PSU, PPT, or specific gravity?

[ahiggins]

I’ve always used specific gravity but with the new Milwaukee reader....what’s the best to use? Do I need to take temp into consideration if it’s an atc?
Just trying to understand the why behind it.

 

[AllSignsPointToFish]

I think they're all about the same. Just different expressions of the same levels of salinity.

I always use the SG method. It's more common than the rest (at least in my experience), and even people who use PPT will recognize SG units.

Here's a good article that covers the topic:

http://reefkeeping.com/issues/2006-12/rhf/#6

 

[homer1475]

When I mix my water I prefer ppt(much more precise), but typically talk and write in SG since thats what most of us recognize.

 

[ahiggins]

When I mix my water I prefer ppt(much more precise), but typically talk and write in SG since thats what most of us recognize.

It don’t you have to take temperature into the equation to convert? Or does the atc do it for you?

 

[homer1475]

ATC does it for me.

I use and prefer my Hanna pen for salinity, just so darn easy to use. I do on occasion, when I get a weird reading(I weigh my salt so I know exactly how much to add to my water for the correct SG) will double check with a properly calibrated refractometer.

 

[Silver14SS]

It’s like reading a thermometer in Fahrenheit or Celsius, both measure temperature just different scales.

ATC should be automatically reflected in all of the measurements, that’s what the A in automatic temperature compensation is

Here’s a calculator if you want to play around and get more familiar with the different scales. https://reefapp.net/en/maintenance/calculator/unitconversion

 

[EMeyer]

While Silver is absolutely right - they are just different scale - I will argue we should all stop using SG, because it is fundamentally misleading. PPT for life.

Consider this. Imagine you mix a batch of saltwater and accidentally add 25% more salt than you should.

Salinity (measured in ppt or psu) will be 25% higher than normal (about 44 ppt). It looks like a big difference (44 vs 35) because it is!

Specific gravity will barely increase, to 1.0332. Thats less than a 1% increase in specific gravity from that of normal seawater (1.0264). This looks like a tiny difference, in terms of raw numbers, but its not. Its the same 25% increase in salinity, just expressed on a misleading scale.

I get that seasoned users who have been around a while understand 1.0332 is actually a very high salinity when they see it. But I think to new users, it looks like a tiny change. Cmon, we're talking about the second digit after the decimal point here

 

[ahiggins]

While Silver is absolutely right - they are just different scale - I will argue we should all stop using SG, because it is fundamentally misleading. PPT for life.

Consider this. Imagine you mix a batch of saltwater and accidentally add 25% more salt than you should.

Salinity (measured in ppt or psu) will be 25% higher than normal (about 44 ppt). It looks like a big difference (44 vs 35) because it is!

Specific gravity will barely increase, to 1.0332. Thats less than a 1% increase in specific gravity from that of normal seawater (1.0264). This looks like a tiny difference, in terms of raw numbers, but its not. Its the same 25% increase in salinity, just expressed on a misleading scale.

I get that seasoned users who have been around a while understand 1.0332 is actually a very high salinity when they see it. But I think to new users, it looks like a tiny change. Cmon, we're talking about the second digit after the decimal point here

That’s what got me a few months ago when my refractometer went on the fritz. I decided to go with Milwaukee but after looking at the ppt values, I read it’s all about temp. Seeing as how the atc does it on its own, I didn’t know how to go back to SG after getting the ppt reading. Or do I even need to?
1.025 sg (what I keep my reef) at 78degrees is a different sg at a different temperature. That’s what I have gathered anyway. Is that wrong?
Will ppt change from temp to temp?

 

[Bayareareefer18]

Jake Adam's would cringe at this question lol

 

[SeaDweller]

I use the Milwaukee and use PPT; I've found if the metal plate is warm or at least close to what your tank temp is, its reading is more accurate.

 

[Randy Holmes-Farley Verified Community Expert]

That’s what got me a few months ago when my refractometer went on the fritz. I decided to go with Milwaukee but after looking at the ppt values, I read it’s all about temp. Seeing as how the atc does it on its own, I didn’t know how to go back to SG after getting the ppt reading. Or do I even need to?
1.025 sg (what I keep my reef) at 78degrees is a different sg at a different temperature. That’s what I have gathered anyway. Is that wrong?
Will ppt change from temp to temp?

There is no real change in ppt with temperature since it is derived from the ratio of the salt mass to the total mass.

BUT, the change in specific gravity is also very small. It comes from the change in both the seawater volume and freshwater volume with temperature. Both are small, and in some ranges, are changing similarly so the actual SG is hardly changing. Here's some data from one of my articles:

The specific gravity of natural seawater (S =35) compared to freshwater at the same temperature is
1.0278 at 3.98°C
1.0269 at 60°F
1.0266 at 20°C
1.0264 at 77°F

The thing that throws most people is that the typical devices for measuring salinity in any units of measure are themselves VERY dependent on temperature, and they all need corrections. It's not the units of measure, its the devices!

 

[ahiggins]

There is no real change in ppt with temperature since it is derived from the ratio of the salt mass to the total mass.

BUT, the change in specific gravity is also very small. It comes from the change in both the seawater volume and freshwater volume with temperature. Both are small, and in some ranges, are changing similarly so the actual SG is hardly changing. Here's some data from one of my articles:

The specific gravity of natural seawater (S =35) compared to freshwater at the same temperature is
1.0278 at 3.98°C
1.0269 at 60°F
1.0266 at 20°C
1.0264 at 77°F

The thing that throws most people is that the typical devices for measuring salinity in any units of measure are themselves VERY dependent on temperature, and they all need corrections. It's not the units of measure, its the devices!

Those darn devices!