Specific gravity vs temp question

[Aquarist007]

I can't seem to wrap my brain around this statement:

"Acceptable ranges but be careful where your Specific Gravity is when your temperature approaches those higher levels. Many inverts, Anemones in particular, are not very good osmolators. A specific gravity of 1.024 is not going to work for them if your temperature is 82F. At that temp it will feel like 1.020 and is the main reason so many hobbyists fail to keep these long term."

The above statement was made from this post

Salinity 1.024 to 1.026 for mixed reefs

pH 8.0----8.4

calcium 390--420 ppm

alkalinity 8.0----11 dkH

magnesium 1300-1400

temp 76-84 degrees Farenheit

Wouldn't the specific gravity of salinity increase at a higher temp as water is heated and evaporates more?

 

[Mike31154]

Things become less dense as temperature rises, do they not? I just pressed some Riesling grapes the other day after a -8 degree Celsius overnight temperature. This approaches the ideal temperature for making Icewine. When I measured the SG of the freshly squeezed, still cold juice, it was 1.150. The same juice measured later at closer to room temperature yielded a reading of 1.142.

Certainly with evaporation the concentration of a dissolved solid in liquid will rise, but the same sample measured for SG at different temperatures without evaporation should read lower at higher temperature. Floating hydrometer in a warmer, less dense liquid will sink farther down giving a lower SG reading. Opposite is true when liquid is colder/more dense. Most floating hydrometers are calibrated to be accurate at around room temperature & are usually accompanied with charts to compensate for temperature if sample is measured significantly outside the calibrated range. Not sure how my reasoning applies to a refractometer, but most are temperature compensated anyhow.

Further to the evaporation issue, most of us use ATOs to compensate, no?

 

[disc1]

Oh, I see your confusion. And I'm probably about to make it worse.

First off, it's not about the evaporation. That is a completely separate thing from what we are talking about and yes it will be more of a problem at higher temps.

What we're talking about is the relationship between salinity and specific gravity.

Fish and coral care about salinity. The actual amount of salt that is in a given amount of water. They don't give a hoot about specific gravity. But we've grown so accustomed to the units, that nobody bothers to convert back and forth to salinity, we just throw around the specific gravity numbers.

But we can't measure salinity. We can't. We have to have some surrogate that we can measure and that is proportional in some way to salinity. The three best are specific gravity, refractive index, and conductivity. All three of those things change linearly with changing salt concentrations so if we know any of those three things we know salinity.

Well, here comes part of the temperature part. Specific gravity changes not only with increasing salinity, but also with temperature. So 35ppt is the salinity we want. At 60F that's 1.026. At 80F that's 1.023.

Hold the phone! Are we all running our tanks high? Well, here's more confusion. We report the values corrected to 60F. So if you were to weigh out a sample of tank water and calculate specific gravity you would always seem low because your hydrometer is making a correction for you. They actually fudge the calibration so that it is reading what it would be if it was 60F. But there's the fatal flaw.


SO more confusion from temperature. We not only have to convert for it, we also have to correct for it. All three of those things, specific gravity, refractive index, and conductivity, change with temperature.

Now our nice conductivity meters, most of them automatically correct for temperature. And they are giving you a value that is in conductivity units, if you convert that back to specific gravity you have to assume some temperature (60F) and BAM, the conversion gets taken care of for you.

Our refractometers measure refractive index. Most of them are also automatically corrected for temperature. They will often have a scale on them in units of specific gravity. Guess what. A refractometer CANNOT measure specific gravity. It measures refractive index. A conversion must be made, so a temperature (60F) must be assumed. Again the conversion is taken care of for you, but you need to realize that it is happening.


Now we get to the thing the thing your quote is talking about. The lowly hydrometer actually measures specific gravity. Not that fish give a hoot about specific gravity, but it sure is a convenient way to find the salinity. The problem is that they are not corrected for temperature. They do make the conversion to 60F, but they have to assume that they are starting out at some value. I think it is normally 78F or 72F.

Let's say it's 72F. But let's also say we read our water at 85. Now the specific gravity is lower than expected even though the salinity is the same. So the hydrometer reads low. And we think we need more salt. It is only because the hydrometer is making that conversion from 72F to 60F when it should be using 85F and 60F. And in the end we add salt to this tank and make it in reality hypersaline at 36 or 37ppt.





Moral of the story is: If you use a hydrometer, pay attention to the temperature of your water sample and make sure you know how your hydrometer is calibrated. It can make a significant difference.

 

[disc1]

To make it even more confusing, let's split hairs and think about the hydrometer. What it is doing is finding the point at which it's mass is the same as the amount of water it is displacing. So that also depends not only on the density of the water (its specific gravity) but also on the density of the hydrometer itself. Since that also changes with temperature, the story gets even more convoluted.

The hydrometer expands as it warms. So it displaces more water. So it floats higher. So it gives an erroneously high reading if you perfectly correct for the effect of the water temp. If the hydrometer is designed well, this effect will be minimal. And the best hydrometers will come with a calibrated chart at different temps. It certainly won't overcome the effect of the water temperature, but it does confound the issue.

 

[Aquarist007]

To make it even more confusing, let's split hairs and think about the hydrometer. What it is doing is finding the point at which it's mass is the same as the amount of water it is displacing. So that also depends not only on the density of the water (its specific gravity) but also on the density of the hydrometer itself. Since that also changes with temperature, the story gets even more convoluted.

The hydrometer expands as it warms. So it displaces more water. So it floats higher. So it gives an erroneously high reading if you perfectly correct for the effect of the water temp. If the hydrometer is designed well, this effect will be minimal. And the best hydrometers will come with a calibrated chart at different temps. It certainly won't overcome the effect of the water temperature, but it does confound the issue.

thanks for the very detailed and clear explanation.
Are you saying that if we use a refractometer then the adjustment has been made for each temp of the aquarium water? eg
This is all I use--and I am using it for over 30 tanks which all vary in temp between 76 and 84 and most of them stocked with anemone---so I want to be sure.

BTW
what temp are the acceptable ranges based on for cal, mag and alk?

 

[greg1786]

My tank fluctuated from 79 degrees overnight to 81 degrees during the day. At both temperatures my hydrometer reads 1.024. I keep a mixed reef with soft corals and sps. I also have a Rbta. does this mean that my bta and coral won't survive as long at 81 and 1.024 as they would at 76 and 1.024??


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[bertoni]

For a refractometer, the temperature of the tank water has no significance. The thermal mass of a drop of water is tiny. The refractometer might have some hardware to compensate for the variation in the temperature of the refractometer itself. If that's working, then the room temperature is unimportant, and the only question is whether the device has been calibrated accurately for saltwater.

 

[disc1]

Right. For the refrac it is the temp of the refrac and not the water that matters. But it is giving you the 60 degree numbers you want on the scale. So your conversion to salinity is sound and you're reading the right thing.

It really only becomes a problem with hydrometers.

 

[disc1]

My tank fluctuated from 79 degrees overnight to 81 degrees during the day. At both temperatures my hydrometer reads 1.024.

Right. 2 degrees isn't going to make enough difference. So the hydrometer reads the same in both.

I keep a mixed reef with soft corals and sps. I also have a Rbta. does this mean that my bta and coral won't survive as long at 81 and 1.024 as they would at 76 and 1.024??

No, what it means is that if you measured with the same hydrometer 1.024 at both temps, then the water at 81 actually has more salt in it. The salinity is higher in one than the other.

 

[Mike31154]

More reading material...

http://reefkeeping.com/issues/2004-07/rhf/index.php