nuketopia wrote:No worries, all in good humor.
I agree with almost
everything you wrote.
"Water debit" is just an odd term to express volume flow rate with no basket present. Ie, zero-pressure maximum flow.
The terminology is bizarre, but it matches gicleur, as debit is French for flow and gicleur is French for jet... While water debit is simply a flow rate measurement (thanks for posting how to measure it for others! ), it is also a useful number because it helps define the relationship between the flow through the machine and the pressure drop between the pump outlet and the shower screen. When you measure it, the pressure drop is
pump outlet pressure. This relationship defines the flow coefficient (Cv
) of the entire system, which is the factor that determines what the difference between pump pressure and brew pressure will be on different machines at different flow rates.
Case in point:
A double shot pulled on any machine will have a higher average flow rate and a lower average pressure at the puck
than a single shot pulled on the same machine. It's the puck that controls the flow rate between a single and a double, but the Cv
of the machine determines what the pressure drop between the pump and the puck will be at that flow.
The zero pressure flow rate (aka debit) can probably be compared between machines of similar designs. But of very different hydraulic design, not so much.
Very hard to compare the two. I doubt one "water debit" value is applicable across different machine designs.
My S20 is also prodigious in it's water supply, but that is in factory trim without any jet at all. I first put a 0.5mm jet into the threaded 8mm hole and that calmed things right down. The needle valve I selected allows me to emulate up to a 1.75mm jet on the upper end. With my pump and hydraulic circuit, this gets me into LMLM territory. Regardless of complicated plumbing schemes or any other tricks under the hood, measuring the water debit (if it's fixed) defines the relationship of pressure and flow of the machine. It would be a farce to suggest that just by knowing the gicleur size one could predict with any accuracy what the brew pressure would be at various flow rates, but the water debit doesn't lie... I see no reason why debit by itself won't paint the complete picture of puck saturation and resulting back pressure the puck generates (based on the permeability of the puck, see last paragraph...
) so long as one knows the pressure at the pump when the debit is measured. The only other (massive) variable is the puck itself and the headspace of your machine/basket combination.
It all gets into the dynamics of fluid flow and pressure through an annulus and a particle bed, which is a bit hard to sort out.
The pressure ramp of a puck of coffee is determined first and foremost by the puck itself (grind, height and surface area), and then by the combination of headspace volume and the water debit. The initial fill is easy, as it is just your water debit flow rate times the number of seconds it takes to fill the headspace sufficiently to start building pressure. Empirical testing suggests that 25 - 30ml is a typical volume that needs to be dispensed before drops start forming on a 58mm group with ~2mm headspace and an 18g dose. So we have the total volume pretty well figured out. How fast you dispense that volume determines how fast the pressure rises, and how much of that total volume the puck absorbs before building back pressure. I agree with your assessment that the difference between 0.6mm and 0.8mm jets would be small-ish.
Slow fill rates tend to stay steady longer and the pressure ramps up slower. Take a 2ml/s debit: Unimpeded by the puck, it would take 14s to deliver 28mls and saturate the puck and give you beads on the bottom of the basket. Instead though, the puck does
impede the flow, and the pressure starts rising slowly and the flow rate starts slowly falling. The puck generates the pressure because water is flowing through it, and in response, the flow into and through the puck starts dropping. You dont see this drop in DE1 flow charts, because the pumps just pump harder to maintain the target flow rate, but it's present in all conventional machines. But in the real world (because apparently DE1s aren't "real"
), you get 2ml/s for about 5 to 8 seconds and then, depending on the grind, the flow rate then drops off to around 0.5ml/s over the next 10 to 15 seconds as the pressure rises, at which point drips form. Lower debits can take 30 or more seconds to saturate the puck and may not even build pressure at all if your grind isn't fine enough.
Faster flowing fill rates get through this process quicker, but experience the same basic dynamics. There is an initial fill where only the headspace is filled and the rate is steady prior to pressure building. Then the puck starts generating back pressure and the flow rate decreases. The same volume of water gets absorbed by the puck, but the spike in pressure happens faster, there is more dwell time between pressure being generated and flow beginning, and the resulting shot pours slower...
This last statement is really the crux of using water debit as a tool to adjust brewing parameters, as the rate at which water is introduced to the puck (water debit) has a very strong influence on the permeability of the puck once the pour begins. By making subtle adjustments to your water debit, you can control how fast the shot pours and then alter your grind or dose in ways that your machine would otherwise not be able to support.
Once again, this post is far too long and annoyingly over-technical.