WHY "PPT"?
Lo and behold: there I am, using a refractometer to measure the salinity of a water sample the FLOW team had just collected at the Bolsa Chica tidal inlet, as part of our weekly citizen science activities.
I am often asked about the meaning of the title I chose for this website.
What does “parts per thousand” mean?
Much like percentage, parts per thousand (ppt) is a ratio often used to refer to the concentration of solutes in solutions, such as salts in water (i.e. salinity). “The average salinity in ocean water”, you will read in most introduction to oceanography textbooks, “is 35 ppt”. But in reality, “hardcore scientists” do not use ppt as a unit for the measurement of salinity.
A brief and salty history lesson
Up until the 1980’s, scientists used to measure salinity in parts per thousand. Oceanographers would use titration-based techniques to determine the amount of one of the seawater constituents, such as chlorine or bromide, and then extrapolate that result to calculate the overall amount of salts dissolved in any given water sample. Salinity measured in this old-fashioned way used ppt as its unit.
For all practical purposes, salinity given in ppt corresponds to a ratio of mass: in other words, if you take 1 thousand parts of pure, filtered seawater at 35 ppt, and you evaporate all the water out of that, then you would be left with 35 parts of salts (so, from 1,000 grams of seawater, you would get 35 grams of salts; in 1,000 lbs of seawater you would get 35 lbs of salts, and so forth).
The titration-based methods were gradually replaced by the use of electrical conductivity, a more practical and accurate way to measure salinity. The scale used for salinity measured by electrical conductivity is called the practical salinity scale (PSS) and the resulting salt content measurement is not supposed to have any units (which can be quite confusing, and so we sometimes see folks using PSU, to indicate that the salinity is measured in practical salinity unit).
More recently, scientists introduced a whole new system to measure salinity called thermodynamic equation of seawater 2010 (TEOS-10) which uses Absolute Salinity (i.e. mass fraction of salt in seawater) as opposed to Practical Salinity to describe the salt content of seawater. According to this new system, measurements of water salinity should be followed by a unit again: g/kg.
So why did I choose a name that refers to a unit of salinity that is not even used anymore?
Well, because it is!
In fact, ppt is still THE unit used by most people measuring salinity around the world! Salinity is one of the most basic variables we measure when conducting any kind of monitoring activity in coastal bodies of water, and most of us use a simple piece of equipment called refractomer to do so.
Refractometers usually provide the user with a measurement of salinity in ppt in addition to a measurement of density or specific gravity. These measurements are extrapolated by the equipment from a reading of the sample’s refractive index. Light propagates differently through different water densities. The saltier the water, the denser it is, thus the more light refracts (or “bends”) as it passes through the drop of water that you place over the refractometer’s sensor.
While official oceanographic research has (as it should) strived to develop more accurate and precise ways to measure salinity, most folks engaged in Citizen Science and volunteering activities still use this simple, cheap, easy-to-carry and easy-to-operate piece of equipment called the refractometer. Therefore, to this day, most salinity readings being recorded around the world are still in ppt!
Which finally brings us to the actual answer for why I chose this name for the website:
because it symbolizes the inherent differences between "official" science and citizen science while reminding us of the fact that these two "types of science" can and should continue to co-exist and complement each other.
What does “parts per thousand” mean?
Much like percentage, parts per thousand (ppt) is a ratio often used to refer to the concentration of solutes in solutions, such as salts in water (i.e. salinity). “The average salinity in ocean water”, you will read in most introduction to oceanography textbooks, “is 35 ppt”. But in reality, “hardcore scientists” do not use ppt as a unit for the measurement of salinity.
A brief and salty history lesson
Up until the 1980’s, scientists used to measure salinity in parts per thousand. Oceanographers would use titration-based techniques to determine the amount of one of the seawater constituents, such as chlorine or bromide, and then extrapolate that result to calculate the overall amount of salts dissolved in any given water sample. Salinity measured in this old-fashioned way used ppt as its unit.
For all practical purposes, salinity given in ppt corresponds to a ratio of mass: in other words, if you take 1 thousand parts of pure, filtered seawater at 35 ppt, and you evaporate all the water out of that, then you would be left with 35 parts of salts (so, from 1,000 grams of seawater, you would get 35 grams of salts; in 1,000 lbs of seawater you would get 35 lbs of salts, and so forth).
The titration-based methods were gradually replaced by the use of electrical conductivity, a more practical and accurate way to measure salinity. The scale used for salinity measured by electrical conductivity is called the practical salinity scale (PSS) and the resulting salt content measurement is not supposed to have any units (which can be quite confusing, and so we sometimes see folks using PSU, to indicate that the salinity is measured in practical salinity unit).
More recently, scientists introduced a whole new system to measure salinity called thermodynamic equation of seawater 2010 (TEOS-10) which uses Absolute Salinity (i.e. mass fraction of salt in seawater) as opposed to Practical Salinity to describe the salt content of seawater. According to this new system, measurements of water salinity should be followed by a unit again: g/kg.
So why did I choose a name that refers to a unit of salinity that is not even used anymore?
Well, because it is!
In fact, ppt is still THE unit used by most people measuring salinity around the world! Salinity is one of the most basic variables we measure when conducting any kind of monitoring activity in coastal bodies of water, and most of us use a simple piece of equipment called refractomer to do so.
Refractometers usually provide the user with a measurement of salinity in ppt in addition to a measurement of density or specific gravity. These measurements are extrapolated by the equipment from a reading of the sample’s refractive index. Light propagates differently through different water densities. The saltier the water, the denser it is, thus the more light refracts (or “bends”) as it passes through the drop of water that you place over the refractometer’s sensor.
While official oceanographic research has (as it should) strived to develop more accurate and precise ways to measure salinity, most folks engaged in Citizen Science and volunteering activities still use this simple, cheap, easy-to-carry and easy-to-operate piece of equipment called the refractometer. Therefore, to this day, most salinity readings being recorded around the world are still in ppt!
Which finally brings us to the actual answer for why I chose this name for the website:
because it symbolizes the inherent differences between "official" science and citizen science while reminding us of the fact that these two "types of science" can and should continue to co-exist and complement each other.
Salinity, measured in parts per thousands with a $15 dollar-refractometer and by a Citizen Scientist- someone who may not have a degree in science, but who has received some training and is willing to help collect valuable data, that can then be used by scientists to make sense of the world- may not pass the most rigorous standard tests that are necessary to validate ALL research out there, but it is more than good enough for most water quality monitoring programs conducted around the world.
And good enough oftentimes is a very good thing. In this particular case, it is certainly better than no data at all!
By the way, since 1 kg= 1,000 grams, the unit of salinity measured by TEOS-10 (i.e. g/kg) is also a ppt ratio!
