Salinity

[rusty dowell]

I thought it was a simple question

 

[rusty dowell]

The first pic is at 78°
The second pic is at 70 room temperature
The question is.
What is my salinity?

 

[Randy Holmes-Farley]

The first pic is at 78°
The second pic is at 70 room temperature
The question is.
What is my salinity?

Sometimes the world is more complicated than it seems.

The two values you quoted are within the claimed error of the other two devices, and since you have likely not tested the hydrometer for accuracy, all we can assume is that is accurate at 77 where it is designed, which is close enough to 78. Use that value.

 

[rusty dowell]

I’m simple
What is my salinity?

 

[Randy Holmes-Farley]

I’m simple
What is my salinity?

I doubt that is the question you want answered, but your salinity is about 35 ppt.

If what you want to know is what is the specific gravity, it is about 1.0265 (hard for me to see the level in the photo)

 

[rusty dowell]

This is the original question
Does temperature affect PPT?
At 78° I’m at one 1.026
At 70° room temperature I’m at 1.027

 

[KStatefan]

This is the original question
Does temperature affect PPT?
At 78° I’m at one 1.026
At 70° room temperature I’m at 1.027

So if you correct for a hydrometer that was calibrated at 77° F your specific gravity is 1.0261

 

[rusty dowell]

Thank you
I just didn’t know what number to shoot for when I adjusting

 

[Randy Holmes-Farley]

This is the original question
Does temperature affect PPT?
At 78° I’m at one 1.026
At 70° room temperature I’m at 1.027

And I have said over and over: temperature has absolutely zero effect on salinity (ppt).

Ppt and specific gravity and hydrometer readings are all different things with different properties.

 

[rusty dowell]

Listen, thank you for all the great advice
But I don’t understand how the glass hydrometer says 1.026 at 78
And when the room temperature is at 70, it’s at 1.027
With the Hannah salinity checker 1.026 at 78
And when you check it at room temperature, it’s 1.027
And same as the refractometer



So the original question was am I shooting for 1.026 at 78° or 70°

 

[Randy Holmes-Farley]

You salinity is perfectly fine. Reef tank salinity is not critical. Yours is currently at about 35 ppt, which matches the whole ocean average and is the target I recommend.

You do not appear to be getting the points I have made over and over about your measurements, so I’m going to stop trying to get you to understand the answers to questions you are asking and I’ll make it ultra simple:

Target a hydrometer reading of 1.025 to 1.027 at 78 deg F with the tropic Marin hydrometer and the tank salinity will be perfectly fine. (Assuming the hydrometer is working properly, which I have no reason to doubt).

 

[rusty dowell]

I completely 100% positively understand
I understand my salinity my specific gravity is good
I’ve been a reefer for 25 years
I asked a basic question. Does specific gravity change due to temperature?
I measure my salinity/specific gravity, using three different tools

I also agree measuring the salinity/specific gravity at 78is the proper answer

 

[KStatefan]

I asked a basic question. Does specific gravity change due to temperature?

This has been answered previous in this thread but yes specific gravity of sea water changes with temperature.

 

[EricR]

Just to clarify the goal here, is it something like this:
New saltwater mix is unheated so at 70d (room temp) and you'd like to use that floating hydrometer for checking that?

If so, you'd need to use a temperature correction chart like noted in post# 27.
*no temp adjustment needed for tank water (assuming hydrometer is calibrated at 77d)

 

[Randy Holmes-Farley]

This has been answered previous in this thread but yes specific gravity of sea water changes with temperature.

Well, very, very little.

 

[Randy Holmes-Farley]

I completely 100% positively understand
I understand my salinity my specific gravity is good
I’ve been a reefer for 25 years
I asked a basic question. Does specific gravity change due to temperature?
I measure my salinity/specific gravity, using three different tools

I also agree measuring the salinity/specific gravity at 78is the proper answer

And I answered it repeatedly.

Glass hydrometer readings change a lot with temp.

Specific gravity hardly changes at all with temperature.

If you are interested in exact numbers and gory math and chemistry details, I discuss that here:

Chemistry and the Aquarium: Specific Gravity: Oh How Complicated!

This month, Randy explains and reviews specific gravity.

reefs.com

From it:

Temperature of the Standard​

Unfortunately, the world of specific gravity is not as simple as described above. Different fields have apparently chosen different standard temperatures. In addition to the 3.98 °C standard, others include 20 ° C (68 °F) and 60 °F (15.6 °C). A quick look through several laboratory supply catalogs shows many examples of hydrometers using each of these two, and a few odd ones thrown in for good measure (such as 102 °F for milk). Most authors writing about marine aquaria assume that people are using the 60 °F standard, but in reality many aquarists are not, and in some cases they don’t even know what they are using. Some hobby hydrometers use other standards, with 77 °F being quite popular (used by Tropic Marin, for example).

The density of pure water at 20 °C is 0.998206 g/cm3, and at 60 °F it is 0.9990247 g/cm3. While these seem close to 1, and are often simply claimed to be 1.00 in many contexts, the difference can be substantial. For example, the specific gravity of natural seawater (S =35) is 1.0278 using the 3.98 °C standard, 1.0269 using the 60 °F standard, 1.0266 using the 20 °C standard, and 1.0264 using the 77 °F standard. [I calculated these based on tables of the density of seawater, different tables may present slightly different densities that might then result in slightly different specific gravities]. While these differences are small, they are real. They arise because the density of pure water and seawater change in slightly different ways with temperature. Seawater becomes less dense faster than pure water as the temperature rises. This effect may relate to the interactions between ions and between ions and water in seawater that are broken up as the temperature rises, but that’s beyond the scope of this article.

Unfortunately, it has been my experience that many aquarists quoting a specific gravity measurement for their tanks do not know what standard is being used by their hydrometer. Most quality lab hydrometers will have the standard used written on them or their supporting documents. Some hobby hydrometers, however, make no mention of the standard used. Note that there is NO “correction” table that can convert readings at temperatures other than the standard temperature unless you know the standard temperature. If you don’t know it, using such a table will not give accurate values, and may give a value farther from the truth than the uncorrected reading.

Temperature of the Sample​

As if the confusion about the temperature of the standard were not enough, the temperature of the sample is also a variable. Many quality lab hydrometers also have the expected sample temperature indicated directly on them. This is referred to as the “reference” temperature. In a great many cases (though not all), the standard temperature and the reference temperature are the same: either 60 °F or 20 °C. This is often written as “60 °F/60 °F”, though it is sometimes written simply as “Temperature of Standardization: 60 °F”. If a hydrometer is used at the reference temperature, no special corrections are necessary (though the answer one gets will depend a bit on the standard temperature chosen by the manufacturer as described above).

Why does the temperature of the sample matter? There are two reasons. One is that the hydrometer itself may change its density as a function of temperature, and thus give incorrect readings at any temperature except that for which it is specifically designed (i.e., it floats higher or lower as its density changes). Unfortunately, unless you have a table for your specific hydrometer (which is rarely supplied), this effect cannot be corrected by a table because of the different materials of construction of hydrometers. Various types of glass and plastic
are used for hydrometers, and each will have it own particular change in density as a function of temperature. It should be noted that this effect is substantially smaller for glass hydrometers than the second effect described below because the change in density of glass with temperature is 8-25 times smaller than the change in density of aqueous fluids.

The second reason that the sample temperature is important is that the sample itself will change its density as a function of temperature. For example, the density of seawater (S = 35) changes from 1.028 g/cm3 at 3.98 °C to 1.025 g/cm3 at 20 °C to 1.023 g/cm3 at a typical marine aquarium temperature of 80 °F. Since the density of the sample is changing with temperature, the measured specific gravity will also change, unless this is taken into account.

The impact of temperature on the density of the sample can be corrected in a table, assuming that one knows how the density of the sample would change with temperature (which is well known for seawater), and also that one knows the temperature of standardization of the hydrometer. For example, if you have a hydrometer calibrated for 60 °F/60 °F, then you will be correcting for the difference in density between the sample at 60 °F, and the temperature at which you measured it. If the actual sample were measured at 86 °F, then the correction is the ratio of the density of seawater at 86 °F (approximately 1.0217 g/cm3) divided by the density at 60 °F (approximately 1.0259 g/cm3), or 0.996. Thus a specific gravity reading, or more correctly, a hydrometer reading, of 1.023 would be corrected to an “actual” reading of 1.027.

Again, if you do not know the temperature of standardization, you are out of luck, and a correction using a table is as likely to cause bigger errors, as it is to correct any. Likewise, using a “correction” table that does not specify what it is intended to correct is equally risky.

Some marine hobby hydrometers claim to be accurate at all temperatures (68 – 85 °F; these include SeaTest, Deep Six, and eSHa Marinomat). Such a device can be designed, if its change in density as a function of temperature were exactly the same as seawater at all temperatures. Two of these tested below (the SeaTest and the Deep Six) do a fair job of temperature correction, but in fact slightly overcorrect.

 

[rusty dowell]

The difference between 1.026 and 1.027 isn’t a little
Not in my opinion

 

[Randy Holmes-Farley]

From the above discussion, we can pull out the values for specific gravity vs temp:

the specific gravity of natural seawater (S =35) is 1.0278 at 3.98 °C ​

1.0269 at 60 °F ​

1.0266 at 20 °C , ​

and 1.0264 at 77 °F ​

 

[Randy Holmes-Farley]

The difference between 1.026 and 1.027 isn’t a little
Not in my opinion

Ok, but you still misunderstand. The real specific gravity did not change with those temp changes. The hydrometer reading did.

 

[KStatefan]

Well, very, very little.

Yes but he said he wanted the basic answer.