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How do refractometers really work
- Thread starter 2poor2reef
- Start date
Understood Iron. But exactly how does it measure salinity. I thought it measured the difference between how light passes through saltwater versus pure (distilled) water. Couldn't other aspects of the sample water affter the refraction also? Or do I fundamentally misunderstand the process?
All sorts of translucent stuff will bend light differently depending on molecular arrangement and density.
(Like those old grade-school science projects...
A stick poked downward into a container of water will appear broken at the water's surface, precisely because light reflected from the stick, passing through the water back up into air has been bent.)
Saltwater and not-so-salty water are different materials in this sense, with different densities, and therefore dfferent refractive influences on light.
If you know what the proper refraction index (which tells you how much it bends light) for proper saltwater is, then a device measuring refraction can be used to confirm that you indeed have proper saltwater. (Certainly, temperature affects a liquid's density, and will affect its refractive effect on light.)
In many refractometers, light is bounced off the sample, and the device differentiates between light bounced right off the sample's surface, and light that has penetrated and THEN bounced back. Refraction will ensure a difference in the angles at which those two kinds of returned light come, and the device uses that difference to determine the sample's refractive index (Snell's Law, and all that.)
hth,
horge
(Like those old grade-school science projects...
A stick poked downward into a container of water will appear broken at the water's surface, precisely because light reflected from the stick, passing through the water back up into air has been bent.)
Saltwater and not-so-salty water are different materials in this sense, with different densities, and therefore dfferent refractive influences on light.
If you know what the proper refraction index (which tells you how much it bends light) for proper saltwater is, then a device measuring refraction can be used to confirm that you indeed have proper saltwater. (Certainly, temperature affects a liquid's density, and will affect its refractive effect on light.)
In many refractometers, light is bounced off the sample, and the device differentiates between light bounced right off the sample's surface, and light that has penetrated and THEN bounced back. Refraction will ensure a difference in the angles at which those two kinds of returned light come, and the device uses that difference to determine the sample's refractive index (Snell's Law, and all that.)
hth,
horge
A refractometer doesn't measure salinity, it measures the refractive index of the sample relative to air, as horge as said.
Other dissolved things in the water can also alter the refractive index. But most "other things" are small relative to the amount of salt you put in there.
FWIW, I use a refractometer to measure specific gravity in my brewing hobby, only in that case the gravity reading is caused by the dissolved sugars... not salt.
Other dissolved things in the water can also alter the refractive index. But most "other things" are small relative to the amount of salt you put in there.
FWIW, I use a refractometer to measure specific gravity in my brewing hobby, only in that case the gravity reading is caused by the dissolved sugars... not salt.
A refractometer measures how much light is bent (refracted) when passed through a sample versus air (as previously mentioned). Just about anythingthings dissolved in the sample will affect this measure.
FWIW- A hydrometer that measures specific gravity is measuring basically the same thing only in a different way: it measures the density of the sample relative to the density of pure water. A specific gravity of 1.025 means that for a given volume of the sample it weighs 1.025 times as much as the same volume of pure water. It doesn't matter what is dissolved in the sample.
What is important here is the relative amounts of whats dissolved in the sample. The amount of NaCl (Salt) in the water is much, much higher than anything else. This is why these devices work to approximate the amount of salt dissolved in the water. In fact, another measure of salinity is ppt (parts per thousand) where most other things dissolved in the water is measured in ppm (parts per million) or ppb (parts per billion). In natural seawater, salt is 30 times more abundant than the next most prevalent substance, magnesium.
Ernie
FWIW- A hydrometer that measures specific gravity is measuring basically the same thing only in a different way: it measures the density of the sample relative to the density of pure water. A specific gravity of 1.025 means that for a given volume of the sample it weighs 1.025 times as much as the same volume of pure water. It doesn't matter what is dissolved in the sample.
What is important here is the relative amounts of whats dissolved in the sample. The amount of NaCl (Salt) in the water is much, much higher than anything else. This is why these devices work to approximate the amount of salt dissolved in the water. In fact, another measure of salinity is ppt (parts per thousand) where most other things dissolved in the water is measured in ppm (parts per million) or ppb (parts per billion). In natural seawater, salt is 30 times more abundant than the next most prevalent substance, magnesium.
Ernie
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