Fire engine pumps do. The Total Pressure Master system (automatic electronic pressure governor) uses a sensor on the discharge manifold which outputs a 0-5V signal to translate water pressure into voltage, thereby controlling the truck's engine speed. There are two options: RPM and Pressure mode. Pressure mode varies the engine speed to get a consistent pressure, RPM mode maintains a constant engine RPM, monitoring the discharge pressure up to 30 psi.
We use pressure mode most often in fire attack operations, RPM mode when drafting in pump relay operations where the pump using RPM mode is not also the fire attack pump. RPM mode controls the volume of water into the pump in a draft operation to prevent pump cavitation if the fire attack lines suddenly demand more than the supply, ensuring that the volume going out (edit) doesn't outpace the supply.
And someone is there monitoring the whole operation!
There sure better be! 
Deep pockets for that pump. Just so you know. If you want to pump water straight up 24 feet, you're looking at a constant water flow to maintain prime. That's a 3/4 HP pump on your best day, with a full container of water, already primed at 33 feet head pressure in a column, reliably.
My suggestion would be to check your local construction equipment rental house and ask them, and test it on rent before you buy.
The standard PWM signal of an Arduino board is 970Hz and 0-100% dutycycle.
You’re searching for a pump that can be regulated with this signal.
Maybe you can take the cheapest pump and switch it on for a few seconds in several minutes?
(Effectively PWM on a much longer timescale.)
The plants won’t care either way.
Not at 8m elevation loss. Add in friction loss in an unprimed pump with low head pressure on the static water supply (rain barrels are only 4 feet tall or so), not going to happen. No, I can't remember the math (knew it for the test, 20 years ago - don't judge me!) but enough experience with submersible pumps in construction, ag , pool maintenance and firefighting tells me OP needs a solid electric sub pump if not a gas powered trash pump to do this at that elevation and getting the system to run at balcony height, for the cost and effort, honestly, just water your plants from a faucet at the same elevation. Take that to mean what you will.
I mean, I can get a centrifugal one that claims 8m for 25-30 EUR. I don't need 8m, I just need "noticeably better than the really feeble ones" so I've got some hope of delivering water for hanging baskets (2.5m up).
I didn't say I need to go up 8m, I just need about 2.5m - 3m, so I was going to get an 8m centrifugal one on the assumption that with all the loss it would be okay.
Yeah, if all else fails I'll do this. I'd like to have a bit of flow control (drippers vs spray ends etc.) if I can, but as it seems a bit fraught, I might just go with on/off and be done.
The pump selection comes down to the motor that drives the pump and the pump type. Using a non positive displacement pump the slower the RPM the greater the slip and the lower the efficiency. The speed/control range is very limited but It could work. A positive displacement pump will change volume in directly relation to the RPM.
Not all motors like PWM and it could possibly fry them. The VFD (Variable Frequency Drive) is a good choice but again will the motor tolerate it. They work great on three phase but there are problems with split phase motors which have an additional winding to get them to start.
You need to find a pump that will do the job with a motor that you can control. This is not an easy task but there is a lot of good advice given by others to help you. Good luck!
Oh, my misunderstanding.
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