What do Sg and PPT Mean? How They Measure Salinity - A Simple Explanation & Handy Calculator
Welcome to Simple Reefs. We are a brand new website designed to take the stress and expense out of starting your very own marine aquarium. We’ve been talking about marine aquarium Salinity a lot, lately. Today we are going more in depth on What do Sg and PPT Mean? How They Measure Salinity.
We also have a really handy calculator towards the bottom of this page that I developed to convert SG to PPT and PPT to SG. You might never need it but why not bookmark this page in case you ever do? Having the ability to convert units could be quite useful in certain situations. Particularly if you are reading salinity with a refractometer but the salinity you are referencing is in PPT.
Note: Please keep in mind that Simple Reefs is a very new website. Some information may be incomplete, some links may not be active, and some articles may be unfinished. Please bear with us while we build.
How Do We Measure Salinity?
We discussed the importance of salinity in a marine aquarium just a few days ago. If you need to catch up, feel free to check that article out. Suffice to say, Salinity is one of the most important parameters in your marine aquarium.
Every creature in your tank depends on it and keeping your salinity sufficient and stable is key to a healthy tank. We have an article coming up that will help you decide exactly what salinity you need your aquarium to be depending on what type of aquarium you have.
In our conversation on the importance of salinity, we were introduced to the terms Specific Gravity (SG) and Parts Per Thousand (PPT). These terms are rather ubiquitous in the marine aquarium hobby. This makes me wonder, how many people have the question what do SG and PPT mean and how do they work when measuring salinity?
After all, we become so familiar with the terms that we just make sure that our salinity matches and then move on. If you are wanting to gain a better understanding of how Specific Gravity and Parts Per Thousand work, as well the answer to the question “which is better?”, you are in the right place.
I should point out, however, that we aren’t going to be talking about the devices used to measure salinity in this article. I don’t want to run into thousands of words so that is coming up. In this article, we are focusing on the units that we use to measure Salinity.
What Do SG and PPT Mean? A Measurement of Salinity
Salinity can be measured in a number of different ways. The end goal is always the same, we are looking to find the amount of dissolved salt in the saltwater.
When it comes to marine aquariums, the most common terms you will see used are SG (specific gravity) and ppt (parts per thousand). While there are other ways to measure (Conductivity, units per ml, etc), SG and PPT are the usual standard in marine fishkeeping. Even digital devices that measure salinity using conductivity will typically convert the numbers to SG or PPT.
Both of these terms reference the amount of salt that is concentrated in the water. They just do it in slightly different ways. One is slightly better than the other but both will arrive at the same result and one is slightly more applicable to budget setups. We are all about saving money on Simple Reefs, after all.
Measuring Salinity Using SG (Specific Gravity)
The way specific gravity (SG) works is to measure the current density of saltwater compared to the density of a reference liquid like fresh water. You can almost think of it like one of those old fashioned scales with two plates. You know the type?
They judge weight by placing the object they want to weigh on one side of the scale with a calibrated weight on the other side. Imagine on one side of the scale you have a bucket of freshwater and on the other you have a bucket of saltwater.
It’s not at all scientifically accurate but you kind of get the picture. We are comparing the density of the saltwater against the density of freshwater to find the difference. The difference should equate to the amount of salt in the saltwater.
It is important to remember that the density of liquid changes based on temperature. So specific gravity has to be read at the temperature specified by the equipment made to read it. In the case of marine aquariums, this is typically 25 degrees Celsius. This introduces an outside variable that makes SG a less desirable way of measuring salinity.
So, for example, pure, fresh, water is always going to have a specific gravity of 1.000 at 4∘C. When you measure salinity using a device like a refractometer, it will be calibrated for a certain temperature, usually between 20-25∘C. This means that it will treat the specific gravity of pure water at 25∘C to be 1.000 even though it is actually a little less. This will give the measuring device a benchmark to calculate from.
As we add salt to that fresh water, the water is going to become more dense as it is now saturated with salt. We can then measure that density, making sure it is at the temperature that our device was calibrated to, and compare it to the density of pure water – 1.000. This gives us a measure of the amount of salt in the water.
SG Is A More Common Budget Measurement
Specific Gravity is a more common measurement when it comes to budget setups and there is a good reason for that – refractometers. Refractometers are the cheapest and most reliable way to read your aquarium’s salinity. Refractometers traditionally read salinity in SG, it’s that simple.
SG is a ratio so doesn’t technically have units. We always write it with a decimal, though. As mentioned earlier, freshwater is 1.000. In a marine aquarium, we are typically aiming for between 1.023 and 1.026SG when measured at 25∘C (77∘F).
Note the 0 in the middle, don’t make the mistake of attempting to aim for 1.23, Temperature is crucial to an accurate measurement, as well. Your device should say clearly what temperature it is calibrated for and you should always read your water at that temperature.
If you want to get into layman’s terms, specific gravity could be describe as a way of measuring the weight of the saltwater at a certain temperature. That isn’t scientifically accurate because weight and density are different things but I think it helps to make things a little easier to understand. If we had 1 litre of saltwater mixed up to 1.026SG at 25∘C, it should weigh 1,026 grams.
Measuring Salinity Using PPT (Parts Per Thousand)
PPT (parts per thousand) is a direct measure of mass. In this case, it quantifies the mass of the dissolved salts in a given solution. The solution here is saltwater so we are looking for the mass of salt dissolved within the saltwater.
The “thousand” part of parts per thousand refers to the saltwater itself (the solution) and the measurement is in grams so 1,000 grams of saltwater. The parts per thousand, in simple terms, means there are that many grams of salt in every 1,000 grams of saltwater. For instance, 35 ppt means there are 35 grams of dissolved salt in every 1,000 grams of saltwater solution.
Take note, here, of the nuance in that measurement. We aren’t measuring the mass of salt in freshwater. We are measuring the saltwater itself.
It’s not 35 grams of salt in 1000 grams of pure water, but rather 35 grams of salt combined with 965 grams of water to make 1000 grams of saltwater solution. This direct measurement makes PPT less susceptible to temperature variations compared to SG. As you may have guessed, PPT is a more scientifically accurate and desirable measurement.
A Practical Example
Let’s put all of that into a practical example as if we are mixing water. If you are performing a 10% water change on an aquarium with 250 litres of water, you will need to prepare 25 litres of new saltwater.
When mixing, it’s crucial to remember that saltwater is denser than freshwater. While 1 litre of pure freshwater weighs approximately 1 kilogram, 1 litre of saltwater at 35 ppt (which has an SG of approximately 1.026 at 25∘C/77∘F) weighs about 1.026 kilograms.
To calculate the salt needed for 25 litres of 35 ppt saltwater:
- Determine the mass of 25 litres of saltwater: To do this we need to remember that 1 litre of 35ppt saltwater weighs 1,026 grams at 25 degrees. We then need to do a simple calculation – 25 x 1026 which will give us the weight of 25 litres of saltwater which is 25.65kg or 25,650 grams.
- We can now calculate the rate of salt needed per 1,000 grams of salt water simply by dividing 35 by 1000 which gives us 0.035.
- We then take that amount of salt, 0.035, and multiply it by the weight of the saltwater we measured earlier so 0.035 x 25,650 which gives us the result 897.75 which is the amount of salt we will need to add to 25 litres of pure water.
Practical Tip: When mixing, you typically measure out your desired volume of fresh RO/DI water (e.g. 25 litres), then add the calculated amount of salt (approx. 898 grams for this example). The salt will dissolve, slightly increasing the overall volume of the solution, but your primary goal is to achieve the correct salinity for that volume.
Converting SG to PPT – A Handy Calculator
Specific Gravity and Parts Per Thousand can be converted back and forth pretty easily. Especially if we assume that 35PPT equals 1.026SG at 25 degrees Celsius which should be accurate. While there isn’t always a need for this, some websites and salt makers are pretty stubborn and only show one measurement. If that’s the case, you can use this handy calculator to convert back and forth.
In Short
Specific Gravity (SG) and PPT (Parts Per Thousand) are terms used to reflect the amount of salt currently dissolved in saltwater. While PPT is a more accurate and reliable measurement, budget conscious devices like Refractometers tend to measure in SG. Therefore it helps to become familiar with both terms. They can be converted between each other using the handy calculator above. Thanks for reading and spending your time at Simple Reefs.
