conductivity question
- Thread starter josephv
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Conductivity is a measure of how well a given sample conducts electricity. Chemically pure H2O is a terrible conductor of electricity. In the real world, however, water is one heck of a solvent, and it's rarely encountered at that level of purity. RO/DI comes pretty darned close, though, so its conductivity should be very low.
Conductivity relates to Total Dissolved Solids (TDS) because dissolving things into water increases its conductivity. As the tds of a water sample increases, so does the conductivity. Luckily for us, the relationship between sea salt concentration and conductivity has been firmly established (probably by thousands of readings of each property on seawater samples of various concentrations).
Another major way we measure salt content in water is specific gravity. The various elements dissolved
in seawater do not have the same density as water. 100 mls of pure water weighs less (or has lower mass) than 100 mls of water containing a bunch of extra salts. Pure water weighs exactly as much as pure water, or 1.000. All other specific gravity readings are a ratio of the density of your sample compared to the density of pure water.
Lastly, there's refraction. Light passing through a bunch of water molecules is bent, or refracted, at a particular angle. Light that passes through a bunch of molecules and ions of water, sodium, chlorine, etc. is refracted at a different angle. This is what refractometers actually measure.
For sea salt in water, the four measurements have been correlated to each other. That's why you can have a specific gravity scale on a refractometer, and a tds readout on a conductivity meter.
Of course, by the time I get done typing all this, someone will have answered the question already...
Edit: Wow. Nobody did. Anyway, according to some Google searching, the conductivity of raw seawater averages about 53 milliSiemens (aka milliMhos) per centimeter. I'm finding DI cartridge manufacturers claiming they can achieve 0.1 microSiemens per cm, but I haven't found a general statement that "RO/DI water should have a conductivity less than x".
Conductivity relates to Total Dissolved Solids (TDS) because dissolving things into water increases its conductivity. As the tds of a water sample increases, so does the conductivity. Luckily for us, the relationship between sea salt concentration and conductivity has been firmly established (probably by thousands of readings of each property on seawater samples of various concentrations).
Another major way we measure salt content in water is specific gravity. The various elements dissolved
in seawater do not have the same density as water. 100 mls of pure water weighs less (or has lower mass) than 100 mls of water containing a bunch of extra salts. Pure water weighs exactly as much as pure water, or 1.000. All other specific gravity readings are a ratio of the density of your sample compared to the density of pure water.
Lastly, there's refraction. Light passing through a bunch of water molecules is bent, or refracted, at a particular angle. Light that passes through a bunch of molecules and ions of water, sodium, chlorine, etc. is refracted at a different angle. This is what refractometers actually measure.
For sea salt in water, the four measurements have been correlated to each other. That's why you can have a specific gravity scale on a refractometer, and a tds readout on a conductivity meter.
Of course, by the time I get done typing all this, someone will have answered the question already...
Edit: Wow. Nobody did. Anyway, according to some Google searching, the conductivity of raw seawater averages about 53 milliSiemens (aka milliMhos) per centimeter. I'm finding DI cartridge manufacturers claiming they can achieve 0.1 microSiemens per cm, but I haven't found a general statement that "RO/DI water should have a conductivity less than x".
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One more note: When it comes to RO/DI units, many of the manufacturers don't report the conductivity of the purified water, but its resistance. Instead of siemens or mhos per cm, their specifications are given in ohms (or megaohms).
Resistance is the inverse of conductivity. As resistance goes up, conductivity goes down. That's probably why an older unit of conductivity was the mho (the inverse of ohm). Since 1971, the official SI unit has been the siemens (singular and plural both end in s), but ohms are still frequently used in electronics.
Resistance is the inverse of conductivity. As resistance goes up, conductivity goes down. That's probably why an older unit of conductivity was the mho (the inverse of ohm). Since 1971, the official SI unit has been the siemens (singular and plural both end in s), but ohms are still frequently used in electronics.
josephv
New member
Now its time to get to the meat of the matter. all of our fresh and salt water friends measure the conductivity or tds of water we put into our tanks but does anyone have info on what the numbers inside the display tank should be. If so how does conductivity or tds relate to our our waters viability in sustaining our mini Eco systems, What should our tds or conductivity be at optimal levels inside our tanks? What number should dictate a water change.From what I have studied we should do a water change once a week around 10 %. That seams like throwing money away just because we don't know any better.Please forgive my ignorance.
I'm not sure that setting a target conductivity/tds number for water changes is the way to go. Water changes are not just about lowering tds; they also replenish calcium, magnesium, iodine, and other elements that are depleted by corals and other critters.
Another issue, especially in salt water, is that we deliberately dissolve a lot of solids into the water. Once you've added artificial salt mix, I'm not sure that you could distinguish "bad" water from "good" water with a conductivity meter.
When water comes out of an RO/DI its conductivity should be as close to 0 as possible. Any source of conductivity is bad. As soon as we add salt to this "pristine" water, I think our chance of distinguishing desirable sources of conductivity from harmful sources rapidly approaches zero.
Another issue, especially in salt water, is that we deliberately dissolve a lot of solids into the water. Once you've added artificial salt mix, I'm not sure that you could distinguish "bad" water from "good" water with a conductivity meter.
When water comes out of an RO/DI its conductivity should be as close to 0 as possible. Any source of conductivity is bad. As soon as we add salt to this "pristine" water, I think our chance of distinguishing desirable sources of conductivity from harmful sources rapidly approaches zero.
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