Calibrating My Refractometer.

[thefirethief]

I've got an SG reading of 1.004 in my new tank despite the fact that I added the salt (reef crystals) about a week ago and used the measurements on the box as a rough guide to how much I was adding. So I'm thinking I must not have calibrated my refractometer right.

The part I'm struggling to understand in the instructions manual is the part about calibration and it says 'make sure the ambient room temperature is correct for the solution you are using (20C/68F)'. Does that mean the room temperature and solution need to be 20C for it to be calibrated properly? Does the Temperature Compensation system affect this process?

I'm lost. According to the readings I've been getting I'll need to add about another 4KG of salt to 110L of water.

Any ideas?
Mark

 

[BigC]

The instructions that came with my Deltec refractometer states that you calibrate it using RO water and set the instrument to zero. then by adding your salt mix this should give you a relevant reading. The instrument is temperature conpensated.
Regards
BigC

 

[Dorkhedeos]

ive heard it was better to get a solution that you know the salinity of to calibrate it instead of using ro

 

[Danno]

SkiFletch gave me this information in my journal, it might help you out too:

Ok I'll paraphrase Randy's directions for you guys

1) Procure 2L soda bottle or anything else that holds 2L of water
2) Measure into the bottle 1/4 cup of plain old iodized table salt
3) Add 1 more teaspoon of salt to the bottle
4) Add 2 liters of RO water to the bottle
5) Shake vigorously until dissolved
6) Let stand to equillibrate to room temp

The resulting solution has a salinity of 35ppt or a specific gravity of 1.026 at 25 degrees C. So, if you make that solution and your Hydrometer tells you say 1.022, it is reading .004 below the standard and should be normalized. So then you take that same hydrometer and measure your tank and get 1.024 for a reading, that in actuality is 1.028 and too high... Make sense?

 

[thefirethief]

Thanks a million for the input everyone. I've actually worked out that it's my arithmetic that is way out. Added about a tenth of the salt I was supposed to.

ive heard it was better to get a solution that you know the salinity of to calibrate it instead of using ro

I took my refractometer to the LFS I bought it from and the guy tested it on a few tanks there. I had calibrated it right so RO water must work fine. I'm guessing if your RO unit is working correctly then the SG will be zero.

problem solved anyway.

 

[SkiFletch]

I'm guessing if your RO unit is working correctly then the SG will be zero.

Actually it has to do with the quality and orientation of the prism and lenses inside the refractometer. In general if the refractometer is a quality unit, RO calibration works very well. If it's a bargain chinese knockoff type (like I have ), then its better to calibrate with a salt solution with the instructions Danno so helpfully dug up for me

 

[n3ont3tra]

Only question about the instructions is how much is two liters? At the verrry top of the bottle, where if you add any more it will over flow? Or down a few inches where the soda is normally filled to? Or is it so small of a difference that it doesn't matter?

 

[Danno]

I would grab a measuring cup (liquid) and measure out of the liters in there and then add it to another container.

 

[n3ont3tra]

Good idea.

google says 2 liters = 8.45350564 US cups

So as soon as I finish off the Dr. Pepper I'll try it out.

 

[SkiFletch]

The instructions I linked above were created by Dr Randy Holmes-Farley for filling the container to the top which is just slightly above 2L.

 

[n3ont3tra]

Thanks Ski.

 

[Musho3210]

2 liters with salt or 2 liters without? salt raises the volume......

 

[Dorkhedeos]

2 liters with salt or 2 liters without? salt raises the volume......

ive never heard of salt raising the volume. i believe the only time salt takes up volume in water is when it hasnt dissolved yet.

 

[SkiFletch]

salt has volume too, and even in aqueous solution it still increases the volume

 

[Musho3210]

dissolving means the molecules spread out. If the you put in 10 salt molecules (i'll try to keep the numbers low) and you have 100 water molecules. Then in total you will have 110 molecules. Nothing came out when you added the salt. So salt will have volume. If you dissolve a lot of oxygen or if you dissolve a lot of carbon dioxide in your tank, the amount of molecules will raise the total tank volume (not enough to notice though since there is so little)