Refractometer, not sure how to use?

[Tigerarmy]

Just got a refractometer, no instructions, not sure how to use it. I get a reading of 1.026 on my hydrometer and get 4 on my refractometer. I was told to multiply that number by 4 and that would give me my SG? If that's the case I'd be sitting at 1.016?
Here is a link to a meter that is almost identical to mine.
http://www.sciencefirst.com/vw_prdct_mdl.asp?prdct_mdl_cd=78-620

[slakker]

which model or brand... mine reads the salinity without doing any math.

But typically, just have to calibrate it with pure water to get "1" then read with tank water...

[christyf5]

Ok so typically with most refractometers you have SG running down the left side of the scale when you look through, and salinity running down the right.

You should not be getting a reading of "4" that would mean you hardly. Test it with RODI water or bottled water would work and make sure that the reading is either 1.000 SG or 0 salinity. If it isn't use the little screwdriver to move the blue line to that mark. Then do your test making sure you have enough water in the "chamber" and the lid is down tight enough that the water covers the blue grid of the chamber.

Not sure about multiplying by 4. Never heard of that.

Hope this helps :biggrin:

[Tigerarmy]

It only goes up to 28% and it has no salinity on the left hand side. It does zero out, and I get the same reading of 4.

[christyf5]

I'd return it, its not going to do anything for you as most SW tanks need a salinity of 28-32ppt.

I don't think its a salinity refractometer. Seems kind of odd to me that you don't have the full scale on it.

[Canadian]

Furthermore, if you do end up getting a refractometer more appropriate for a saltwater tank don't use RODI, distilled water, or the water provided with the refractometer to calibrate it. Spend the extra 4 or 5 bucks and get some of the PinPoint 53.0 mS Calibration Solution so you can ensure more reliable readings in the range that you will actually be running your tank.

[Tigerarmy]

It is a salinity refractometer. I got it from work. It's used for measuring salinity in water that comes out of oil wells.

[midgetwaiter]

It's not the right kind of refractometer, the scale is way to big. Seawater averages 35 parts per thousand which would be 3.5%. That is equivalent to 1.0265 SG.

I'm not sure if that will measure the refractive index of seawater correctly though. It should be close enough if you keep it in between 3% and 4% but keep in mind that 3% is about 1.023 and 4% is about 1.030 so you may be better off with something that is more precise.

[Tigerarmy]

The refracto I got has lines in between 3 and 4.Is there a mathmatical equation I can use to find out SG from ppt?

[midgetwaiter]

The refracto I got has lines in between 3 and 4.Is there a mathmatical equation I can use to find out SG from ppt?

Not an easy one, there is no linear relationship between specific gravity and salinity. Given that you aren't even measuring the salinity directly but instead are reading the refractive index of the sample and then use that to determine the salinity it would never be reliable. I can give you a guess but it's predicated on a big assumption. If the refractometer you have is designed to measure the refractive index of sodium chloride solutions then this should be close. If not it's not then this might not be anywhere near correct.

The refractive index of 35ppt seawater is the same as a 36.5ppt solution of sodium chloride. You can get a seawater equivalent reading by multiplying your measurement by .96

Take the number you get and refer to table 3 in this article:
http://www.reefkeeping.com/issues/2004-06/rhf/index.php