salinity and refractometer question

jgvergo · Feb 11, 2016

Busa1340 said:
I believe they all say calibrate with RO water.. But I like the fact with the solution ur calibrating it at exactly 35ppt or 1.026 which is what we want in our reefs.. Just my .02.

This makes very good sense scientifically. It dials in the accuracy of the device ad leaves precision as the only remaining issue.

Randy Holmes-Farley · Feb 11, 2016

Every refractometer will claim to be properly calibrated with distilled water, because that is the way the are made to be used.

Unfortunately, many are not actually made to be used to measure seawater, but rather sodium chloride solutions.

So even if perfectly made and perfectly calibrated with pure fresh water, they will be off. How much they will be off depends on whether you are measuring in ppt or in specific gravity units.

There are now a few brands of refractometer on the market designed specifically for seawater.

IF they are PERFECTLY made, they can be calibrated with pure fresh water, and will measure seawater correctly. These types will always say something like "True Seawater Refractometer".

BUT every refractometer (except certain digital ones which electronically prevent this type of calibration) will be properly calibrated for measuring seawater when calibrated with a 35 ppt standard, even if they are designed for sodium chloride solutions, and even if they are not perfectly made (which, IMO, may be a concern for the cheaper models).

So there's no reason to not use a 35 ppt standard (assuming it is reliably made, I have published a recipe, but other recipes work too) and there are several reason to use it.

treedog5 · Feb 11, 2016

I'm use the cal solution but can't get one of my refractometers to cal to save my life. Anyone ever had this problem?

Mfreddy · Feb 11, 2016

Thanks Randy. I needed a full explanation.

Randy Holmes-Farley · Feb 12, 2016

treedog5 said:
I'm use the cal solution but can't get one of my refractometers to cal to save my life. Anyone ever had this problem?

What happens exactly?

treedog5 · Feb 12, 2016

Once I put the solution on the eye piece I can't get the scale to go above 0. If I put saltwater on the eye piece it will go up to about 10. It was working fine before I tried to cal it...

Randy Holmes-Farley · Feb 12, 2016

treedog5 said:
Once I put the solution on the eye piece I can't get the scale to go above 0. If I put saltwater on the eye piece it will go up to about 10. It was working fine before I tried to cal it...

Which solution? Fresh water or 35 ppt standard?
You cannot adjust the screw enough to get to proper calibration?

Wiz · Feb 12, 2016

I would like to add the swing arms are complete garbage!! I had four at one time and everyone read differently. and then I got my refractometer and found all four were wrong. Personally I would toss that thing.

Randy Holmes-Farley · Feb 12, 2016

FWIW, if you have a gram scale, you can make a DIY solution using table salt to check any of the devices we use:

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

treedog5 · Feb 12, 2016

Yes I'm using the 35pt solution.. And yes I can't get the scale to get a true reading.

Randy Holmes-Farley · Feb 12, 2016

treedog5 said:
Yes I'm using the 35pt solution.. And yes I can't get the scale to get a true reading.

So it can only get to 10 ppt and not higher? I've never heard of that before. I didn't even realize one could move it that far off of calibration.

Are you sure you are using it correctly?

See if these directions help:

http://www.jbjlighting.com/pdfs/refractometer_instructions.pdf

If that fails, I might try to take it apart (of possible) and see if something inside is clogging it or some such thing.