HELP! SG vs Temperature

[Joe del Rey]

I am quite confused right now.
I've read over the web that temperature affects the value of specific gravity and might be actually different form the readings.
Since I don't have much budget on hand right now, I am using a hydrometer (arm-swing type) for the mean time. Here's the reading I got:
SG: 1.021
Salinity: 28
Temperature: 32°C

What is the actual SG of my tank?

 

[Hemmdog]

I’m not sure but that’s way to hot for marine animals.

 

[lapin]

This might help you understanding temp and how it effects your reading
https://www.algone.com/salinity-and-specific-gravity-guidelines-for-the-saltwater-and-reef-keeper

 

[Joe del Rey]

This might help you understanding temp and how it effects your reading
https://www.algone.com/salinity-and-specific-gravity-guidelines-for-the-saltwater-and-reef-keeper

thank you! this is so informative!
Just this morning I did some more readings. The hydrometer reading was 1.022 and my tank water temperature is at 29.44°C. So I guess the actual SG of my tank is 1.0252! (Correct me if I'm wrong.)

 

[Randy Holmes-Farley Verified Community Expert]

The question is complicated, beyond what most reefers care about. Actual specific gravity changes only a little with temperature. Depending on the device you are using to measure it, however, the device may need to be at a specific temperature or be corrected. Some correct automatically and some do not.

Fortunately for you, most swing arm hydrometers, whether accurate or not, do seem to automatically correct for temperature effects pretty well.

 

[Randy Holmes-Farley Verified Community Expert]

thank you! this is so informative!
Just this morning I did some more readings. The hydrometer reading was 1.022 and my tank water temperature is at 29.44°C. So I guess the actual SG of my tank is 1.0252! (Correct me if I'm wrong.)

Likely wrong. [emoji23]

 

[Joe del Rey]

Likely wrong. [emoji23]

HAHAHA I knew it.
I am still sooooo confused TBH. [emoji23]

 

[Saltyreef]

To accurately test the SG with a refractometer, you'd need to let the temps between the refracto, sample and ambient air equalize.
A swing arm hydrometer is a great tool once its been error checked and marked with an accurate refractometer.
I'd also lower that tank temp to 25°

 

[Joe del Rey]

To accurately test the SG with a refractometer, you'd need to let the temps between the refracto, sample and ambient air equalize.
A swing arm hydrometer is a great tool once its been error checked and marked with an accurate refractometer.
I'd also lower that tank temp to 25°

I see. Thanks for the advice

 

[Joe del Rey]

UPDATE
Here's the latest reading @ 30°C

 

[Randy Holmes-Farley Verified Community Expert]

I’d just add that a sg of 1.022 is low, IMO.

 

[Joe del Rey]

I’d just add that a sg of 1.022 is low, IMO.

Yes, I am fully aware of that.
I actually bought new saltmix and mixing it right now in a 5 gallon bucket. I'm planning to rise the SG slowly within a few days.

 

[Saltyreef]

I would still check this against a calibrated refractometer. Cheap and easy way, have your LFS test your salinity. If its between 1.025 - 1.026 then make the mark on your hydrometer where the needle currently sits at 1.025 or 1.026.
If its too high or too low, adjust slowly over a few days and have them test again until they say its spot on. Otherwise buy a refractometer and do it yourself.
I trusted a swing arm hydrometer for over 5 years and found out that my salinity was up around 1.030. Explained a lot of issues i was running into. Countless hours of scratching my head.

 

[Joe del Rey]

I would still check this against a calibrated refractometer. Cheap and easy way, have your LFS test your salinity. If its between 1.025 - 1.026 then make the mark on your hydrometer where the needle currently sits at 1.025 or 1.026.
If its too high or too low, adjust slowly over a few days and have them test again until they say its spot on. Otherwise buy a refractometer and do it yourself.
I trusted a swing arm hydrometer for over 5 years and found out that my salinity was up around 1.030. Explained a lot of issues i was running into. Countless hours of scratching my head.

Thank you for the suggestions. I guess I'll get my own refractometer once I have enough funds. And I am shocked to see that 1.030 specific gravity BTW. [emoji50] Now I am having trust issues with hydrometer. I've heard many times that it's not as accurate as refractometers, but it's the only thing I could afford for now.

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Three weeks ago, I went on a beach vacation and brought my hydrometer (how weird is that? [emoji23]) and while no one's around, I secretly measured the SG of the sea (because I don't wanna look any more weirder than this [emoji23]). And to my surprise, the reading was 1.017! So I did 3 more readings and the results were the same. I was baffled and scratched my head. Does the water within the shallow areas (9 ft deep) usually have lower SG? I am having trust issues with my hydrometer now [emoji28]

 

[Saltyreef]

Thank you for the suggestions. I guess I'll get my own refractometer once I have enough funds. And I am shocked to see that 1.030 specific gravity BTW. [emoji50] Now I am having trust issues with hydrometer. I've heard many times that it's not as accurate as refractometers, but it's the only thing I could afford for now.

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Three weeks ago, I went on a beach vacation and brought my hydrometer (how weird is that? [emoji23]) and while no one's around, I secretly measured the SG of the sea (because I don't wanna look any more weirder than this [emoji23]). And to my surprise, the reading was 1.017! So I did 3 more readings and the results were the same. I was baffled and scratched my head. Does the water within the shallow areas (9 ft deep) usually have lower SG? I am having trust issues with my hydrometer now [emoji28]

Natural sea water is between 1.026 and 1.027 SG depending on what temp the sample is IIRC.
I would venture to guess your hydrometer is reading low. I wouldnt be trusting it until checked against a calibrated refractometer and marked as mentioned in my previous reply.
Have you tested any other elements in a freshly mixed bucket of salt? Usually the calcium, alkalinity and magnesium readings compared to the mixing instruction parameters on the bucket would be a telltale sign of salinity being too high or too low.
I would bring a bottle of your tank water to a local fish store and have them run the gamut of tests including salinity or SG reading.
Then you can mark your hydrometer and not worry about getting a refractometer right now.

 

[Hemmdog]

What’s your reason your running your tank at 30C ? That is really hot.

 

[Joe del Rey]

Natural sea water is between 1.026 and 1.027 SG depending on what temp the sample is IIRC.
I would venture to guess your hydrometer is reading low. I wouldnt be trusting it until checked against a calibrated refractometer and marked as mentioned in my previous reply.
Have you tested any other elements in a freshly mixed bucket of salt? Usually the calcium, alkalinity and magnesium readings compared to the mixing instruction parameters on the bucket would be a telltale sign of salinity being too high or too low.
I would bring a bottle of your tank water to a local fish store and have them run the gamut of tests including salinity or SG reading.
Then you can mark your hydrometer and not worry about getting a refractometer right now.

I see. I'll head to the LFS later for them to test my tank water sample Thank you so much. I've learned a lot from you

 

[Joe del Rey]

What’s your reason your running your tank at 30C ? That is really hot.

I live in a tropical country. It's already summer here and it's really really hot at noon. Although the tank temperature would slowly drop to 27°C as the sun goes down.
My tank is just FOWLR.