4 instruments 4 different salinity readings...What is my actual salinity?

[SaltwaterScoop]

I purchased Hanna's new HI981520 Hanna Marine Monitor pH, Marine Salinity, Temperature and it initially read really low salinity on my tank so I panicked because the difference between it's reading and my refractometer's reading was significant. My refractometer was old so I decided to buy a few instruments to test my salinity to see what it actually is (got a new BRS refractometer too).

Here are the results:

1. HI981520 Hanna Marine Monitor pH, Marine Salinity, Temperature - Calibrated with Hanna 35 ppt calibration fluid at 77 degrees Fahrenheit
Listed Accuracy = ±1.0 ppt between 0.0 ppt and 40.0 ppt
Reading = 34.4 PPT at 77 degrees Fahrenheit

2. Glass High Precision Hydrometer
Listed Accuracy = .001 sg
Reading = 1.0266 sg / 35.3 PPT at 77 degrees Fahrenheit

3. BRS Refractometer with LED light - Calibrated with their 35 PPT Refracto Juice at 77 degrees Fahrenheit
Listed Accuracy = ???
Reading = 1.0279 / 37 PPT sg at 77 degrees Fahrenheit after 45 seconds

4. Plastic Coralife Refractometer from Amazon (seasoned for 24+ hours in tank water before reading)
Listed Accuracy = +.001 sg
Reading = 1.0286 / 38 PPT sg at 77 degrees Fahrenheit

Which one should I trust?

 

[taricha]

I love a good swing arm hydrometer, very repeatable. But the actual numbers on the scale are usually pretty off. You have to make your own known specific gravity solution and calibrate it. I don't know about the tiny glass hydrometer.
But I would trust the two instruments that you actually use a calibration fluid with.
You can mix up a known specific gravity solution, and mark it with the two hydrometers, then you could know how they are performing. But without that I would not trust them.

 

[JimWelsh]

#1 and #2 agree with each other, within published uncertainties. I'd trust #2.

 

[SaltwaterScoop]

I love a good swing arm hydrometer, very repeatable. But the actual numbers on the scale are usually pretty off. You have to make your own known specific gravity solution and calibrate it. I don't know about the tiny glass hydrometer.
But I would trust the two instruments that you actually use a calibration fluid with.
You can mix up a known specific gravity solution, and mark it with the two hydrometers, then you could know how they are performing. But without that I would not trust them.

I had never seen the glass hydrometers but they are supposed to be high precision. It was way bigger than I expected and incredibly delicate.

#1 and #2 agree with each other, within published uncertainties. I'd trust #2.

This is what I was leaning towards as well.

 

[CK00020]

I float mine in a 250 ml grad cylinder just for safety

 

[SaltwaterScoop]

I float mine in a 250 ml grad cylinder just for safety

That's a good idea! Do you use yours for your "trusted" salinity?

 

[CK00020]

If it’s 77 degrees

 

[vtecintegra]

Only one I trust is the glass hydrometer.

 

[Randy Holmes-Farley]

I'd check all of them, including the glass hydrometer, in a known standard (DIY or high end commercial) designed for each device, if I wanted to have confidence in them.

That said, I expect the salinity is most likely 35-36 ppt, and is fine.

 

[KStatefan]

That's a good idea! Do you use yours for your "trusted" salinity?

I did after I mixed 450 ml (17.3 grams morton table salt) of Randy's Calibration Fluid to verify it was correct.

 

[Dennis Cartier]

I would trust #2, the TM High Precision hydrometer. It is what I use as my standard. It requires no calibration and should always be accurate as long as the water is 25C / 77F. To be confidant in it's reading, buying a second one to compare to and / or mixing up some of Randy's Calibration Fluid to verify against is a good idea. A tall cylinder for measuring outside of the tank, both to protect the delicate instrument, and to make it easier to read, is also a good idea.

 

[chipmunkofdoom2]

34.4 and 35.3 are very close. I'd trust either #1 or #2.

This is just my opinion, but it's unfortunate that the hobby has coalesced around refractive index as the default method to measure salinity. I've never had a refractometer that wasn't a hassle to use or keep calibrated. In my experience, drop-in measuring tools like the hydrometer above or conductivity meters are not much more expensive than quality refractometers and give much more reliable readings.

 

[srobertb]

34.4 and 35.3 are very close. I'd trust either #1 or #2.

This is just my opinion, but it's unfortunate that the hobby has coalesced around refractive index as the default method to measure salinity. I've never had a refractometer that wasn't a hassle to use or keep calibrated. In my experience, drop-in measuring tools like the hydrometer above or conductivity meters are not much more expensive than quality refractometers and give much more reliable readings.

Agree. I think it’s a great tool for making sure salinity stays stable, to watch for numbers creeping up and down, and to make sure new saltwater is in the ballpark but trying to get and hold a perfect 35 feels like a fools errand.

 

[MinnieMouse2]

I use the swing arm hydrometer and double check it after the water change with the Tropic Marin Glass High Precision Hydrometer. I totally gave up using refractometers ( garbage that do not stay callibrated) and the Hanna is expensive and they do break down. The only one I really trust is the glass high precision hydrometers and buy a good one. There are cheap models around, just say no. They use these glass hydrometers in the medical industry, so that got to be good. I think they use it for kidney dialysis treatments. My swing arm works well, usually after I double-check it is right on. You get use to using them and where you need to be.