samuellaw178 wrote:For lever users, I find that the 'water debit' is pretty much analogous to how quickly/slowly you are raising/pushing the lever
I think I agree, though what you are really describing is how
a given water debit affects the ramp up rate. It just so happens that in order to achieve this ramp up or that ramp up, you need to have this water debit or that water debit with a given puck and headspace. Does that make ANY
samuellaw178 wrote:As a result, the head space (possibly containing compressible air) may affect the rate of pressure build up, so you really need to fix that to be able to compare. Also, if you know how much headspace you've got and how much water you're pumping through, you can pretty much 'flood' the group immediately, then slow down the pump (to have a slower pressure build up), to achieve Slayer-like shots. In this context, the water debit is pretty much non-relevant.
I'm convinced compressed air is critical to the ramp rate. Here's a quote from the ramble
, because it's easier than typing the same thing again, or even paraphrasing:
(for the record, I the drops don't start falling faster in most cases. Instead, the puck drinks more and more slowly as it becomes further saturated.)
Jake_G wrote:Through whatever process that is currently in use, water enters the group, is dispersed by the jet breaker/shower screen and begins falling through the headspace and lands on the surface of our puck. The initial flow rate at this point is equal to the water debit of the machine. The puck, which is totally dry until these first drops start pummeling it, starts drinking up. More drops fall, and the puck keeps on drinking until drops are falling faster than it can drink them.
At this point, water begins accumulating on top of the puck. I had always envisioned that the water comes down like a piston, pushing the air out of the headspace through the puck, but in this slow motion shot, it seems readily apparent that the rain drops are falling through the air gap between the shower screen and the puck and that once a pool of water accumulates on top of the puck, the air begins to be compressed at the top of the group. As more drops fall, this compression continues, and a pressure gauge mounted in the group would start registering pressure.
Ok. I bring that up for a reason. Imagine a rigid brewing apparatus with a fixed volume of 750ml with a fully choked puck in the portafilter and NO
air in it at all. It's 100% full of water at zero gauge pressure. What happens when you add 1 ml of water? Many folks try doing the math of 751 ml in a 750 ml container and figuring it's only a 0.13% increase in volume, so times that by 1 bar and... wrong. Trying to cram 751ml in a 750ml container results in a pressure spike equal to a ruptured weld or an open safety valve. (Side topic: this is why we have expansion valves on heat exchangers. They would explode or otherwise fail in grandiose ways without some way to vent the added volume that won't fit in them when the water heats up.)
Even if the puck is flowing some water through it, if you try to push more water in than is flowing out, you will have and immediate spike to max brew pressure in the absence of air.
Ok. No air means instantaneous step change in pressure with a pump driven machine that is stuffing water into the group. So, let's say you have 16ml of air at the top of the puck. If you compress that to 8ml, you now have 8ml of water in the head space and 8ml of air at 1 bar of pressure (technically at 2 atmospheres of absolute pressure, but it's close... ). Alright, let's add 4ml more water to the headspace. The puck drinks some, too, but let's say 4ml gets added on top. Now you've got 12ml of water and 4ml of air at 3 bar (4 atmospheres abs, we're doubling here
). Now add another 2ml and you've got 14ml water and 2 ml air at 7 bar gauge pressure. The reason I drag this out is because this incremental flow of water into the headspace (and the puck) is what gives you the pressure response with pump machines. The puck provides the back pressure and the compressed air provides the cushy pressure source instead of a wall. The pump just supplies water, and the OPV or bypass valve simply limits the maximum pressure the puck can generate. Water debit tells us the maximum rate at which this could happen and also tells you how much slower it starts happening as the pressure rises.
Levers work fundamentally different in that your arm, or the spring, or both supply a force to a column of water. The puck determines how much back pressure to provide and how much water will flow through it, while your arm or the spring combined with the surface area of the piston determine what the maximum available pressure is to force the water through. To your point, you can push hard and seal up a puck, or you can ease up and soften the puck to influence the shot time. This is analogous to controlling how quickly the air in the headspace gets compressed on a pump machine. And I suspect it is this ramp, more than what happens first, that gives you the overall control over the shot and resulting flavor. How quickly it gets compressed is a function of flow rate, headspace volume and the rate at which the puck drinks up the water. Since the flow rate slows down as the pressure rises in a pump machine, we don't have a good way of quantifying what the flow rate actually is at any given point prior to measurable flow coming out of the portafilter. What we can
measure, is water debit, and water debit tells us what the flow will be at any measured puck pressure and what the puck pressure will be at any measured flow rate into the group. That's a pretty neat trick!
Flooding the headspace and then choking the flow to abate a rampant pressure rise is a very doable strategy. But it's hard to do without darned good instrumentation. What if you leave the flow too high for too long? What if you pull the flow back too much? Water debit is easy and repeatable. Flush for a moment, dial in the flow and then pull your shot. It's simple. But I'm all for trying different variations if there is reason to suspect they may provide better results.
samuellaw178 wrote:In my understanding, a rotary pump has a constant flow rate profile regardless of pressure whereas an Ulka pump has a decreasing flow rate with increasing pressure. So obviously those two are not comparable at all and is something that is helpful to keep in mind when experimenting using a nominal water debit/flow rate.
This is a misconception. Rotary pumps have the same pressure vs flow curves as vibe pumps, it's just that this curve sits out at 10 times the normal average flow rate for espresso, whereas vibe pumps are right on the ragged edge, so we bump up against the flow limitations of vibe pumps far more frequently.
Also bear in mind that in either case, it is only the maximum available flow rate that tapers with pressure. Once a rotary pump or vibe pump has lifted their respective OPVs, they are effectively the same device. It's only when the shot flow outruns the available pump flow that you see the pressure vs flow relationship show up on the pump side of the equation. A good example is that the water debit on a vibr pump machine might be 6ml/s but the pump might only be putting out 4 bar. That's ok as long as you know what the pressure is when measuring the debit, as the real value is in understanding the relationship between the flow and pressure drop between the pump and the group. The relationship is based on the square of the flow. 2x the flow means 4x the pressure drop. In the little example above. 6ml/s at 4 bar means that a lungo shot flowing at 3ml/s would have a 1 bar pressure drop at the group on average and a 1.5ml/s normalle would have only a 0.25 bar average drop from what is reading on the pump gauge. The fill rates do get weird with vibe pumps because the while the pressure drop and flow rates can be understood, the max often starts off lower when the debit is higher. From a repeatability standpoint alone, setting the debit low enough that the pump hits full brew pressure when measuring water debit is helpful, but it's no big deal as long as you understand at what point the pump pressure starts climbing on a given machine.