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Pyureeeeeee!
At least I'm pretty sure that's the case....
It's so close to half the rated input that a few percentage may be in the wash. My case really isn't a good test.
I just tried swapping between a fluid regulator set to 1 B/t and a huge titanium pipe and within my ability to do stopwatch numbers they have the same output (500kEU in 12 seconds).
They both took 49 seconds to fill 2 MEU. I'm fairly confident in these results considering there's a 4% difference between (38400 mB/s) / 2 and 20 B/s and if I had a one second error that would still only be 2%.
https://imgur.com/a/TJy0R
Also these are the excess numbers. In all cases I'm running 16 MV centrifuges for the pahoehoe lava with 100% duty cycle (there's a buffer built up, typically only about 10 run on average) as well as the fluid regulators (which is under the noise floor here). Don't worry 40 B/s of SHS produces more than 2040 EU/t. Two sets of turbines are running: one has a single fluid regulator between the LHE and the turbine and the second one has a fluid regulator and the variable block tested here. All four turbines are using tungstensteel rotors (20000 mB/s optimal flow, 115% efficiency).
New test with the excess lava completely drained and the entire system at a nominal steady state. In all cases I verified that the rate that SHS was being pulled from the LHE matched how much was produced. A webpage stopwatch with alt+tab (gonna say about half second percision) was used to measure the time to charge a 10 MEU lap xtal.
fluid regulator run 1: 243.883 s, 2050 EU/t
fluid regulator run 2: 244.101 s, 2048 EU/t
huge titanium pipe run 1: 272.564 s, 1834 EU/t
huge titanium pipe run 2: 271.721 s, 1840 EU/t
huge tungstensteel pipe run 1: 274.074 s, 1824 EU/t
huge titanium pipe run 3: 271.170 s, 1844 EU/t
huge tungstensteel pipe run 2: 271.637 s, 1841 EU/t
That's a roughly 10% difference. A bit more than the expected 4% but maybe I messed up in an assumption of how much was lost or in the testing methodology (I tried to pay pretty close attention to the details here though).
In conclusion: I think you're right lol.
After trying with a huge tungstensteel pipe: if my only issue was the half throughput due to internal tank size then I wouldn't be losing 10% on the huge tungstensteel... I'm thinking some is lost due to sloshing between the input hatch and the pipe. I wouldn't think this is possible since typically machines are one way valves for fluids. I guess hatches are an exception. Huge titanium run 3 uses a shutter set to output only on the pipe side facing the input hatch.
Nevermind that was wrong too. huge tungstensteel pipe run 2 uses a shutter input only facing the fluid regulator and a shutter output only facing the input hatch. If this doesn't work I can only assume GT pipes suck.
It gets weirder. At this point I'm no longer thinking any fluid is getting lost since I've looked at the return distilled water pipe and it's still full (I made every pipe segment's tank be full). If steam were being lost then this should not be full. Perhaps there's some issue with when the flow rate of the turbine is calculated relative to when the GT pipe moves liquid (and how much liquid is moved since it has this fill up/empty mechanic).