What are you using to test salinity?

[Dan1788]

Really just wanting to get an idea of what everyone is using to test their salinity and how accurate, precise, consistent it is. How often do you have to calibrate it? Any negatives? I recently purchased the Milwaukee digital refractometer and I'm pretty upset with it for how much it cost. I'm constantly calibrating it, it can test 2 different levels with pressing read twice back to back, constantly .002 over. Looking for different options.

I have a Milwaukee and it's just over a year old and it's broke gives all sorts of reading's. I need to get it repaired but not sure where to get it done.
I'm in the UK.

 

[ClownWrangler]

The measure/weigh/boil off/reweigh process is theoretically the most accurate, but you need to be insanely precise with every single step. As every single student who's taken gen chem or organic chem lab will tell you, even minor errors can have a dramatic impact on the result.

I have put plenty of thought into measuring salt by weight instead of volume. The problem is that sea salt mixes are extremely hygroscopic because they contain potassium salts. This means that if you weigh 1kg of salt when you first open a bucket and weigh out another kg after it's left open for a day or even opened numerous times briefly, the second time you will have less salt and more absorbed moisture. Eventually I just settled for an even ½ cup per gallon for an approximate sg of 1.024, then after fully dissolved, add 1tsp/gal per 1.001 change in SG needed. It has saved me a tone of time chasing numbers as it’s always close enough after that single adjustment.

 

[Reefinmike]

The measure/weigh/boil off/reweigh process is theoretically the most accurate, but you need to be insanely precise with every single step. As every single student who's taken gen chem or organic chem lab will tell you, even minor errors can have a dramatic impact on the result.

I cant argue with that! the TM glass hydrometer is significantly easier than dealing with all that mess.

 

[hds4216]

I have put plenty of thought into measuring salt by weight instead of volume. The problem is that sea salt mixes are extremely hygroscopic because they contain potassium salts. This means that if you weigh 1kg of salt when you first open a bucket and weigh out another kg after it's left open for a day or even opened numerous times briefly, the second time you will have less salt and more absorbed moisture. Eventually I just settled for an even ½ cup per gallon for an approximate sg of 1.024, then after fully dissolved, add 1tsp/gal per 1.001 change in SG needed. It has saved me a tone of time chasing numbers as it’s always close enough after that single adjustment.

Yeah, I do not believe there is any practical way to measure salt by weight for the purposes of our hobby without access to lab-grade equipment and storage methods.

 

[Reefinmike]

I have put plenty of thought into measuring salt by weight instead of volume. The problem is that sea salt mixes are extremely hygroscopic because they contain potassium salts. This means that if you weigh 1kg of salt when you first open a bucket and weigh out another kg after it's left open for a day or even opened numerous times briefly, the second time you will have less salt and more absorbed moisture. Eventually I just settled for an even ½ cup per gallon for an approximate sg of 1.024, then after fully dissolved, add 1tsp/gal per 1.001 change in SG needed. It has saved me a tone of time chasing numbers as it’s always close enough after that single adjustment.

RHF has a good article using table salt to create standards. I think he practically laughed at me when I asked about iodine or water impurities in NaCl. sea salt blends can be up to 17% or so water so that should always be accounted for...

 

[don_chuwish]

.

 

[Clownfish_Boy]

Hydrometer. Never needs calibration, and is accurate enough. for gov't work....

 

[Timfish]

Like my other testing methods I use multiple different manufactueres. My main goto is the Hanna digital but beside calibrating it and a refractometer I'm checking them against an hydrometer. Like Clownfish_Boy said, "accurate enough" and since it's going to measure ~.002 SG* low it can be a fairly accurate check against checkers that need to be calibrated.

*As I understand it the typical hydrometer is testing saltwater that's made just with sodium chloride and will test seawater with all it's additional stuff about ~ .002 SG low.

 

[DE FISH]

I just use a standard cheap refractometer I calibrate every time I use it with Ro water reading 0 TDS. I would however recommend a digital meter such as the hanna for more precision.

 

[Randy Holmes-Farley Verified Community Expert]

Has anyone heard or used the Tropic Marin hydrometer? Supposedly extremely accurate with zero calibration needed. Only downsides are size and how fragile it is.

High Precision Hydrometer

Ensure that your equipment is reading correctly and your aquarium is at the right salinity with Tropic Marin's High Precision Hydrometer. Many tank inhabitants are sensitive even to minor changes of the water conditions. Unfortunately as we've all experienced, salinity reading equipment as...

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It is likely a fine device. I had one that was accurate, but I will note that some people touting the accuracy have never actually verified if it is true or not.

But they have substantial drawbacks and I think conductivity meters are much more user friendly option. Temperature corrections needed, the fact that it needs deep water to float in, cannot be used in otehr settings (such as limewater potency),and the fact that it is not easy to read at a glance while doing other things (say, when acclimating new creatures) made me rarely use mine.

I use and liked an Orion Model 128 conductivity meter with a 4 electrode probe.

 

[Randy Holmes-Farley Verified Community Expert]

I just use a standard cheap refractometer I calibrate every time I use it with Ro water reading 0 TDS. I would however recommend a digital meter such as the hanna for more precision.

That might be inherently wrong to calibrate that way, but knowing the exact salinity may not be needed for many folks.

 

[Randy Holmes-Farley Verified Community Expert]

I have put plenty of thought into measuring salt by weight instead of volume. The problem is that sea salt mixes are extremely hygroscopic because they contain potassium salts. This means that if you weigh 1kg of salt when you first open a bucket and weigh out another kg after it's left open for a day or even opened numerous times briefly, the second time you will have less salt and more absorbed moisture.

The main problem is not the potassium, but the fact that magnesium and calcium are usually added to salt mixes as hydrates to begin with, and the magnesium ones (magnesium chloride hexahydrate and magnesium sulfate heptahydrate) may be close to 50% water by weight.

Salt mixes made with 35 grams per liter typically come out to 28-30 ppt. Craig Bingman showed that here decades ago:

Aquarium Frontiers Feature

 

[Randy Holmes-Farley Verified Community Expert]

Yeah, I do not believe there is any practical way to measure salt by weight for the purposes of our hobby without access to lab-grade equipment and storage methods.

Nothing you can do about it would matter. The salinity would be very low right out of the box, even with perfect equipment, as I show above.

 

[DE FISH]

100% agree I am hoping to get a meter when funds allow myself.

 

[Randy Holmes-Farley Verified Community Expert]

Nothing- and I mean absolutely nothing comes close to the accuracy, precision, resolution and ease of use as the Tropic Marin precision glass hydrometer.. the only way you can be more accurate is to measure/weigh/boil off/reweigh the salt in the sample. I gave away my refractometer and bought a second backup TM hydrometer in case I break my first one. two years and still intact...

That is untrue.

First, weighing is not a super accurate way to determine salinity. It was discarded as a preferred method decades ago. Not just because it is hard to do, but because it is not accurate.
This is trickier than it might seem, and not worth doing at home, but it can work and was the standard back in the 1800's to early 1900's. There's a detailed historical description in Chemical Oceanography by Millero. It requires pretty high heat (480 degrees C) since you need to dehydrate the salts that precipitate as crystalline hydrates (magnesium chloride, magnesium sulfate, etc.). Unfortunately, that also will convert carbonates and bicarbonates into oxides and hydroxides, causing weight loss. You also lose some chloride, bromide and iodide (as HCl, HBr, and HI and some Cl2 and Br2). Boric acid also vaporizes. All of these require corrections (or prevention) if high accuracy is a goal.


Conductivity is by FAR the most precise and accurate method, and is what chemical oceanographers use. It is the whole basis of the current salinity scale: 35 PSU is seawater that matches the conductivity of a particular specified concentration of potassium chloride (32.4356 grams in a mass of 1 kg of solution (=53 mS/cm).) in water.

 

[JosephM]

It is likely a fine device. I had one that was accurate, but I will note that some people touting the accuracy have never actually verified if it is true or not.

But they have substantial drawbacks and I think conductivity meters are much more user friendly option. Temperature corrections needed, the fact that it needs deep water to float in, cannot be used in otehr settings (such as limewater potency),and the fact that it is not easy to read at a glance while doing other things (say, when acclimating new creatures) made me rarely use mine.

I use and liked an Orion Model 128 conductivity meter with a 4 electrode probe.

I will look into that. I just had a bad experience with the Milwaukee digital refractometer that was already a bit pricey for my budget. I will definitely be looking to purchase a conductivity meter when I can but so far I am pleased with the hydrometer but completely agree on your points.

 

[Adamantium]

It's usually always within +/-.1. I calibrate every month. Once I calibrate, it reads 35ppt salt at 34.9-35.1.

I thought mine was spot on for the longest time, but turns out it was reading low. What it was showing as 35PPT was actually almost 38PPT. Even after multiple calibrations. Now I just have to read it low. About 32PPT is where I have to aim.

I confirmed this by using the Tropic Marin High Precision Hydrometer.

 

[wonroc]

i taste the water.

 

[mike550]

Hanna HI98319 salinity tester. Calibrate it every 30 days. Checked against a recently calibrated refractometer read by someone else.

 

[Jeff Jarry reef]

Milwaukee