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 Verified Community Expert]

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.

 

[330Bob]

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?

You calibrated with different solutions and one of those is off. Use one solution to calibrate with hopefully one that is actually 35ptt. The refractometer and Hanna should be able to check the two calibration fluids you have.

If both calibration fluids check 35 ptt on the refractomter at same temp 35ptt the water in your tank if really 35 would read 35 not 38. With the info provided #1 and #2. But it could be the cora life and refractometer are correct and the 1 and 2 are off. On the refractometer do not read the SG ignore that and read and calibrate using just the ppt ignore the sg. If the calibration fluid say 35ppt set that so the line passes through the 35ppt grid line.

 

[Randy Holmes-Farley Verified Community Expert]

You calibrated with different solutions and one of those is off. Use one solution to calibrate with hopefully one that is actually 35ptt. The refractometer and Hanna should be able to check the two calibration fluids you have.

If both calibration fluids check 35 ptt on the refractomter at same temp 35ptt the water in your tank if really 35 would read 35 not 38. With the info provided #1 and #2. But it could be the cora life and refractometer are correct and the 1 and 2 are off. On the refractometer do not read the SG ignore that and read and calibrate using just the ppt ignore the sg. If the calibration fluid say 35ppt set that so the line passes through the 35ppt grid line.

I would not assume that a solution designed as a 35 ppt standard for a conductivity meter will read 35 ppt on a refractometer.

 

[Dan_P]

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?

Trust no one and nothing. Pick your favorite instrument, obtain a reference standard for it, calibrate it, and then stick with it. Check the instrument several times a year at least.

 

[330Bob]

I would not assume that a solution designed as a 35 ppt standard for a conductivity meter will read 35 ppt on a refractometer.

I don't have an electronic unit but why not use actual sea water to calibrate one? In the end you are testing the actual water in a aquarium and water taken out of the ocean that is a true 35ppt should check 35ppt on any unit even electronic ones.
As for the calibration juice used on electronic devices, I assumed it was sea water (I try to never assume). Its index of refraction could be different from sea water at various temps which would be a problem using it on a refractometer.

Your point is well taken, and as you know, assumption is the mother of of all ....

 

[Randy Holmes-Farley Verified Community Expert]

I don't have an electronic unit but why not use actual sea water to calibrate one? In the end you are testing the actual water in a aquarium and water taken out of the ocean that is a true 35ppt should check 35ppt on any unit even electronic ones.
As for the calibration juice used on electronic devices, I assumed it was sea water (I try to never assume). Its index of refraction could be different from sea water at various temps which would be a problem using it on a refractometer.

Your point is well taken, and as you know, assumption is the mother of of all ....

It would be perfect to buy a calibrated seawater standard, but they are quite expensive. it is easier and cheaper to make a standard with known refractive index, conductivity, or density (depending on the device requiring calibration).

Seawater salinity is actually based on potassium chloride solution conductivity, so using KCl or NaCl for that one is a no brainer.

The change in refractive index of NaCl solutions matching 35 ppt seawater with temp is very close to the change in actual 35 ppt seawater refractive index with temp, since it is mostly NaCl to begin with. So the error is not going to be significant when using a hobby device with a fairly large error anyway.

That said, some companies, such as American marine/Pinpoint make their conductivity standard as a seawater mimic (contains the major ions), so it doesn't have that issue either.


Here are some standards:

Seawater Standards & Salinometers Archives

Salinometers & Seawater Standards for salinity calibration, nutrient analyses, laboratory determination of Dissolved Oxygen & quality assurance.

osil.com

 

[330Bob]

Randy I just noticed you did an article on refratometers. I just looked at it. I appreciate the amount of effort you put into it, I enjoyed it nice work!