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I haven't seen this before so sorry if it is already known. I came up with a pseudo-method of monitoring a heat exchanger to power inserters without complex circuitry or timers etc.

Basically you connect 1 tank to 1 of the Heat Exchangers in your power plant, add a pump to the output and then connect that to the rest of your steam. When the Heat Exchanger is running the tank will hover around 450 steam, (for a 2x2 setup. This value depends on how much steam your station is currently using), as the pump is not fast enough to completely drain the tank but as soon as the Heat Exchanger cuts out the steam level will fall to almost nothing really fast. Connect the tank to your fuel inserters using red wire and set them to run when the steam level is less than 1 for example. As soon as the fuel is inserted the steam level will jump straight back up again, quick enough that the inserters only have time to place one fuel cell into the reactor. You can use a level greater than 1 if your plant is running at full capacity and you can't afford the 15 seconds down time, (if you have reserves of steam it won't be a problem), just bear in mind that a value greater than 10 increases the chance that the inserters add more than 1 fuel cell. A setting of <1 is pretty good if the plant isn't running at full capacity and isn't full of steam.

Here's a couple of screenshots of the setup.





If the inserters are inputting more than 1 fuel cell it is down to the fact that you have too much stored steam in your system. To solve this you can create an AND gate where 1 input is as above and the second input monitors the steam level of your other tanks and only outputs when the stored level is say 5k, (this is better as you don't really want to activate the reactor if you have stored steam in your main tanks!).



In the above setup the green decision combinator monitors steam in the tank connected to the Heat Exchanger and pump, the red decision combinator is monitoring the stored steam in the main tanks and the red and green arithmetic combinator, set to AND, outputs blue when it receives red and green. The inserters activate when blue = 1.

The simplest way to control fuel use is to only take the previous spent fuel out when you are running out of steam, and only insert fuel while you are taking spent fuel out. This trivially limits the maximum number of fuel cells in the reactor to one.

Make sure you have less steam engines / turbines than your heat exchangers could power, so you will always produce a small amount of excess steam to fill your tank(s).
And ensure that your reactors never hit 1000 degrees, you can either hold onto the heat in heat pipes / exchangers or build lots of tanks to store all the steam.
Then you can build more reactors for better fuel efficiency without worrying about wasting fuel.

One of my reactor setup's had 48 reactors, which could theoretically provide heat to ~750 exchangers. But due to space / layout constraints, I only managed to get enough heat (and water) to about 480 heat exchangers. So the reactors would hit a peak of ~900 degrees each fuel cycle. But this is fine, since I could store a few million units of steam in tanks and I had quite lot of heat piping.

For a super-simple starter reactor throttle, that is good. And slightly more efficient that I'd been doing it myself, since I wasted a box and an inserter to feed the reactor inserter one cell at a time from a large stockpile.

But really, for any serious reactor there is plenty of time for (the bots to build from blueprint) a fancier fish logic control system.
Eg: Insert fuel, then wait for empty cells to be removed, then wait for efficiency conditions (fresh cells available for all reactors & water and steam at good levels), then wait for SCRAM signal to be low, then repeat.

Adding logic is really cheap.
My mid-game (2x2 = 300% efficiency) reactor blueprint comes with baked in steam/water light gauges, and a segment display showing MW and GJ stats. It waits for enough stored water and enough space in the steam takes for a full burn cycle, but that allows it to begin power production even when only partly assembled. The logic finishes long before the heat exchangers and tanks are built.
My high end reactor has additional pumps, gateway tanks and logic to keep the MW display accurate even when the reactors are burning and steam is being injected into the storage.

Note:
You can connect up existing tanks with logic wires easily and cheaply by stamping a blueprint with wires over them. Wire up four yourself, and then copy and stamp out the blueprint to add more wires to everything. This makes wiring all your steam tanks together quite easy even when designing, so you don't need a special test tank with pump. Just measure the entire system, and divide by 10256 to get GJ stored.
Take that same measurement, and do some memory buffers & subtraction to get a MW stat. Use that to alert you when you need a newer and/or bigger reactor to handle peak loads.

Wiring up all your steam tanks is a good idea regardless; you won't need to worry about any lag, because your steam buffer is fully measured and you know if there is enough room for a burn cycle worth of energy. As soon as there is, burn!