Found these online, useful for EC to PPM conversion etc...I use an HM TDS 3 meter, the cheaper one. Had serious issues in the beginning of this grow with getting accurate measurements with this pen. The problem turned out to be me. So with that being said, several weeks ago my head fell out of my ass and I finally set some ground rules for my testing.
I never test IN the reservoir.
I draw the same amount out into a test cup every time.
I let the meter set in the test solution through the whole test cycle till it beeps, wait a minute or so, and release the hold, if it doesn't move, I have my number.
I use Nova nutes, so rinsing in hot water is a must, and a once a month 30 min soak in bleach/water cleans the probes.
As long as I follow these rules like OCD, I get really accurate measurements and my plants flourish.
The key i guess is time when it comes to PPM testing...give it time.
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I also keep a log book. Helps a lot to see the trends on paper.