DWC Topping off res PPM questions

[Moonspell]

Hellow everyone!

I've got some questions regarding topping off reservoir with water, but not with plain PH'd water, but I want to increase PPM level to some point. This is my second dwc grow, but first was just to test if I can pull it off so I was totally clueless what I was doing, yet it was kind of successful. Now I know a thing or two about dwc but still far away from perfection.. From what I've read that I should never adjust nutes in res when plants in it - the point is I can't remove plants from res.

Let me explain what I have in mind:

Got 26 litre res, approx. 15L of water, 2 plants in res - they started to drink 2-2.5 litres per day.
When fresh res was prepared I mixed it at 600ppm (100ppm tap water included, so 500ppm of nutes).
1 day later ppm is at 570 - ok, they've eaten more than I gave them. I've read I should add back what they've eaten. I'm using Canna Hydro A+B for hard water, even tho I've got soft water (adding mono Cal + mono Mag to make up for it).
On the back of the bottle it says 40ml/10l A + 40ml B, so I've made some tests how it increases PPM for different nute strenght (results WITHOUT 100ppm tap water)

33% A+B = 290ppm
50% A+B = 420ppm
66% A+B = 570ppm
100% A+B = 880ppm
150% A+B = 1180ppm
200% A+B = 1440ppm

Then mixed 1 litre 1540ppm (tap water ppm included from now on) into 20 litre 900ppm (so 1 litre from 21 litres is ~4,75%) and it increased to 940ppm. So ppm changed for ~4,5% up.

My questions are:

Are my calculations right ?
Why ppm seem to lose on strenght the more I add to res ? Shouldn't 200% str. be equal to 2x 100%? Seems to me strenght of nutes of 100% + 66% str (880ppm + 570ppm = 1450ppm) = 200% (1440ppm).
If I want to go up from 600ppm to 750-800ppm, water in res 12 litre (3 litres missing) - how do I calculate how much PPM to add to 3 litre of 100ppm water to bring nutrient solution from 600 to, let's say 800 ? And would calculation be different if I want to later increase ppms from 800 to 1200 considering ppm to % of nutes strenght isn't linearly scaled ?

I'd be grateful for any help or clarification and I hope that this post isn't complete gibberish since I'm high AF!

 

[Scientific]

I'm all for the methodical, scientific approach, but you might be overthinkin' it.

You've already had a successful grow. If you keep everything the same as last time, you can just bump the nute concentration 10% and then carefully watch the leaf tips for yellowing.

That way you're starting from a known-good process and just tweaking one variable slightly, ready to fall back at the first sign of toxicity, or even to increase strength if all goes well.

> From what I've read that I should never adjust nutes in res when plants in it - the point is I can't remove plants from res.

I don't understand. Why not? A plant is happily growing in soil and the farmer comes along with the manure spreader and increases the available nutrients slightly and the happy plant responds. Same thing with gently increasing the concentration in the reservoir. Yeah, there will be a slight change in the osmotic pressure and all that, but small changes shouldn't be a problem.

The good news is you were successful the first time. (No small accomplishment. ) Now you can make small adjustments and see what happens.

Good luck!

 

[Scientific]

Are my calculations right ?

You don't show any calculations, just your results.

To validate your process, you'd have to list exact quantities used and any other relevant data.

I would expect that if you get X PPM by adding Y ml to 10 liters of pure water (so no funny variables), you'd get 2X PPM by adding 2Y ml to 10 liters of water. (A normal person would say, "If you double the fertilizer, you'd expect to double the PPMs.")

If you're not, maybe the hard water formulation is to blame? (Like with some weird buffering process? Call the manufacturer on that one.) Are you giving the mixture plenty of time to equilibrate? (Like even standing overnight?)

Again, I'd suggest just goosing the juice 10%, but being a similarly analytical type as you, I had to comment on your process and calculations.

Good luck and happy growing!