Refractometer Calibration

[GiuseppeDC]

Hi Randy

Based on a recent thread on here I decided to calibrate my refractometer using the home made calibration solution you had mentioned. Well if I did things right then i'm in big trouble. My meter is way off.

This is what I did. Made the solution as specified. Measured the salinity of the solution on the refractometer and found it to be way off. Adjusted my refractometer to be a specific gravity of 1.026 and then tested my aquarium water. My SG was 1.016. Which blew me away since I though that I've been keeping it at 1.024.

Before I go and start raising the salinity I wanted to ask you 2 things.

1. The meter model is a RHS-10ATC that I bought from Marine depot a few years ago. It's an all black and aluminum unit with a dial for adjustment. There is no manufacture specified in the documentation. Did you by any chance test this unit for its acuracy if so how accurate was it in your testing?

2. The documentation for the refractometer says that it shows specific gravity and concentration of parts per thousand salt in water. Is what they refer to as specific index actually the specific index or the refractive index that you mention in the article?

Thanks
Giuseppe

 

[Gudwyn]

My initial try at calibrating my refractometer was way off. I had added 2.0L of RODI to the salt. But I was supposed to add a full 2L bottle (which is more than 2.0L).

In the end, I did some math and added 500ml to 1tbs of salt. Which is probably wrong as well, but calibrating to that solution to 35ppt made the RODI water read zero on my meter. So I felt good and went away happy.

If it doesn't feel right, don't do anything to your tank salinity. Do the calibration again using a new solution or borrow somebody elses refractometer for a cross check.

 

[GiuseppeDC]

Gudwyn

Ok..I did the exact 2 litter thing.. so I guess I should go and get a bottle of soda and re-test.

I agree not to make any changes right away. Everything in the tanks is fine and if my salt is really low then it has been like this for a long time.

I just called a few people that I know locally to see if I can borrow their testing equipment.

Thanks

 

[dandydan]

I was infod by Aqua Medic who supplied my Refractometer that R/O Deionised water is good enough to calibrate to 1.0.

Distilled would have been better but cant seem to find it over here now.

 

[GiuseppeDC]

Well this is turning out to be very interesting. I re-mixed the calibration solution using the proper full 2litter bottle of water and the salt. My refractometer still showed about .004 off.

I borowed someone elses refractometer, his is the type with a blue color and a screw for adjustment. His meter showed the calibration solution at 1.026 and my tank water between 1.024 - 1.025. Which is in line with what I thought I had.

I'm starting to think that my refractometer is no good.

 

[Randy Holmes-Farley]

Yes, do follow the directions exactly, as it is designed for the full bottle, not 2 L, and you must use table salt, not salt mix salt.

I borowed someone elses refractometer, his is the type with a blue color and a screw for adjustment. His meter showed the calibration solution at 1.026 and my tank water between 1.024 - 1.025. Which is in line with what I thought I had.

I'm starting to think that my refractometer is no good.

It may be off, or just need to be calibrated with the salt solution only.