How to calibrate a refractometer?

[Recife]

Or better still, how do I know it needs to be calibrated?
I just got mine from marine depot and I want to make sure the value reported is accurate? There are no instructions.

[GL]

Get some distilled water and test it, it should read 0. If not there should be a small screw used to make adjustments.

[mg426]

Take a look in the chemistry forum. Randys artcles on making a calibration (homemade refract. ) Standard

[bigfruits]

look under the foam in the refractometer case. there should be instructions.

[Amphiprion]

Unfortunately, I would not automatically assume that the manufacturers calibrated it properly in factory, so it could be off even when using distilled or RO water. I second mg426 in checking it against Randy's standards in the chemistry forum
Homemade Calibration solutions (http://www.reefkeeping.com/issues/2004-06/rhf/index.php)

[GL]

<a href=showthread.php?s=&postid=7421879#post7421879 target=_blank>Originally posted</a> by Amphiprion
Unfortunately, I would not automatically assume that the manufacturers calibrated it properly in factory, so it could be off even when using distilled or RO water. I second mg426 in checking it against Randy's standards in the chemistry forum
Homemade Calibration solutions (http://www.reefkeeping.com/issues/2004-06/rhf/index.php)

Unless your a chemist I think this method leave way too much room for error. Even Randy says how much the measuring devices can vary.

I mean look what you need to do:

For a rougher measurement in the absence of an accurate water volume or weight measurement:

1. Measure ¼ cup of Morton's Iodized Salt (about 73.1 g)
2. Add 1 teaspoon of salt (making about 79.3 g total salt)
3. Measure the full volume of a plastic 2-L Coke or Diet Coke bottle filled with purified fresh water (about 2104.4 g)
4. Dissolve the total salt (79.3 g) in the total water volume (2104 g) to make an approximately 3.65 weight percent solution of NaCl. The volume of this solution will be slightly larger than the Coke bottle, so dissolve it in another container.

[[Notice added post-publication: the standards described here that use Coke bottles are subject to variation in the volume of a 2-L Coke bottle. It has recently come to my attention that such 2-L bottles can vary in total volume, and that this can lead to at least a 1 ppt error in the salinity of the standards matched to seawater salinity of 35 ppt. Standards made with accurate measurements of salt and water should still accurately match 35 ppt.]]


Your better off setting to 0 using distilled water, then find someone else with a refractometer or two even and double check it.

I believe in the KISS method, and mixing your own leaves way too much room for error.

[mg426]

Post it in the chemistry forum see just what randy says about the whole thing.

[Recife]

Will do.

Maybe if I brought it to my LFS and asked them to compare mine to theirs would help?

I'm a bit worried because my hydrometer also showed 1.025 and the refractometer is showing 1.030!
Everything in the tank looks very healthy and the SG has been the same for over 2 months.

[Amphiprion]

Do not use the 2L plastic bottles. There are several places where you can get accurate measuring instruments (a common cookware measuring cup that has Litres as a unit). This is usually more than accurate enough. If you have access to lab equipment (or someone who can do it for you), then all the better. I found that even with the supposed "precision" factory settings, that calibrating to freshwater still usually results in a +/- .002 error.

[MCary]

Do what I do. I work in an hospital lab and have calibration and control solutions that I check mine with.

Mike