Hydrometer & Salinity

[Ken S]

I started reefing about 35 years ago and got out about 15 years ago now I'm back with a 200 gallon tank. I have always used a swing arm hydrometer cheap type thing. The guys at my lfs were needling me about using those so I brought some water up with me to have them test it with their refractometer. They came up with the exact same measurement as my swing arm 1.024, so I was needling them but I decided to buy one of those large Tropic Marin float glass hydrometers and I'm using it in a 10,000 ml graduated cylinder. Please see photo attached it is showing my salinity at 1.0277. Am I reading this thing correctly? The temperature of the water according to my Inkbird is 76.9° when I put it in the cylinder and I took the measurement immediately.

I am setting up a 50 gallon WC for this weekend. Would someone kindly tell me what salinity I should adjust my WC water to safely bring this down to 1.025. Or should I do several smaller water changes spaced several days to a week apart so I can bring it down slowly? What is the best way to go about this?

Thank you,
Ken

 

[Fish Fan]

@Randy Holmes-Farley

Wow, certainly looks quite high. I thought those floating bulb hydrometers, when used in a graduated cylinder, were supposed to be spot on. I'm interested to see how this turns out.

 

[Randy Holmes-Farley Verified Community Expert]

@Randy Holmes-Farley

Wow, certainly looks quite high. I thought those floating bulb hydrometers, when used in a graduated cylinder, were supposed to be spot on. I'm interested to see how this turns out.

Maybe it is. :)

 

[Fish Fan]

Maybe it is. :)

Right!

What would you suggest comparing the glass hydrometer to?

 

[Randy Holmes-Farley Verified Community Expert]

You can make a standard to test both of your hydrometers:

https://www.reef2reef.com/ams/reef-aquarium-salinity-diy-calibration-standards.956/
If it is high, there’s no rush to bring it down, and I’d stop at 35 ppt, sg = 1.0264

 

[Fish Fan]

You can make a standard to test both of your hydrometers:

https://www.reef2reef.com/ams/reef-aquarium-salinity-diy-calibration-standards.956/
If it is high, there’s no rush to bring it down, and I’d stop at 35 ppt, sg = 1.0264

I was trying to figure out how many tiny bottles of calibration fluid it would take to fill that graduated cylinder :winking-face:

Thanks for your help, Randy!

 

[Ken S]

You can make a standard to test both of your hydrometers:

https://www.reef2reef.com/ams/reef-aquarium-salinity-diy-calibration-standards.956/
If it is high, there’s no rush to bring it down, and I’d stop at 35 ppt, sg = 1.0264

I will create a standard solution this weekend, test both hydrometers & post back with the results.

Thank you for the article, Randy! You are a gentlemen & a scholar!

 

[Randy Holmes-Farley Verified Community Expert]

I will create a standard solution this weekend, test both hydrometers & post back with the results.

Thank you for the article, Randy! You are a gentlemen & a scholar!

You’re welcome.

Happy Reefing!

 

[Ken S]

Hello, I made the standard solution today. Over the weekend when the wife and I were getting out Christmas doecorations, I found a box with some of my old, old, old reefer stuff in it and found a Sea Test swing I bought 35 years ago and used for the entirety of the first reefing stint. The BoxTech one I just bought about 10-12 months ago. Interesting findings.

Pic 1 is both swing arms side by side with my tank water at 76.5 degrees.
Pic 2 is the glass hydrometer in the standard solution at 76.1 degrees. It is cold in my basement and it cooled a bit between the time I got the water into a bucket with the salt, mixed it, filled the graduated cylinder, walked to my bar (then back to the fish room because I forgot the hydrometer), got out the hydrometer and took the reading.
Pic 3 is the standard solution in both swing arms.
Pic 4 is the glass hydrometer in my tank water at 76.5 degrees.

Here is how I analyze the results:
1. I would read the glass hydrometer in the standard solution at 1.0268. Agreed? So knowing this, I should deduct .0004 from all future readings, yes? Such a small difference, I would think I could ignore it, yes?
2. I would read the glass hydrometer in the tank water at 1.0278. Agreed? So knowing this and the .0004 difference, I should bring my tanks specific gravity down to 1.0260, per the glass hydrometer?
3. I'll remove 3g every few days (240g system) and put in RO water until it comes down to the desired level. Sound like a good plan?

I don't really know what to think of the swing arm readings.

 

[Randy Holmes-Farley Verified Community Expert]

The SeaTest isn't bad. Only low by 1 ppt. the other seems well off.

It's a bit hard to read the hydrometer at that angle, but your values seem fine. Are you reading the flat water level, not the meniscus that climbs up the hydrometer a tiny bit?

 

[Ken S]

The SeaTest isn't bad. Only low by 1 ppt. the other seems well off.

It's a bit hard to read the hydrometer at that angle, but your values seem fine. Are you reading the flat water level, not the meniscus that climbs up the hydrometer a tiny bit?

Actually, I don't recall. I am going to repeat the entire process this holiday weekend and I'll report back findings with better photos.

 

[Ken S]

Hi Randy, I took another salinity reading and what I hope is a better Pic. To me, it looks like the meniscus climbs up all the way around, so I am reading the line under the meniscus.

Looking at the attached photo, reading the bottom just below the meniscus, I see 1.0270. Do you see the same?

The water temp in the graduated cylinder was 75.2 degrees. That Tropic Marin hydrometer is calibrated for 77 degrees, so how would I compensate for the difference? Is there a formula?

I plan to make another standard solution later today or tomorrow & calibrate again. Last result was my hydrometer was off by -.0004. So based on that my current salinity is 1.0266 plus the adjustment for the temperature difference between 77 degrees & 75.2 degrees (whatever that is?). Agreed?

Thank you!
Kenneth

 

[fishyjoes]

To me, it looks like the meniscus climbs up all the way around, so I am reading the line under the meniscus.

I read the hydrometer by looking through the cylinder with the water line at eye level - makes it easier to see the real value

 

[Ken S]

I read the hydrometer by looking through the cylinder with the water line at eye level - makes it easier to see the real value

Yes that makes perfect sense, except I make a poor choice purchasing a partially opaque, plastic graduated cylinder. I will be getting a glass one. I have a 10,000ml cylinder, I've heard this can be done with this hydrometer & a 5000ml cylinder. Can anyone confirm that?

 

[KStatefan]

I use a 500 ml glass graduated cylinder. There is an online calculator at Hamza's Reef to adjust for temperature.

​

 

[Randy Holmes-Farley Verified Community Expert]

FWIW, I show and discuss a meniscus issue here:

https://reefs.com/magazine/chemistry-and-the-aquarium-specific-gravity-oh-how-complicated/
from it:

A Tropic Marin hydrometer showing the meniscus rising to about 1.0260, but the actual reading is about 1.0265.

 

[Kilman805]

Yes that makes perfect sense, except I make a poor choice purchasing a partially opaque, plastic graduated cylinder. I will be getting a glass one. I have a 10,000ml cylinder, I've heard this can be done with this hydrometer & a 5000ml cylinder. Can anyone confirm that?

I use this vase, 3 in diameter, 16 in tall. It’s big enough, clear, and cheaper than glass graduated cylinders.

https://www.amazon.com/gp/product/B008NUQCHO/
Amazon product ASIN B008NUQCHO
I tested my Tropic Marin hydrometer with Randy’s calibration solution and it was spot on.

 

[fishyjoes]

Yes that makes perfect sense, except I make a poor choice purchasing a partially opaque, plastic graduated cylinder. I will be getting a glass one. I have a 10,000ml cylinder, I've heard this can be done with this hydrometer & a 5000ml cylinder. Can anyone confirm that?

From your first picture it looks like you can see through the plastic well enough.
I have a 500ml cylinder with the same partially opaque plastic and can see through it just fine.