Salinity

[Neptune777]

Well, I was browsing around the forums and noticed people talking about properly calibrating your refractometer. As it turns out Randy Holmes Farley does not recommend using RODI water to do a calibration. He put out an article on how to make your own standard solution using Mortons Table Salt and how to calibrate at close to natural sea water levels (rather than to zero).

Here is the article:
http://reefkeeping.com/issues/2004-06/rhf/index.php


I just tested mine and my Refrac was way off!!!! I have had my salt levels really low thinking I was right on. Instead of 1.026 I was at 1.020 . My QT tank I thought was at 1.009 but it was actually at 1.003....fish seem fine and I am sure the ICH is dead ;) but I will adjust up now knowing I was off.

I would like to verify if my measurements are now correct...

Any one else looked at this?

[JMBoehling]

Dave,

Interesting read. What does your RO/DI measure now? I hope 0.00 :)

Later,

Jim

[J. Montgomery]

Yeah, I've read the article but I've been too lazy to make the reference solution. Maybe your post will motivate me to do that.

[surfnvb7]

isn't the calibration solution essentially just ro/di water that has been distilled? :confused:

my refractometer is about 4 years old, and came with some calibration solution in a little eye dropper bottle. i get around to testing it maybe once a year and its always within +/- 0.003

[J. Montgomery]

Update: I just made the reference solution (1 tsp of Morton's salt) dissolved in 161 mL of RO/DI water (measured with a graduated cylinder). My refractometer was reading 1.028, I've now calibrated it to 1.026. The tank water is now reading 1.023, I guess I'll be raising it through water changes over the next few weeks.

Jim, my refractometer now registers below zero for RO/DI water.

[J. Montgomery]

double post

[Neptune777]

<a href=showthread.php?s=&postid=11499744#post11499744 target=_blank>Originally posted</a> by J. Montgomery
Update: I just made the reference solution (1 tsp of Morton's salt) dissolved in 161 mL of RO/DI water (measured with a graduated cylinder). My refractometer was reading 1.028, I've now calibrated it to 1.026. The tank water is now reading 1.023, I guess I'll be raising it through water changes over the next few weeks.

Jim, my refractometer now registers below zero for RO/DI water.


Cool! Mine does the same thing when I measure just RODI or Distilled water (they both read below zero).

Jim my TDS is at zero, I checked that as well (thanks :) )

I made a batch using the coke bottle method, and using the 161 mL + 1 tsp of salt method and they both read 1.026 in the now calibrated refractometer.

[JMBoehling]

Hmmmm.... Just did this.. My refractometer is reading 1.035... Just want to make sure I'm doing this right.. 1 Teaspoon of Iodized Tabel salt disolved into 161 Ml (Little more than 1/2 cup) of RO/DI should measure 35 PPM (1.02645)

Is that right.. If so, man my reef is actually a brackish lagoon :)


Jim

[Neptune777]

<a href=showthread.php?s=&postid=11500101#post11500101 target=_blank>Originally posted</a> by JMBoehling
Hmmmm.... Just did this.. My refractometer is reading 1.035... Just want to make sure I'm doing this right.. 1 Teaspoon of Iodized Tabel salt disolved into 161 Ml (Little more than 1/2 cup) of RO/DI should measure 35 PPM (1.02645)

Is that right.. If so, man my reef is actually a brackish lagoon :)


Jim



That sounds right Jim. So your tank salinity is actually running at 1.017 instead of 1.026 by what the standard solution is saying right?

[Neptune777]

All, here is the thread that caught my attention originally....folks have had the same results that we are experiencing.

http://archive.reefcentral.com/forums/showthread.php?s=&threadid=1276603&perpage=25&highlight=Refractometer%20below%20zero&pagenumber=1

[JMBoehling]

Do you think Kroger brand Iodized table salt is the same as Mortons?

[J. Montgomery]

<a href=showthread.php?s=&postid=11500413#post11500413 target=_blank>Originally posted</a> by JMBoehling
Do you think Kroger brand Iodized table salt is the same as Mortons?
Yeah, I'd imagine that it's close enough.

[JMBoehling]

Thanks for this post Dave. You never stop learning in this hobby!

This is crazy.. What are the long term effects on SPS and fish of running your salinity so low... I can imagine my Salinity has been 10 points low for a year or so...

Jim

[Neptune777]

<a href=showthread.php?s=&postid=11500458#post11500458 target=_blank>Originally posted</a> by JMBoehling
Thanks for this post Dave. You never stop learning in this hobby!

This is crazy.. What are the long term effects on SPS and fish of running your salinity so low... I can imagine my Salinity has been 10 points low for a year or so...

Jim

:thumbsup:

I'm curious to see what response your corals have after making the adjustment...keep us posted.

[J. Montgomery]

Get the salinity up to a true 1.026 and the calcium and other elements will also increase! w00t!

[will_w]

Thanks for bringing this to our attention. I don't think that I had calibrated my refractometer since I orignally purchased it.

I just tried this on mine. The refractometer measured 1.030 with the standard. However, when I measure my RO/DI water it shows below zero. That leads me to be concerned over our ability to properly measure the water and a true 1 tsp. of salt. It seems like a slight variance in your volumes could yield significant inaccuracy in the calibration fluid.

I question the calibration fluid being that all of us are appearing to be calibrated low. I don't want to over-salt my tank, so I think I will try and invest in a little true calibration fluid if one of the LFSs carry it. I wouldn't start adjusting salinity until you know your calibration fluid is accurate.

Does anyone know if any of the LFSs carry salinity calibration fluids?

[Neptune777]

Hi Will,
I have similar concerns so I posted in the Chemistry Forum to see if Randy or Boomer can shed some light:

http://archive.reefcentral.com/forums/showthread.php?s=&threadid=1283818

[twon8]

i got some of the calibration standard from neptune systems. was less than $3.

[will_w]

Twon, Did a local store carry it?

[J. Montgomery]

<a href=showthread.php?s=&postid=11500767#post11500767 target=_blank>Originally posted</a> by will_w
That leads me to be concerned over our ability to properly measure the water and a true 1 tsp. of salt.
Will, thats a good idea to see how a commercially produced calibration solutions compares to the home made version.

I used a graduated cylinder to measure the water, same as we use in the lab. I used a level tsp to measure the salt, and Randy states in his article that most measuring spoons are extremely accurate. So I feel pretty confident in my measurements (and I made two different solutions).

When I get a chance, I'll use the balance in the lab to accurately weigh out both the salt and the water.

[Neptune777]

Hey Anthony,
Any chance you can make the DIY solution and compare to your purchased standard to see if they match?

Also, after calibrating with the commercial solution does your refractometer read below zero with pure RODI water?

[surfnvb7]

just out of curiosity, how does the refractometer reading of 1tsp of table salt differ from 1tsp of the salt you actually use in your tank?

seems to me you are dealing with apples and oranges. b/c the salt in the standard is allot "cleaner", its essentially only NaCl, vs the tank water has many other ions and different kinds of dissolved salts in the water that *could* effect the refractometers reading.

[J. Montgomery]

<a href=showthread.php?s=&postid=11500946#post11500946 target=_blank>Originally posted</a> by surfnvb7
just out of curiosity, how does the refractometer reading of 1tsp of table salt differ from 1tsp of the salt you actually use in your tank?
Like you said, the Morton's iodized salt is a uniform composition so if you weight multiple teaspoons you'll get more precise values than you would with Instant Ocean which is composed of multiple salts of various sizes, weights, and densities. And you're trying to make a calibration solution thats a specific concentration.

[will_w]

Al, I think that is why you use Iodized Salt. There should not be other additives so one teaspoon is a fixed amount of salt. With an aquarium mix you would have other additives that could affect the reading. That is why different salt manufacturers may need slightly different amounts of salt in the same volume of water to make the same specifice gravity water. I think ;)

[Neptune777]

Here is a quote from another of Randy's articles to explain more why table salt is OK to use:

"To provide a standard for refractometers requires a solution whose refractive index is similar to normal seawater. Seawater with a salinity of 35 ppt has a refractive index of 1.3394. Likewise, the refractive index of different sodium chloride solutions can be found in the scientific literature. My CRC Handbook of Chemistry and Physics (57th Edition, Page D-252) has such a table. That table has entries for 3.6 and 3.7 weight percent solutions of sodium chloride that span the value for normal seawater. Interpolating between these data points suggests that a solution of 3.65 weight percent sodium chloride has the same refractive index as 35 ppt seawater, and therefore can be used as an appropriate standard (Table 5)."

Here is the complete article (different from the other one I linked earlier):
http://reefkeeping.com/issues/2006-12/rhf/index.php

[surfnvb7]

gotcha ;) i'm surprised ya'lls refractometers didn't come with any kind of calibration standard.

this is the one i have (you can see the little bottle that comes with it in the picture), also looks like they sell the bottles separately.
http://www.premiumaquatics.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=REFRACT-HD&Category_Code=

my tank water wast tested at 1.024 before calibration
tested my 1.000 calibration solution, came in at 1.001, then readjusted refractometer to 1.000

then made up the 1.026 standard which came in at 1.027
then tested the ro/di which comes in at exactly 1.000

then tested the tank water again, comes in at about 1.0235


thats good enough for me...i like to keep mine around 1.023-1.024 for some odd reason. :rolleye1:

[twon8]

i got it online at aquariumsystems.com
http://www.aquariumspecialty.com/catalog/product_info.php?cPath=49_107_64&products_id=244

i haven't calibrated the refractometer yet, seems i have lost the tiny screwdriver. i can try and make up some homemade standard, but it really won't tell us much as my margin for error may be more or less than others

the only salt i have right now is kosher salt, so it will have to wait a bit.

[twon8]

dp

[Neptune777]

Yeah, I wish mine did come with a standard solution :( Now I am wondering if it is indeed off as much as the DIY solution is telling me. Randy really knows his stuff and I would be surprised if his formula was incorrect (as many have used it and have had similar results that we have seen) and nobody has contested it (not that it means anything ;) ). There is also the possibility of operator error here as well but I made the solution using both methods and got the same results.

Josh, I can't wait to see what you get for results at the lab when you weigh everything precisely (thus eliminating the margin of error).

[Neptune777]

Randy and the Chemistry gang recommend using only the Pinpoint Calibration fluid (53ms I think). I will be ordering some as well to see how it compares.

[Charlie Davidson]

All this just got me thinking... (smell the smoke?) and I desided to read instuctions (again after 10+yrs), btw, I had always cal. by ro/di :eek2:
Another thing to make sure of-----I looked at my refrac.. it is ATC (auto temp compensation)
I have never let water sit for more than 10 sec before I read it...
Just tried it - at 5 sec it was 1.025---- at 25sec it was 1.023 :eek:
I just added some salt, but plan to look at this again (2 or 5)

[brward5]

Calibrating with RO/DI is better than not calibrating at all. But there are errors in the way the refractometer is built that makes the error level different across the range of measurements. By calibrating to a solution that is near seawater (35ppt) you are calibrating the refractometer around the range you wish to measure. Same as when you calibrate a pH probe for saltwater use, you use 7 and 10 solutions, not 7 and 4.

[Neptune777]

<a href=showthread.php?s=&postid=11504482#post11504482 target=_blank>Originally posted</a> by brward5
Calibrating with RO/DI is better than not calibrating at all. But there are errors in the way the refractometer is built that makes the error level different across the range of measurements. By calibrating to a solution that is near seawater (35ppt) you are calibrating the refractometer around the range you wish to measure. Same as when you calibrate a pH probe for saltwater use, you use 7 and 10 solutions, not 7 and 4.


The part about calibrating within the range you are shooting for sounds logical, and how this might give you an error in a range outside of where you calibrated (why my re-calibrated refractometer now reads below zero with straight RODi).

I have some pinpoint 53mS solution on the way to do a comparison in the next couple of days.

[Neptune777]

<a href=showthread.php?s=&postid=11504416#post11504416 target=_blank>Originally posted</a> by Charlie Davidson
All this just got me thinking... (smell the smoke?) and I desided to read instuctions (again after 10+yrs), btw, I had always cal. by ro/di :eek2:
Another thing to make sure of-----I looked at my refrac.. it is ATC (auto temp compensation)
I have never let water sit for more than 10 sec before I read it...
Just tried it - at 5 sec it was 1.025---- at 25sec it was 1.023 :eek:
I just added some salt, but plan to look at this again (2 or 5)

Hi Charlie,
Did you calibrate this time with RODI or with the DIY standard solution?

[Neptune777]

Cool! We got a response straight from Randy:

"Should the refractometers read below zero with RODI after calibration with a standard solution?

A perfectly made refractometer would do so. Many of the hobby refractometers appear to not be made correctly, so the calibration at 0 ppt with fresh water does not ensure correct calibration at 35 ppt, and likewise, if correctly calibrated at 35 ppt, it may not read fresh water to be 0 ppt. Since you typically use it at 35 ppt, that's's where you want it to read correctly."

[Charlie Davidson]

Bward missed what I said and mean.
:eek2:= I have been doing wrong for years!
I also wanted to post that the on ATC models you may need to wait 30sec. (or is that on non-ATC?) (read instuctions, even if you think you know how to use)
I have not made my own solution but -- like you and Will.. I do plan on ordering some soon..
( I do not trust my measuring skills) :lol2: :lmao:

[JMBoehling]

Guys,

I went to my local fish store and compared my refractometer to theirs, and our read the same. Remember , mine read 1.035 with the calibration solution, and my reef and theirs reads my water at 1.026.

I am ordering a real calibration set just to make sure mine is right on....

Boy am I confused now :)

Later,

Jim

[JMBoehling]

Just ordered some Pinpoint 53ms Calibration Fluid. I'll post the results when it arrives.

[Neptune777]

I'm pretty sure the LFS's calibrate theirs using RODI water as well.

[J. Montgomery]

I just made three different solutions.
1. 3.65 (weight %) sodium chloride (Morton's Iodized Salt)
2. 3.65 (weight %) sodium chloride (Fisher Chemicals)
Both of these solutions were prepared by dissolving 3.65g of salt in 96.35g of dI water from the lab (18 megaohm purity). I had calibrated my refractometer last night using the 1 tsp salt dissolved in 161ml of RO/DI water. Both of these solutions measure 1.024

The third solution was prepared by dissolving 6.20g (1 tsp) NaCl in 161g (mL) of dI water. This solution measured 1.025

I've now calibrated my refractometer to solution #2, and I've saved a few aliquots of it for future use.

[Neptune777]

Josh, thanks for checking this!

So while the DIY is not 100% perfect it is pretty darn close....much closer than calibrating with pure H2O. Looks like these cheap Refractometers do have a fairly large margin of error if you do not calibrate in the zone you are looking to test.

[J. Montgomery]

<a href=showthread.php?s=&postid=11507312#post11507312 target=_blank>Originally posted</a> by Neptune777
So while the DIY is not 100% perfect it is pretty darn close....much closer than calibrating with pure H2O.
I concur. I'd like to see how this solution compares to the salinity standard once someone gets a hold of one.

[will_w]

Sounds like some LFSs need to start carrying calibration solution as I think a stampede is headed over to get some. :)

[Neptune777]

My Pinpoint 53mS solution arrived today. I will be doing a comparison when I get home this afternoon.

[Charlie Davidson]

I am looking for to seeing ---
BTW did you order from premium?
You know after some thought, I know calbrating with ro/di was not " wrong" because it was the way most people did it.. (still in Dr Fosters catalog) Still I tryed to keep it somewhat consistant/ stable.
WE just may not have be getting 100% correct results..
I do remember that my reef did best at 1.027... Wondering if it was really 1.023--024--025,, if properly calibrated :confused:
Jim, Please let us know results also.. I was very concerned because of your "brackish water results"

[Neptune777]

I tested the pinpoint calibration 53mS solution and it matched perfectly the DIY solution I made the other night using the 2L Coke bottle method. Both read precisely 1.026 after I calibrated the other evening. So if you read straight RODI water the Refractometer reads below zero.

My conclusion: RODI will not give you an accurate calibration and could cause your salinity to be off by a fairly large margin. I trust the DIY solution now that I compared it to the commercially lab prepared standard offered by Pinpoint.

[Charlie Davidson]

Is it allways off on the low end? What was ready when calibrated with RO/DI?
Sorry I am full Of ????? and other "stuff" :lol:

[J. Montgomery]

Dave, cool to hear! I never doubted Randy's homemade solution; he wouldn't have written an article about it if it wasn't solid. It seems these hobbyist refractometers aren't accurate over their entire scale, which isn't too surprising. Thanks for starting this thread and getting me off my b*tt to get on the right track with my salinity. :thumbsup:

[Neptune777]

<a href=showthread.php?s=&postid=11521801#post11521801 target=_blank>Originally posted</a> by Charlie Davidson
Is it allways off on the low end? What was ready when calibrated with RO/DI?
Sorry I am full Of ????? and other "stuff" :lol:


Hi Charlie,
I had already calibrated using Randy's DIY Salinity Standard the other night. If you calibrate correctly (i.e. in the range you are looking to test) then you will get incorrect readings with RODI water thus showing below zero on the scale. This is normal and can be expected due to the margin of error these refractometers apparently have. In other words calibrate in the NSW range with a known standard and we will be covered.

[JMBoehling]

Bravo Dave! I ordered the 53mS solution as well. Guess I'll use that to calibrate with since my measuring skills aren't very sharp.

Hard to believe that many of us may have messed up to first and most important part of our hobby, salinity... :)

Question> How far below 0.00 does your refractometer indicate when you test RO/DI now?

Thanks,

Jim

[J. Montgomery]

<a href=showthread.php?s=&postid=11522757#post11522757 target=_blank>Originally posted</a> by JMBoehling
Question> How far below 0.00 does your refractometer indicate when you test RO/DI now?
I'd guesstimate my refractometer to read 0.098 with RO/DI water (should be 1.000)

[JMBoehling]

That's pretty far below the line isn't it. It just freaked me out when I measured water and it was way under the 1.000 level after calibrating it.

Now I hope the Biologist and Chemist that tested NSW on the Reefs calibrated their refractometers correctly :)

Later,

Jim

[J. Montgomery]

<a href=showthread.php?s=&postid=11522961#post11522961 target=_blank>Originally posted</a> by JMBoehling
That's pretty far below the line isn't it. It just freaked me out when I measured water and it was way under the 1.000 level after calibrating it.
For my refractometer, thats just two dashes (each dash is 0.001) below the 1.000 mark. Not too far below the line. I don't think marine biologists are getting their refractometers from Marine Depot! :lol:

[LB]

I just checked mine with a 2liter and 1/4 cup + 1 teaspoon and it read 39ppt. Kind of crazy this hasn't come up before!!!

On the upside think of the thousands of dollars saved in salt :D

[icantremember]

I ran across your thread after searching for refractometer calibration. I made up a batch of the DIY test solution per Randy's guidelines, calibrated, checked RO/DI water and checked my tank. 1.020!! I was way off using my RO/DI as a calibration point before. Thanks for the heads up on this topic. A big thank you to Randy as well!