Understanding Conductivity Readings

chipmunkofdoom2 · Oct 4, 2017

I currently measure salinity using conductivity with an American Marine Pinpoint Conductivity meter. I would like some help understanding some readings I'm seeing.

Last week I was mixing saltwater using Red Sea blue bucket. The salt mixes to around 1,245 ppm Mg, 425 ppm Ca and 8.45 dKh carbonate alkalinity at 52 mS. Before adding the new water to my tank, I typically increase Mg to around 1,350 ppm using a MgSO4/MgCl2 solution. I also increase Ca to around 450 ppm using a CaCl2 solution. After I added Mg and Cl solutions the new salt water, I noticed the conductivity had risen. Before dosing, it was around 51.6 mS. After dosing Ca and Mg, it was around 52.0 mS.

What is causing the increase in measured conductivity? Are the chloride ions from the MgCl2 and CaCl2 combining with free sodium in the water and forming more salt, and thus raising salinity? Or is the conductivity simply higher because Ca and Mg in general add to conductivity without increasing salinity?

For background, when measuring newly mixed saltwater, I shut off all the pumps so flow and potential electrical leakage do not affect the readings. I always let the probe temperature compensate for at least three minutes, and set a timer to be sure of that.

Thanks!

Randy Holmes-Farley · Oct 4, 2017

Salinity, both by definition, and as measured by all of the different devices that aquarists use, counts all of the ions that are in the water.

Magnesium, calcium, chloride, and sulfate, all contribute to salinity and to conductivity, so yes, it does rise when you supplement these things.

Randy Holmes-Farley · Oct 4, 2017

These have more:

Reef Aquarium Salinity: Homemade Calibration Standards by Randy Holmes-Farley - Reefkeeping.com
http://reefkeeping.com/issues/2004-06/rhf/index.htm

Using Conductivity to Measure Salinity
AquariumFish.com :: Aquarium Fish International :: The monthly magazine for both novice and expert aquarium hobbyists.

chipmunkofdoom2 · Oct 4, 2017

Thank you very much Randy. I had read the first article (I actually use a DIY standard to calibrate my meter), but I hadn't seen the second one. It's a shame that it's no longer up, it has a lot of good information.

I think my confusion arose from the definition of "salinity" as we discuss it in the hobby. When we talk about salinity and what it should be in a reef tank, I think I was mistakenly understanding that my aquarium water has quite literally 35 grams of salt (NaCl) per every liter of water. I was not aware that we were measuring the concentrations of all ions and not just plain salt.

Thanks again for the information.