TDS VS Conductivity

[LadyTang2]

Some meters state they measure conductivity but not TDS. But Randy wrote a while back "conductivity of 0.5 to 1.5 mg/L TDS."
- Can TDS be measured by a conductivity meter then it's just like a unit conversion or? If so what would 1 ppm tds be in conductivity?

If one has a good conductivity meter, is a low reading on that just as useful in terms of thinking about low tds when measuring RO water? So one can basically use the conductivity in place of a tds meter or are there advantages to getting on that claims it can do TDS?

 

[Righteous]

Yes. TDS meters are conductivity meters. To get the TDS you’d take the measured microsiemens/cm and multiply it by a conversion factor (I think it’s around 0.7 for seawater)

Just be careful with your units. If your conductivity meter is displaying ppt, then even low readings will equate to high TDS, so you’d need a reading in microsiemens to get accurate low range TDS readings. With apex for example you’d need the separate PM2 module to get it to read in that range.

 

[Randy Holmes-Farley Verified Community Expert]

Right, all TDS meters work by measuring conductivity. But not all "TDS meters" can read high enough to measure aquarium salinity, and not all reef hobby "conductivity meters" or "salinity meters" can read low enough to monitor an RO/DI.

As with most chemistry things that reefers use, nothing is perfect, and the conversion of conductivity units (like mS/cm and uS/cm) cannot exactly be converted into ppm TDS without knowing exactly what the chemicals in the water are. That is true because, for example, 1 ppm of potassium chloride has a somewhat different conductivity than 1 ppm of sodium sulfate.

I describe these details here:

What is TDS?

http://www.reefkeeping.com/issues/2004-04/rhf/feature/index.htm

 

[LadyTang2]

Right, all TDS meters work by measuring conductivity. But not all "TDS meters" can read high enough to measure aquarium salinity, and not all reef hobby "conductivity meters" or "salinity meters" can read low enough to monitor an RO/DI.

As with most chemistry things that reefers use, nothing is perfect, and the conversion of conductivity units (like mS/cm and uS/cm) cannot exactly be converted into ppm TDS without knowing exactly what the chemicals in the water are. That is true because, for example, 1 ppm of potassium chloride has a somewhat different conductivity than 1 ppm of sodium sulfate.

I describe these details here:

What is TDS?

http://www.reefkeeping.com/issues/2004-04/rhf/feature/index.htm

What would an acceptable reading in uS or mS be for rodi water? And what would a really good reading be out of a multistage rodi system with new blocks and all? Can you guess what 1ppm might be in uS or mS?

If you had a good conductivity meter that could go down to measuring 1μS (that is possible right) then would that be good enough for checking RODI water?

 

[Randy Holmes-Farley Verified Community Expert]

Yes. A high quality conductivity meter is good enough to read 1 uS/cm as different from 0 uS/cm or 1 ppm TDs as different from 0 ppm TDS. The best ones can accurately read far below 1 ppm TDS or 1 uS/cm.

 

[LadyTang2]

Yes. A high quality conductivity meter is good enough to read 1 uS/cm as different from 0 uS/cm or 1 ppm TDs as different from 0 ppm TDS. The best ones can accurately read far below 1 ppm TDS or 1 uS/cm.

Thanks! Can you link an example of one that can go below 1uS?

Any idea on how what an acceptable uS reading would be for rodi?

 

[Randy Holmes-Farley Verified Community Expert]

Thanks! Can you link an example of one that can go below 1uS?

Any idea on how what an acceptable uS reading would be for rodi?

Depends. With an inline meter, 0 is the goal and 1 uS/cm is starting to deplete.

But CO2 alone from the air can start to bring up conductivity, so taken in a container after the fact, then 0 ppm TDS or 0 uS/cm is harder to attain.