Warning: salinities may be higher than you think

[kris walker]

I missed this before, but there is a correction factor that you need to account for when you are taking specific gravity (s.g.) readings. In many introductory books, it says the s.g. should be 1.022-1.024 for marine aquaria. But many on this board have suggested raising it up to 1.026, which is said to be closer to natural seawater. While 1.026 is equal to 35 ppt salts (natural seawater), it is only equal to that after the temperature correction.
At 75 F, 1.024 is 1.026, which is 35 ppt.
At 80 F, 1.024 is 1.027, which is 36 ppt.
At 85 F, 1.024 is 1.028, which is 37 ppt.
So your tank may be saltier than you think.
I just wanted to share with those who didn't already know about this conversion issue.
kris

 

[steamboat1569]

Very good point, Kris. I found this out years ago, and have always adjusted for it. There are some web sites out there with a temp. salinity calculator. It's a good tool. Most hydrometers are calibrated to salinity at 65 degrees F. How many tropical reefs do you know that stay at 65 degrees?
Don't ask me why they do this.

 

[kris walker]

I guess partially calibrated is better than none. LOL. I have the Aquarium Systems "SeaTest" meter, which appears to not be calibrated at all (or at least it doesn't say on the meter). I still like the meter though, despite an occassional bubble-sticking problem.
cheers,
kris

 

[piscesblue]

I thought you only had to adjust the reading when using a floating hydrometer. The plastic ones with the needles measure it a different way. I don't know, something I read a long time ago.

 

[steamboat1569]

Whether it be a floating, digital, deep 6, or whatever, it must have some standard to go by. Unless your hydrometer reads temp. as well as salinity and compensates for it (I have never seen one), than it must be compensated by the user. I have a floating hydrometer, and on the package it states that it is calibrated at 65 degrees. I have also used a Deep 6 and I compensated for it also.

 

[bang guy]

If you wait about 30 seconds on the plastic swing arm type they will adjust for temperature. Unfortunately most are wildly inaccurate right out of the box. If you calibrate them with a refractometer every six months or so and make note of the error factor they should be fine.
You must still lookup the S.G. reading and match it with temp in order to get the correct Salinity though.
There is a chart of S.G. / Salinity near the bottom of this article: <a href="http://www.animalnetwork.com/fish2/aqfm/1997/nov/features/1/default.asp" target="_blank">http://www.animalnetwork.com/fish2/aqfm/1997/nov/features/1/default.asp</a>

 

[bluemarlin]

So, 1.023-1.024 is good then?

 

[u235a4]

I agree on the 1.023 -1.024 blue, i have notice that my anemones seem to be the happiest around that at water temp in the 80 to 82 range

 

[kris walker]

Bang guy, how does waiting 30 sec correct for the temperature factor? And then why correct for it again with the table? Did you mean that if you wait 30 seconds the swing will hone in on the reading more accurately (waiting out minor buoyancy fluctuations)?
There appears to be some slight inaccuracies in some of the tables out there. The numbers I used in the first post are a little off of the numbers predicted by the table Bany guy refers to. The table looks more accurate, but I'm not sure (I'm at work, and don't have the original numbers in front of me).
kris

 

[bang guy]

Originally posted by Kris Walker:
<strong>Bang guy, how does waiting 30 sec correct for the temperature factor? And then why correct for it again with the table? Did you mean that if you wait 30 seconds the swing will hone in on the reading more accurately (waiting out minor buoyancy fluctuations)?
</strong><hr></blockquote>
Hi Kris,
The important parameter to find is the Salinity. A hydrometer can only find the specific gravity and after thet we need to lookup the salinity based on temperature. Marine animals don't really care what the s.g. is, only the salinity.
If I muddied the waters I appologise.
Waiting 30 seconds will allow the swing arm to reach temperature equilibrium. This extra time will allow the arm to expand depending on the temp. If the water is warmer it will expand more and read higher. Try it... Take a reading immediately and another 30 seconds later. There is a limit to the temp adjustment (somewhere in the 90's I think) and it's not 100% accurate but not waiting for the adjustment is even more inaccurate.
Like I said in the earlier post. Most of these hydrometers are inaccurate and need to be compared against a refractometer (or better) a couple times a year to find out how far off they are.

 

[kris walker]

Now I understand. Thanks for the elaboration.
kris

 

[richard rendos]

65 F is ridiclulous for a hydrometer to be calibrated at. I have talked to both Marine Enterprises and Aquarium Systems and they both say the swing arm types are calibrated at 78 F because that is close to the average tank temperature. They also agree that they are not very accurate. For better accuracy, there are refractometers.

 

[steamboat1569]

Why would it be ridiculous to be calibrated at 65 degees. Most ocean water is not tropical. It is cold water - 75 degrees to 55 degrees.
It says on the back panel of my hydrometer - "calibrated at 65 degrees". This is the ocean avererage mean temp. tropical's are only a small part of the ocean.
What would you do if you had an arctic tank. I have seen these. Lots of special equipment. You think reefs are hard, try an arctic tank.