Decent machines (the company called Decent Espresso, to be clear. It’s confusing) use a vibe pump to do cycle-by-cycle water flow measurement. They claim it’s substantially more accurate and responsive than any water flow meters on the market at low/high flow rates.
I own a Decent and I'm quite familiar with its in's and out's.
The Decent uses some math to estimate the flow. Their math is actually quite solid, within a specific range, but if you stray from that then it get less accurate. So most people brew espresso at about 1-2ml/s, if you have the machine calibrated there then it's fairly accurate. If you brew a fast "turbo" style shot, say about 3-4ml/s, it's less accurate unless you set the calibration to be more accurate there.
This actually does end up more accurate than many machines, however some have volumetrics that are much more precise like the Unica Pro. And I suspect the new "Bengle" from Decent will have more accuracy in flow calculation but that's yet to be seen.
