Flow is affected by SG of the fluid, but we're talking ~4% change from STP to boiling, so I figure if the water is hot, you should be good to go!

Good call on the effect of flash boiling.

C_v (the Coefficient of Flow) is a physical property of a fluid "device" (valve, orifice, system of pipes, etc...). There are some assumptions made as to when you can calculate C_v and apply it to predict performance under different circumstances. One such assumption is that the flow is not mixed...

C_v is calculated as the flow rate (in Gph) times the square root of the specific gravity divided by the pressure drop (in PSI). This number remains constant and is a predictor of the pressure drop across the gicleur at various flow rates.

Thus, if you know the flow rate, you can predict the pressure drop across the gicleur. Pump pressure minus this drop gets you puck pressure. But also, measuring puck pressure gets you puck pressure... I would think that measuring it will be more accurate than calculating it but the calculation error should be small. I'll run some numbers and see if my brain hurts less after looking at your data again.

Cheers!

- Jake

Tonefish wrote:So final question, the delta P is Lelit gauge pressure minus atmospheric?

For water debit, yes. It will be Lelit gauge pressure minus puck pressure when pulling a shot. No difference should mean no (or extremely low) flow.

Tonefish wrote:Puck pressure? Won't puck pressure vary from gauge pressure at the top of the puck to atmospheric pressure at the bottom of the puck?

EDIT: I'm vaguely recalling some plots with reversed labels.

Yeah, I vaguely remember that, too...

ok, plugging in your water debit numbers:


As long as the OPV is closed, the pressure and flow relationship during a flush are driven by your pump curve, which is a little on the low side (flow should be more like 220mL/30 seconds @ 3.5 Bar or pressure should be more like 6 Bar at 180mL/30s. See pump curve below...) but this could be due to the PI chamber "stealing" some of the water during the test. @ 3.5 Bar, the PI valve should be cracked, so that's a definite possibility... However, the shape of the chart above is based on the C_v of your gicleur. If you get a smaller gicleur, the pressure drop will increase, and the pump will follow its curve to deliver less water and the shape of the curve above will get steeper (higher pressure) and shorter (less water debit means we stop at maybe 4.5 mL/s instead of 6).

Ignoring the inconsistencies, we can apply the calculated C_v and estimate the pressure drop during a shot if we know the instantaneous flow rate:

This assumes a constant pump pressure of 9 bar. I didn't bother going out beyond 6mL/s, because that's your water debit. Your machine will never be able to exceed that flow, even unimpeded. In fact, if we correct for the capabilities of the Ulka vibe pump:

We notice that above ~4.33mL/s (260mL/min), the pressure begins dropping from our target 9 bar. Drawing a line on our puck pressure chart from 9 bar @ 4.33mL/s and ending at 3.5 Bar @ 6mL/s (our measured water debit), we see what increased flow through the gicleur does to puck pressure when fed by an Ulka. Behold:

OK.

So what this is saying is that IFF your puck is so porous or eroded that it flows 5mL/s (Super Lungo!), the pump will only be able to muster around 6.8 bar to the inlet of the gicleur. The gicleur will then have a pressure drop associated with this flow of just under 2.5 bar, so the resulting pressure at the puck is only 4.4 bar. You can see how the OPV flattens out the pump curve for flow rates of less than 4.5mL/S and leads to much better (or at least predictable) performance in the typical range of espresso brewing.

Cheers!

-Jake