Refractometer

clown-lover

clown-lover

AC Members
May 26, 2007
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Fargo, ND
Well I just received my refractometer (Grins your recommended one) this week and have done some testing. Went and bought distilled water (per suggestions) for calibration and got everything set up.

The good news is that my hydrometer wasn't that far off of my refractometer.. When testing them against each other the hydrometer was at 1.025 and the refractometer was just a hair underneath.

I really do like the ease of use though and thank you Grins and everyone else for getting me on track.
 
LOL.. I never really thought of it like that... but.. yeah kinda.. I do appreciate the help though. I guess I was just lucky though with that hydrometer and that it wasn't that far off.. It makes me feel a lot safer knowing that I my salinity levels weren't horribly off.
 
Thing is that the hydrometer isn't consistent. It could be right this time but different the next. One way to show this is to take several readings with it one after the other. Often you'll get different readings each time.
 
I'll have to do that.. I'll let you know.
 
I highly recommend against the use of distilled and or ro/di water for calibrating refractometers. It works in theory (in an ideal world), if the unit is very accurate in every section of the scale. However, in reality, most are not--even more expensive models--especially as salinity gets higher. My recommendation is to use a calibration solution that matches natural seawater in terms of salinity. It will get you much closer to your target than calibrating with freshwater would.

Here is a handy link/article that tells you how to make it with morton's iodized table salt:
http://www.reefkeeping.com/issues/2004-06/rhf/index.php
 
Thanks for the info Amphiprion I appreciate it.. I'm making a solution right now to see how it checks out against my other readings.
 
Amp - Let me preface this by I respect your opinion a great deal so please read this as my findings performing this task. BTW thanks for the link it was a good read.

While I realize that it is the poor mans way of making a calibration liquid, it my refractometer wasn't that far off from the fresh water calibration method. The difference was 1ppt. Which actually put the refractometer back to where it was set when I received the unit.

I wasn't impressed however when I read the after publication note

[[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.]]

While I realize that 1 ppt is not that great of a difference in the salinity level < .01 sg, the difference in the test that I did against distilled water was 1ppt. I also realize that this test is actually more accurate simply because you never know what the distiller did with the water. This being more important when we have water manufacturers having to label their bottles (This water is the same as what you get from you tap).

If I had a better measurement method other than fill a 2 Liter coke bottle (that I actually had a gram measurement scale) I would have felt better about my overall results. So I'm gonna see if one of the ladies at work who diet have a gram measurement scale that I can use to check the results.

I put my digital thermometer into the solution and ensured a reading of 77 degrees F or 25 degrees C. (I actually used a heater for a while until the temperature raised to that reading.) I did test this against my hydrometer several times as well and against my tank and got different readings. The thing I noticed that you had to be really careful on the hydrometer is to ensure that there were absolutely no air bubbles on the needle (I have the instant ocean kind) at all. That effected the results greatly. However Grins was correct in that if you are not absolutely careful there can be a considerable variance in how a hydrometer performs.

Overall I am satisfied that my refractometer is as close to calibrated that I won't be getting huge errors in my specific gravity for the time being. I would like to find a calibration fluid though that is made for a refractometer so I can check against that.. However as of this point the only ones I have found are for the pinpoint monitor system.
 
I should have added that you don't use the coke bottle. When I made my solution, I used a graduated cylinder (precise to 1 mL( and a mass scale (accuarate to .05 g). That way I knew it would be accurate. If you don't normally have access to that sort of equipment, you could always try your local college/university (or order some stuff online). Most places are more than willing to let you try it. I am glad that your refractometer tested out ok. That is the whole point--not to calibrate it precisely to 1.0264 (though if the solution is accurate enough, that is certainly possible), but to see if it is close. I know with my refractometer, I was off by .004, which is significant when it was calibrated to R.O. water.

I can assure you that if done with accurate equipment, this standard is better than anything on the market. There is a Pinpoint salinity calibration solution, but for whatever reason, it has become too variable. Back when they were more precise, these calibration solutions would match perfectly with the homemade standard. I'll trust my multiple solutions far more than anything I can get anywhere else. BTW, I have also made standards for nitrate, phosphate, and alkalinity. That is how I know my test kits are off :D .
 
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