Salinity Calibration Solutions and why are they different

[pickupman66]

Randy,

I went through my probes last night as part of regular (irregular in my part) re-calibration checks and procedures. one of them is a Salinity Probe used with my Controller. when I made the speficied calibration solution per your article, it began to make my wheels turn. once I was properly calibrated, it really made my head hurt.

WHY do we use different solutions to calibrate the different pieces of equipment? Specifically, why is my Salinity Probe a tsp of salt in 182 ML but my Refractometer is that same measurement of salt in 166.6ML of water?

the main reason I ask is because my Refractometer reads at what my eye perceives as 35PPT yet my Probe that was just calibrated reads 33PT in my water.

 

[dragon99]

The probe is measuring conductivity and the refractometer is measuring light diffraction. Those have different properties.

This article of his discusses the different measurements:
http://reefkeeping.com/issues/2004-06/rhf/index.php

 

[pickupman66]

Dragon,

thank you. so, is my tank at 35ppt or 33ppt? does it make a difference? Should I push it up to 35PPT per the Conductive salinity probe?

 

[dragon99]

Sorry, just realized I linked the to same article you already did.

There's probably a difference in the readings due to slight variations in the solutions you made (ie neither is perfect). Maybe make another batch of standards solution as a double check? Or, if you could buy something like this which should give the same reading on both probe and refractometer:
http://www.bulkreefsupply.com/aqua-craft-refractometer-calibration-fluid.html

 

[pickupman66]

Thanks Dragon,

I guess my question should have been a bit more precise.

for a Refractometer, it is 3.65 weight solution. (1tsp in 163.6 ml)
For a Conductivity probe, it is a 3.29 weight solution. (1tsp in 182 ml)

I guess I dont understant why they are so different. yes they read different ways with different technologies, but they are both striving ot achieve the same number correct?


Which do I trust? LOL>

 

[dragon99]

Yes and no. The NaCl solution you are making isn't the same as seawater, which is why it takes 2 different solutions for the probe and refractometer. When the conductivity of the NaCl solution matches seawater @ 53 mS/cm, it's refractive index doesn't. So you have to change the solution to get 35ppt on a refractometer.

Now, once you've calibrated both with their respective solutions, they SHOULD give you the same reading on your tank. Since they aren't agreeing, one (or both) of your solutions is probably off, or one of the measuring devices is faulty.

 

[pickupman66]

Yes and no. The NaCl solution you are making isn't the same as seawater, which is why it takes 2 different solutions for the probe and refractometer. When the conductivity of the NaCl solution matches seawater @ 53 mS/cm, it's refractive index doesn't. So you have to change the solution to get 35ppt on a refractometer.

Now, once you've calibrated both with their respective solutions, they SHOULD give you the same reading on your tank. Since they aren't agreeing, one (or both) of your solutions is probably off, or one of the measuring devices is faulty.

OK. I see. Ill make some solution up for my Refractometer this evening and see what I can get out of it. If it dont match up, I reckon ill have some soul searching to decide which I want to go by. hopefully the +/- 2 ppt wont harm much. But could it lead to issues keeping sps?

FWIW, I use Seachem Reef Salt in my system.