Just wondering...

[rayjay]

I don't even own a refractometer, but, knowing there are salt AND seawater versions that have different slopes, does the makeup of the calibration fluid affect accuracy?
I've never seen anything about the makeup of commercial calibration fluids, only Randy's DIY version so I was wondering if a seawater refractometer is still accurate when using a calibration fluid of just sodium chloride and not actual seawater salts combined? It's just an old man's curiosity.
(I still use my 24 yr old Seatest Swingarm along with one about 2 yrs newer)

 

[sbash]

In my experience, it depends on the refractometer. I picked up a second hand, high-end, lab refractometer with a lab-grade calibration solution. It needs to be calibrated every time it is used.

I have two cheapo $30 ones that I use most of the time. I have calibrated them once. However, I only use them for the delta between the LFS and my system.

Now, to actually answer your question. Yes, it will technically be inaccurate. At least to some degree; hard to say how much (probably depends on the salt, lol)... Given the random minerals and dissolved solids in seawater, the refractive index will be different than pure water with straight salt.

 

[Martin Kuhn]

Refractometers use the refractive index of a fluid as functional replacement size to measure what we want .... the seawaters density

- if you use a refractometer not for seawater it will give you wrong measurements, even if you calibrate it to "one point correctly" (... all other points below and above will have a certain measurement error. The more away from the point, the bigger)

- if you use calibration fluid "a" (just sodium) to calibrate "one point" of refractometer "b" (saltwater) this should be ok, as long as
- the calibration fluid really has an accurate/known density
- this density is in the area where you want to measure your saltwater(1.026 density @77F )

 

[Randy Holmes-Farley Verified Community Expert]

I don't even own a refractometer, but, knowing there are salt AND seawater versions that have different slopes, does the makeup of the calibration fluid affect accuracy?
I've never seen anything about the makeup of commercial calibration fluids, only Randy's DIY version so I was wondering if a seawater refractometer is still accurate when using a calibration fluid of just sodium chloride and not actual seawater salts combined? It's just an old man's curiosity.
(I still use my 24 yr old Seatest Swingarm along with one about 2 yrs newer)

Not if it is made properly.

Any standard with its refractive index matching 35 ppt seawater is perfect for both types of refractometer, regardless of the chemicals in the solution.

 

[Randy Holmes-Farley Verified Community Expert]

Now, to actually answer your question. Yes, it will technically be inaccurate. At least to some degree; hard to say how much (probably depends on the salt, lol)... Given the random minerals and dissolved solids in seawater, the refractive index will be different than pure water with straight salt.

It is different, but standards take that into account and so have no such inaccuracy. My DIY, for example exactly matches the index of refraction of 35 ppt seawater by being 36.5 ppt (3.65 weight percent) sodium chloride.

Refractometers And Salinity Measurement
http://www.reefedition.com/refractometers-salinity-measurement/

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

 

[rayjay]

Thanks!!

 

[Randy Holmes-Farley Verified Community Expert]

You're welcome.

Happy reefing.