Refractometer calibration

Cygnus · #1 04/11/2004, 02:22 PM

I purchased a refractometer from premiumaquatics, and set it to zero on some RO/DI water. I then saw a post about getting a known higher salinity sample so I did that. It came labeled Pinpoint Salinity Monitor 53.0mS calibration fluid. It came with a table that said 53.0mS should read 1.0259. When I use it in my refractometer, i get between 1.027-1.028 -- closer to 1.028. What should I do now? I am now worried that it is not accurate. What should I do, calibrate it using the 1.0259 sample? Then it will make RO/DI water below zero on the scale.

Thanks for any advice.

mdenigris · #2 04/11/2004, 07:08 PM

It's more important that your meter be accurate at 1.0259 than at 1.000. If you calibrate using the fluid, any water measured that is 1.026 will measure exactly correct. The meter will become more inaccurate as you move from that SG.

If you calibrate at 1.000 using ro/di, than the meter will be exactly accurate when measuring ro/di, and less accurate measuring water at higher SG.

So, I'd calibrate with the fluid and live with the inaccuracy at 1.000

sorenb · #3 04/12/2004, 07:10 AM

The calibration fluid has a conductivitity of 53.0mS, but is it a NaCl solution? If another salt is used in the fluid, the refractive index (measured by the refractometer) may not be what you would expect.

sorenb

Cygnus · #4 04/12/2004, 07:51 AM

I believe it is the calibration fluid sold for the pinpoint salinity monitor probe (electronic device). I did not say whether it was NaCl or not. I think I will just trust the distilled zero and leave it at that. Thanks for the response.

Randy Holmes-Farley · #5 04/12/2004, 08:21 AM

I agree. You need different calibration fluids for different types of instruments. For example, making calibration fluids yourself using table salt requires different amounts:

From an older thread on such standards, by way of example:

Here's a compilation of salinity standards that people can make out of dry table salt and water (using a good balance):

If you want an S=35 standard for specific gravity, try 3.7 weight percent NaCl in water at 20 deg C

If you want an S=35 standard for refractive index, try 3.6 weight percent NaCl in water

If you want an S=35 standard for conductivity, try 3.17 weight percent NaCl in water

Here are some related threads:

http://www.reefcentral.net/vbulletin...threadid=64577

http://www.reefcentral.net/vbulletin...threadid=62670