water sample.
2...the refractometer could be very temperature sensitive (room temperature). so always calibrate right before every use.