# Cheap TDS meter correlation?



## Ma_sha1 (Oct 15, 2016)

My phal skill is OK, but my Paph & phrag skills need improvement! 

I got a cheap TDS meter, something is better than nothing right?


My tab water - 31 ppm
My 1/8 strength k-lite -95 ppm

It seems too low?

Assume calibration is off & I can't recal. The cheap meter. If I correct it by adding 100 to all.


Tab water 131 ppm
K-lite 1/8 - 195 ppm

Does it make sense? 






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## Ray (Oct 15, 2016)

No.

TDS meters, for the most part, are cheap electrical conductivity meters with a built-in conversion factor to TDS. The conductivity of the solution is determined by the content of dissolved ions, and each dissolved ion has its own size and charge, both of which affect its mobility in the solution, hence its conductivity. That means that the same true content (ppm) of two different ions will give two different conductivities, so how can a single conversion factor give a true reading.

That does not mean, however, that the meter is worthless. While the TDS value given is of little use, you can still use it to discern changes.

If you want to develop your own calibration, you need a good EC meter and standard solutions, then compare your meter to the known true readings.


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## Ma_sha1 (Oct 15, 2016)

Thanks Ray.

I can get compare to my next district water report, I think it lists TDS. Many that'll give me an idea how much is off. 

I agree, relative info is still better than no info.


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## PaphMadMan (Oct 15, 2016)

Buy some distilled water. It should be close to 0ppm. You should be able to make a solution of K-lite in distilled water that will have a known approximate ppm. The readings from those will let you determine a reasonable correction for your meter, good enough to monitor any changes in your tap water and fertilizer solution.


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## Brabantia (Oct 29, 2016)

Ray said:


> No.
> 
> TDS meters, for the most part, are cheap electrical conductivity meters with a built-in conversion factor to TDS. The conductivity of the solution is determined by the content of dissolved ions, and each dissolved ion has its own size and charge, both of which affect its mobility in the solution, hence its conductivity. That means that the same true content (ppm) of two different ions will give two different conductivities, so how can a single conversion factor give a true reading.
> 
> ...


Standart solution for EC meter calibration is usally à KCL solution 0.05 molar. KCL is not used for fertilizer production so the calibration is always non accurate. As for exemple: when I measure 500 microSiemens the real conductivity using the conductivity of each component (taking into account there individual conductivity and there concentration) is 400 microSiemens. When we use a conductivity meter we are thus always on the security side for our orchids.

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## Ma_sha1 (Oct 29, 2016)

PaphMadMan said:


> Buy some distilled water. It should be close to 0ppm. You should be able to make a solution of K-lite in distilled water that will have a known approximate ppm. The readings from those will let you determine a reasonable correction for your meter, good enough to monitor any changes in your tap water and fertilizer solution.





Good suggestion but the k-lite dilution charts lists ppm N only, my meter reads ppm of combined particles, TDS. How do I covert ppm N to TDS ppm? 


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## Ray (Oct 29, 2016)

You don't.

Prepare a known ppm N solution using the manufacturer-specified mass-per-volume factor. I suggest that the be 1/2 of what you'll apply to your plants. Measure the TDS with your meter.

Now prepare another solution with your preferred concentration, and a third with double what you'll normally use, and measure them. (If you make the 1/2x and 2x solutions using half the volume of your normal one, you can blend them all together and store them for use.)

You now have a reasonable "calibration curve" allowing you to use your meter to manage your K-Lite solutions going forward.


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## Ma_sha1 (Nov 5, 2016)

Hi Ray,


Good idea if I need further accuracy with K-lite management, but I am satisfied with my 1/8 tiny spoon.

I am trying to determine TDS if my tab water to decide if to change my water source, I guess I can't rely on this meter. 

However, I like your suggestion of deploying serial dilution method.

I think when I get my next MDC water report, I'll compare with the TDS in The report & do some serial dilution with DI water & see if my TDS meter can follow the halving trend. 


PS
I got another Cheap meter, cheap PH meter & that one works great!

It's only off by 0.2 compared with ph7 Calibration solution & I was able to calibrate it with the screw driver that came with it. 


Now I use it to manage my Kovachii ph to 7.5. highly recommend this. I saved my kovachii from Sphrag, this poor thing was sitting in Ph6.5. Without the cheap meter, it would have been slow but sure death.





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