Calibration Frustration - Salinity

-Logzor · Nov 12, 2014

As with most ways of measuring salinity, it completely depends on the calibration of the instrument you're using. I gave up using refractometers, I felt like I could never be confident with the calibration nor were they consistent in their readings. I had purchased two types of calibration solution, both gave different readings.

Same with swing-arm hydrometers, they were never consistent. Never tried a floating hydrometer, seems like this would be a good way to go.

I ended up measuring salinity with a conductivity probe. Since I didn't trust my calibration solution, I mixed up a 1 gallon batch of saltwater using reef crystals, 1/2 cup in exactly 1 gallon of water

I've been using this for a couple years without issue and I've only had to re-calibration once, compared to having to re-calibration my refractometer every time I used it.

Lately I've been thinking, would limewater be the best way to calibrate the probe? Since it maxes out at 10.3 mS/cm then it seemed like a good way to go. I decided to measure a batch of fully saturated limewater just to see how close my conductivity meter would read to 10.3. It ended up coming pretty close, around 10.1 or 10.2.

Just something to think about, I got burned too many times by refractometers. Regarding limewater calibration, what factors do I need to keep in mind? It would be really convenient for me to double check my calibration by just checking against limewater.

Randy Holmes-Farley · Nov 12, 2014

The limewater calibration is probably ok if you do it very close to 25 deg C. Make sure there is plenty of excess solids.

If you have a scale, a salt solution may be better since it is closer in conductivity to what you want to measure.

I give recipes for calibrating or checking all of these devices here:

http://www.reefkeeping.com/issues/2004-06/rhf/

-Logzor · Nov 12, 2014

Randy Holmes-Farley said:
The limewater calibration is probably ok if you do it very close to 25 deg C. Make sure there is plenty of excess solids.

If you have a scale, a salt solution may be better since it is closer in conductivity to what you want to measure.

I give recipes for calibrating or checking all of these devices here:

Reef Aquarium Salinity: Homemade Calibration Standards by Randy Holmes-Farley - Reefkeeping.com

Thanks Randy, my limewater is typically much cooler than 77 since it's in my top off reservoir in my basement. I'm sure it's closer to 68 or maybe even less depending on the month. What can I expect when measuring cooler limewater with the conductivity probe?