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PPM vs TDS question


New Member
I have two meters, one PPM from BlueLab and one cheap one that came with my PH tester, that is labeled as a TDS meter.

Can anyone clarify the difference between PPM and TDS? And why would the BlueLab meter read 750ppm on a 700 scale and the off brand meter read at 1411ppm?

BlueLab options: 500scale, 700scale, EC (I’m using 700 scale setting & EC since I’m using Fox Farm nutrients)

Off brand options: ppm & EC readings

I trust the BlueLab one more since I paid much more $ for it, but I’m just wondering. The recording sheet I’m using has places to record EC, TDS and PPM, so how should I be recording the TDS vs PPM I order to give me the best data to adjust my water?



Nug of the Year: 2016 - Member of the Month: Sept 2015, Nov 2016 - Nug of the Month: Oct 2016 - Plant of the Month: May 2016
Both instruments just measure EC - the conductivity of the water. That’s all they’re capable of doing as far as direct measurement.

They then take that EC number and multiply it by one of several factors to guesstimate an amount of dissolved material. There are different ways of doing it.

I’m simplifying a bit, but TDS and PPM are basically just names for these different factors.

TDS is a term for what you get when you multiply the EC by 500.

PPM is what you get when you multiply by 750.

1 EC is the same as 500 TDS and the same as 750 PPM.

There’s no special reason to use one scale instead of the other. It would be easier and less confusing if everyone just used EC.


New Member
Thanks for spelling it out for me. Starting to see that I may be making this more complicated than it needs to be!

So then the two meters are reading very differently based on that then... the cheap meter is reading basically double the BlueLab one on the same water sample. I’m going with my gut and am going to quit using the cheap one, just record EC and PPM from the BlueLab meter.

Appreciate the help!


Nug of the Year: 2016 - Member of the Month: Sept 2015, Nov 2016 - Nug of the Month: Oct 2016 - Plant of the Month: May 2016
Yes I’d assume that the cheap one is reading wrong.

The main use for the meter is for ourselves. So we can measure things in a relative sense. Did the EC go up or did it go down? Is my feeding mix stronger or weaker than last week?

So in theory your cheaper meter could also work fine as long as it was consistent. But, it showing that far off isn’t a good sign.

Zeppelin forever :48:

Bush Doctor 77

Well-Known Member
TDS stands for Total Dissolved Solids. It is usually expressed in PPM (parts per million) or mg/L (milligrams per liter). 1mg/L=1ppm Instruments aren't very accurate for measuring TDS. In the water labs I worked in, we did it by evaporation or figured it by totaling the substances found in the chemical analysis. All instruments measure EC. Some convert it to TDS for you.
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