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Salinity Calibration with Profilux

Posted 06/19/2011 at 09:03 AM by snorvich

Balancing a refractometer against a GHL salinity probe.

The issue comes from the fact that the vast majority of reef keepers still calibrate their refractometer to 0PPT or RO water. Unless your unit is designed specifically for "Natural Sea Water" (and 99.9999% are not) then you have created a wrong slope to your unit and it is reading inaccurate.

Why? Refractometers are designed in the main to read NACL (natural pure salt, or brine) they are not designed to read NSW.

So whats the difference? Natural salt water is full of multiple trace elements, solids in suspension, things that will change the refractive properties of the water being tested, where as NACL, or pure salt is, well, pure. There is nothing in it to change the refractive index.

So how do i calibrate my refractometer for reef tanks? You need NSW rated solution, however there are only 2 known commercial products I know of that I would trust. That being FM multi ref test solution and pinpoint 53ms, both industry standard NSW rated products.

So now you have your refractometer calibrated to 53ms or 35PPT, we now have something fairly decent to judge the GHL probe against.

GHL Salinity probe calibration.

I will highlight a method that makes sure you have no reason to distrust the probe against your refractometer, however this is not required and only shown here to make sure we have parity.

So for this i will use the same calibration solution used for the refractometer, that being the 35PPT (53mS FM solution) normally you would use the 50mS GHL solution but as I said above I want to keep parity here with no confusions.

Calibratation Process:

1. leave calibration solution in sump for at least 6 hours, this GHL supplied bottle will float.

2. check the calibration solution temp with the GHL probe, allow a good 5 minutes to stabilize, make note of the temp, it should be the same as the tank.

(the most important factor is temperature being equal on all points, probe, calibration solution and tank temp)

3. set the controller calibration to the solution value, in this case 53ms or 50ms for GHL solution

4. select manual temp and input the temperature of the calibration solution when prompted

5. input the tank temp. (hopefully they are the same)

Follow calibration procedure.

after calibrating leave in the solution and wait for it to stabilize and make sure it reads at least within .5 of 53ms or your chosen solution.

If it does not then repeat as you have not got the temp compensation quite right.

once done cross reference between refractometer and GHL probe, but be aware there is likely to be a disparity due to tolerances but it should be within a few points.
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