Conductivity in MOhms (mega ohms?)

[alexmarto]

Hi, this is not reef related but I hope you don't mind helping me anyway.
I work with thermal waters that need to be tested daily. We send the sample to a laboratory and have several parameters tested including conductivity.
The results for conductivity are given in Ohm and that's where I have some doubts.
One example of a value is:

4.38 x 10 -3 Ohm -1 cm -1

If I convert this to MegaOhm x cm I get 4380 MOhm x cm.

We are thinking of purchasing a conductivity meter but 4380 MOhm x cm really blows off the scale of the comercialy available ones.
I'm reltively new at this work and I'm more used to microsiemens/cm or milisiemens/cm. That Ohm-1 x cm -1 is strange to me. Does it sounds right?
Where can i find a table with proper conversions between this units.
I searched reefcentral and found a link posted by Bomber but the link is dead now.

manklit

[Randy Holmes-Farley]

Only very pure water is usually measured in megaohms. Not aquarium or even RO water.

megaohms = 1/conductivity in uS/cm

18 megaohms (very pure water) has a conductivity of 0.055 uS/cm

[Boomer]

I had a real nice post going here then my PC crashed. I'll be short, as I see Randy has replied but missed something

Hob

You have this backwards

.38 x 10 -3 Ohm -1 cm -1

If I convert this to MegaOhm x cm I get 4380 MOhm x cm

That is to the -3 not +3

so, .00438 MÙ.

Resistivity is the reciprocal of Conductivity

1/ Ù + Conductivity, so

.00438 x 1,000,000 ( to convert MÙ to Ù = 4,380 Ù


1/4,380 Ù x 1,000,000 ( for micromho/micor siemens) = 228 uS/uMho or about 1-250 ppm TDS. Seawater is 35,000 TDS. The meter needs to get up down to 19 Ù

1/19 x 1,000,000 = 52,631 uS/uMho. Seawater Std. is 53,025 uS/uM

[alexmarto]

Thanks for the replies.

Just a few more questions if you don't mind, Ù = Ohm?

And the value that i gave should be 4380 Ohm and not 4380 MegaOhm like I said, is that right?

If understand right, uS = microsiemens and uMho is micro mega ohm??? Are you sure? Shouldn't be 228 uS/cm?

Thanks,

manklit

[Randy Holmes-Farley]

Thanks, Boomer.

[Boomer]

Randy must be cat napping

If understand right, uS = microsiemens and uMho is micro mega ohm??? Are you sure? Shouldn't be 228 uS/cm?

No. In the crashed post I explained this. Conductivity is often written in the old style, the word ohm backwards "mho". In more recent times people have gone to naming it after the the the guy that developed the concept, Dr. Siemen.


uMho = micro-mho

micro meg ohm, there is no such thing

micro = 1 millionth of a part

mega = 1 million parts

4380 Ohm and not 4380 MegaOhm like I said, is that right?


Yes, go back an read your post , it is

4.8 x 10-3, which means move the decimal 3 places to the left of the . after 4, so .0048.You move it to the right.If that was the case it should read 4.38 x 10 +3, for 4,380

The "U" is a scripting error. I posted the Greek sign for Omega, which is the sign for Ohm and instead I got the U, my fault. It showed up just fine on my reply but got changed on the actual post. Watch I will do it again

Ù

Go here, you will see what I mean

http://www.sartorius.com/en/biotechn...ivity_en.shtml

[alexmarto]

What I don't understand is why does the lab divide the 0.00438 by (Ohmxcm) and not just multiplies it.

0.00438 Ohm-1 cm -1 is the same as 0.00438/(Ohm x cm) isn't it? Which is not the same as 0.00438 Ohm x cm. Am I right?
So when using (megaohms = 1/conductivity in uS/cm) for conversion purposes, I should have the value in this form: Ohm x cm, don't I?

How can I go from 0.00438/(Ohm x cm) to ?????? Ohm x cm? Or am I just messing up this even more in my head?

I read your link, very informative but I think my dificulties are more related with the conversion between units than understanding resistivity. But thanks again for the link and the help you have provided.

manklit

[Boomer]

They all mean the same thing

Example;

This is sometimes confusing for some

0.00438 Ohm-1 cm -1

The -1 means per

so;

.00438 ohms per centimeter


You will often see this 300 mg -1, which means 300 mg per liter or 300 mg/l

You saw this on the website

The resistivity of absolute pure water is 18.2 M ohms or 0.055 micro-siemens .

Lets do the math.

First I said this

Resistivity is the reciprocal of Conductivity

I have this backwards
But my math was all correct


should be

Conductivity is the reciprocal of Resistivity, so 1 / Resistivity or 1 / ohms



so,
18.2 M ohms


1 / 18.2 megaohms, which we need to convert to ohms 1 ,000,000 x 18.2 =

18,200,000 ohms

1/reciprocal, which is number above

1 /18,200,00 = 5.49 x10 -8

We now need to use umho not mho's, so that value above x 1,000,000

5.49 x10 -8 x 1,000,000 = .0054945054 umho

Or as you noticed the short version

1/18.2 = .055 umho

The same number on the website and that Randy posted, only the above not rounded off to .0055 umho/uS


To make it easier for I finally found that website I was lookin' for


http://www.omega.com/techref/ph-2.html

Another typo eror

228 uS/uMho or about 1-250 ppm TDS

Should be about 100-150 ppm TDS

[Randy Holmes-Farley]

Resistivity is the reciprocal of Conductivity

I have this backwards
But my math was all correct


should be

Conductivity is the reciprocal of Resistivity, so 1 / Resistivity or 1 / ohms

Those mean the same thing, Boomer.

[seven ephors]

>... Those mean the same thing, Boomer.

Obviously, people who works with bombs too much don't know much about reversible operators like 1/(.)

[Randy Holmes-Farley]

Maybe they are trained to not think about reversing wires.

[seven ephors]

>>... 0.00438 Ohm-1 cm -1 is the same as 0.00438/(Ohm x cm) isn't it? Which is not the same as 0.00438 Ohm x cm. Am I right?
...

>... This is sometimes confusing for some
> 0.00438 Ohm-1 cm -1
> The -1 means per
>so;
> .00438 ohms per centimeter

Becareful when you type, Boomer.

4.38*10^-3 Ohm-1 cm -1 means 0.00438 1/Ohm * 1/cm, and 1/Ohm == S, so it means 0.00438 S/cm, or 4.38 uS/cm.
>>... Which is not the same as 0.00438 Ohm x cm. Am I right?...
When you convert uS to MOhm, you need to take the reciprocal of the value (1/0.00438 ~= 228). So they are not the same.

Some aqueous solution has resistivity higher than pure water, but most common lab resistivity meter that I am aware of only goes up to around 30 MegaOhm.

[Boomer]

Those mean the same thing, Boomer.

I know they are, as 1/18.8 = .053 and 1 /.053 =18.8 But most literature defines Conductivity as the reciprocal of Resistivity, with no mention of the other, so I just wanted to stay there with that

Maybe they are trained to not think about reversing wires

Yes, that will be my excuse

Seven

Becareful when you type

Opp's again .....you are correct

[alexmarto]

Sorry but this doesn't seem right to me ( I think).

Seven ephors wrote: ...1/Ohm == S, so it means 0.00438 S/cm, or 4.38 uS/cm

S stands for Siemens doesn't it?
And 0.00438 S/cm is 4.38 mS/cm (milisiemens) and not 4.38 uS/cm (micro siemens)... I believe.

0.00438 S/cm = 4380 uS/cm isn't it?

Thanks a lot to all!!

manklit

[Boomer]

Mat, yes that is correct

And 0.00438 S/cm is 4.38 mS/cm (milisiemens) **and not 4.38 uS/cm (micro siemens)**... I believe.

0.00438 S/cm = 4380 uS/cm isn't it? Correct


We all need to read this, Seven too

Becareful when you type,



We all made errors on this thread or missed things

[alexmarto]

I found a very good online tool to convert these units:

http://www.lenntech.com/unit-convers...tds_engels.htm

I found that 4.38 mS/cm = 2800 ppm!! Very mineralized water!!
Tomorrow I'm going to try my RO TDS meter but I tink it will blow off the scale.

Thanks again for the great help.

manklit

[Boomer]

Mank



Great find on that caculator. I haven't seen that one. I was also lookin' for a converter for Ohms to Mho, noluck yet

Randy will love this one from your site

http://www.lenntech.com/tds-ec_engels.htm

This is a GREAT site, thanks again

[Boomer]

Now look what you did, I will be spending hours on this site

[cyrilp]

Guys,

It should be much simpler!
4.38 x 10 -3 Ohm -1 cm -1 means "4.38 micromhos" = "4.38 microsiemens". Good RO water...

the difference between micromhos and microsiemens
____
cyrilp

[cyrilp]

"mho" is just reversed "ohm" to show 1/Ohm i.e. Siemens

And one more link: Technical Conductivity and Resistivity

[alexmarto]

Guys,

It should be much simpler!
4.38 x 10 -3 Ohm -1 cm -1 means "4.38 micromhos" = "4.38 microsiemens

Its milisiemens not microsiemens, from what I learned here.

Now look what you did, I will be spending hours on this site

Is that good or bad?

manklit

[Boomer]

Cyrlip

I will just add you to the list of;

Becareful when you type

"mho" is just reversed "ohm"

That has already been said above


And one more link:

That link was already posted above


"4.38 micromhos" = "4.38 microsiemens

Its milisiemens not microsiemens or milimhos and not micromhos

See how unsimple it is

We sure made a mess of a simple thing


Is that good or bad?

Well, that depends, it is a good site but is bad that is sucked up half my day. Loooooots of stuff here

[seven ephors]

>...We all need to read this, Seven too

>>Becareful when you type,

Ouch! ops:


>...Its milisiemens not microsiemens or milimhos and not micromhos

Boomer, it is milli-, not mili-. I think it is a Feudian thing for people in the military. Again, "Becareful when you type:

[Boomer]

Boomer, it is milli-, not mili-. I think it is a Feudian thing for people in the military. Again, "Becareful when you type



I think we should all quite why we are still waaaaaaaaay behind

[kbd]

Crumbs....I almost confused myself then ! Resistivity = 1 / conductivity (and yes, vice versa). Eees a miracle...

But you cannot equate conductance (resistance) with conductivity (resistivity). The dimensions are all wrong !

[QUOTE]I found that 4.38 mS/cm = 2800 ppm!! Very mineralized water!! Tomorrow I'm going to try my RO TDS meter but I tink it will blow off the scale[\QUOTE]

Not surprising. Seawater is about 53 mS/cm, and 35 ppt. So 4.38 should be about 3,000 ppm.

kim