salt water conductivity

[snowmansnow]

I stole this from another post....
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Pure water, is a very poor conductor. It would actually make a good insulator but for one major drawback. It is near impossible to keep water pure. The water in our taps has lots of heavy metals, salts and minerals. Those impureties are what make the water a good conductor.

So how does that relate to our tank? We remove all the minerals and heavy metals with our RO/DI correct? Think about the next step. We add salt and a lot of it. So, while I haven't researched it to be sure, I would bet the water in our tanks has better conductive properties than even tap water.
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When lightning strikes the ocean why dont we see a huge kill off of fish / sea life?

B

 

[heathlindner25]

they're not grounded

 

[snowmansnow]

I found the answer but thats not it

 

[heathlindner25]

what you got

 

[snowmansnow]

theres a thing called "skin" effect... which means that lightning has a tendency to travel and spread along the SURFACE of the water. Theres also a thing called "skin depth" which is the depth DOWN that electricity would be conducted.. most of the time fish and other life is under the skin depth and so they aren't affected.
Whod have thunk it
B

 

[rdnelson]

First of all, that looks very familiar

But to answer your question......

When lightning strikes the ocean all the fish in the ocean are suspended in the water column and not touching the earth. It is like a bird sitting on a power line. Unless it touches something connected to the ground no current will flow through it. Before you say "What about the creatures crawling around on the sea floor?" it can be explained as well although a bit more complex.

Electrical theory indicates that electricity will follow the path of least resistance in proportion to the amount of resistance. So if you have two paths of differing resistance, current will flow through both but more will flow through the path with the lower resistance. In the ocean, the salt water is in contact with earth for the entire surface area of the sea floor. The water has a high conductivity (very low resistance). The creatures crawling on the sea floor have a much higher resistance. So when lighting strikes most of the current will flow directly to earth through the water and very little will flow through the high resistive bodies of the creatures.

 

[heathlindner25]

thanks for the info

 

[rdnelson]

SnowManSnow;806726 wrote: theres a thing called "skin" effect... which means that lightning has a tendency to travel and spread along the SURFACE of the water. Theres also a thing called "skin depth" which is the depth DOWN that electricity would be conducted.. most of the time fish and other life is under the skin depth and so they aren't affected.
Whod have thunk it
B

Skin effect (or what I was taught was called "Skim" effect) is a factor. The higher the voltage the more the current tends to flow on the outer surfaces of the conductive materials. Voltage levels in lighting are EXTREAMLY high so yes skin or skim would be magnified I suppose. Close to shore I could see where the bulk of the current could flow on the water surface to the shore line but it seems to me that in the middle of the ocean, the distance would be to great and it would have to flow down through the water column to the sea bed. But I could be very wrong. :doh:

 

[w_hartyjr]

man,you guys have way too much time on your hands:lol2:

 

[heathlindner25]

it's a couple electricians talking.

 

[miami dolfan]

w_hartyjr;807248 wrote: man,you guys have way too much time on your hands:lol2:

And how!!:up:

 

[grouper therapy]

w_hartyjr;807248 wrote: man,you guys have way too much time on your hands:lol2:

Miami Dolfan;807261 wrote: And how!!:up:

You had time to read it

 

[stacy22]

OK, electricity geeks I have a question for you.

I want my Oceanography students to measure salinity in several different ways. One of them is via conductivity. I have seen it reported that t<span style="font-family: Calibri">he conductivity of seawater with a salinity of 35 ppt is 53 mS/cm. </span>
<span style="font-family: Calibri"></span>
<span style="font-family: Calibri">Is there any easy way I can use a simple hand-held multimeter and get something close to this value? I think I would have to actually measure Ohms? then use "conductivity=1/resistance" to get an answer. Does this sound remotely correct? I was playing around with the multimeter yesterday and couldn't dial in any setting to get a value like reported. </span>
<span style="font-family: Calibri"></span>
<span style="font-family: Calibri">The model I have is a CSi/Speco DMR-2500 if it helps. </span>

 

[cdavidson]

stacy22;807372 wrote: OK, electricity geeks I have a question for you.

I want my Oceanography students to measure salinity in several different ways. One of them is via conductivity. I have seen it reported that t<span style="font-family: Calibri">he conductivity of seawater with a salinity of 35 ppt is 53 mS/cm. </span>

<span style="font-family: Calibri">Is there any easy way I can use a simple hand-held multimeter and get something close to this value? I think I would have to actually measure Ohms? then use "conductivity=1/resistance" to get an answer. Does this sound remotely correct? I was playing around with the multimeter yesterday and couldn't dial in any setting to get a value like reported. </span>

<span style="font-family: Calibri">The model I have is a CSi/Speco DMR-2500 if it helps. </span>

I am a mechanical engineer but let me give this a shot. Firstly, Conductivity=1/Resistivity, not 1/Resistance. 53 mS/cm = .053 S/cm. The resistivity is then 1/(.053 S/cm) which is 18.87 Ohm-cm. I think the problem here is that you need to know not only the distance between the two probes but the surface area. Resistance = (Resistivity*Length)/Area. Do you have a way to use two probes with a known surface area and set them a known distance apart?

 

[rdnelson]

Not familiar with that meter Stacy but my guess is that it would not do what you are wanting it to do. While the conductivity will change with changing salinity levels, I suspect it would be minute changes and therefore would take a very precises instrument Even the high dollar Fluke Meters used by most tradesman would not be that precise in my estimation. But I could be wrong. I would say someone like Bill M would be more likely to have an instrument that would be capable of that kind of reading.

 

[stacy22]

cdavidson;807380 wrote: I am a mechanical engineer but let me give this a shot. Firstly, Conductivity=1/Resistivity, not 1/Resistance. 53 mS/cm = .053 S/cm. The resistivity is then 1/(.053 S/cm) which is 18.87 Ohm-cm. I think the problem here is that you need to know not only the distance between the two probes but the surface area. Resistance = (Resistivity*Length)/Area. <span style="color: red">Do you have a way to use two probes with a known surface area and set them a known distance apart</span>?

They will be using the probes in a 10 gallon tank. It's ~ 50 cm long by 21cm front to back. .

Edit:

rdnelson99;807382 wrote: Not familiar with that meter Stacy but my guess is that it would not do what you are wanting it to do. While the conductivity will change with changing salinity levels, I suspect it would be minute changes and therefore would take a very precises instrument Even the high dollar Fluke Meters used by most tradesman would not be that precise in my estimation. But I could be wrong. I would say someone like Bill M would be more likely to have an instrument that would be capable of that kind of reading.

I figured this would be the case. I have this gadget and never get to use it. Us biology/chemistry folks miss out on all the fun.

 

[cdavidson]

Math Error: I should had said 18.87 MOhm-cm.
The problem is still that you need a probe with a known surface area. If you can do that than you can measure the resistance. The effect of salinity on conductivity should use something like a wheatstone bridge and opamp in my opinion to get good data, but you should be able to see a change in resistance with just a standard multimeter.

EDIT: If you have RO/DI and saltwater side by side, you will be able to measure a different resistance.

 

[stacy22]

Thanks for the help guys. Looks like they will just have to measure salinity using floating hydrometer, swing arm hydrometer, and refractometer.

 

[heathlindner25]

way above my pay grade!!

 

[rdnelson]

heathlindner25;807409 wrote: way above my pay grade!!

LOL Mine too but I fake it well. :wow2: