Shout out to Randy! Refractometer calibration thanks

[lawnerd]

I have a refractometer whose instructions called for calibration with RODI water, which I did.

I recently read Randy’s post on salinity, making a salt standard at S35, and calibration with the standard. I did so, and the salinity readings were very different based on this calibration. A three point difference - the RODI calibration was reading the standard as S38!

The recalibrated refractometer now reads the standard at S35 and RODI reads way below the zero point on the scale. Nevertheless, I just ordered a different refractometer because I don’t fully trust the one I have given the range of the scale isn’t correctly reading pure water.

Lesson learned, don’t cheap out on your tools.

 

[Lasse]

In general terms - if you do a single calibration close to the expected value - you should trust these readings. I always calibrate with 35 ppt - not RODI water. A single calibration can never give you the slope of the calibration curve - just that your reading is likely to be true around the calibration point. If you have done a correct calibration solution of 35 ppt - and then reads 35 ppt on your water - trust that you are around that number

Sincerely Lasse

 

[Randy Holmes-Farley Verified Community Expert]

It is normal and expected for a brine refractometer to be off at 0 ppt when properly calibrated at 35 ppt.