Lelit Bianca pressure gauge question.

#1: Post by **philib97** » March 19th, 2023, 11:41 pm

With the blank disk insert in the portafilter, the pump pressure shows just a little over 10, but the gauge at the brew head shows over 11. I don't understand how the pressure down stream can be higher and wonder if one of the gauges is off.

#2: Post by HB » March 20th, 2023, 7:05 am

I agree, the grouphead gauge pressure should always be lower since it's downstream of the needle valve. Do both gauges return to zero when the machine is powered off? Sometimes a gauge needle is pulled against the pin by a vacuum, essentially resetting its zero point.

#3: Post by **JRising** » March 20th, 2023, 8:18 am

Headgauge can be higher because the cold water that gets into the brew boiler has nowhere to escape unless it exceeds the expansion valve pressure. The cold water will be forced into the brew boiler until the brew boiler is at pump pressure. Then the 10 Bar from the pump can no longer flow into the boiler and all flow recirculates over the pump's bypass relief. The new, cool water that did get into the brew boiler will be heated and expand, because the brew valve is open, it will read higher than the pump gauge.

March 20th, 2023, 12:48 pm

Thank you both. First, the needle does go to zero when it is off. @JRising, your explanation makes sense. Though, I would then expect this to be standard behavior. Do all Bianca's (and really all rotary pump machines) show approximately 1 bar greater pressure at the group head over the pump? This also occurs at the beginning of the brew cycle.
--Mike

#5: Post by **JRising** » March 21st, 2023, 12:01 pm

It's been talked about here, before, so people see it often. I don't really look for it any more, but I have made it occur in the past.

Additionally, the pump's bypass valve isn't absolutely perfect. Look how often it's discussed here that they start going way over pressure and need that o-ring lubricated so that static friction doesn't hold it in place and give that pressure spike before the bypass kicks in and limits the pump's outlet pressure... In my mind, that spike is happening before the brew circuit is fully up to pressure (because the pump's output is already more than can get through the gicleur, even with the knob fully, open I believe) but if that spike does occur, that's 11+ Bar for a moment trapped in the brew circuit with the head gauge, while the pump gauge is showing the regulated pressure after the spike has passed.

It doesn't really matter what the maximum pressure gets to in the head while backflushing. The backflush detergent is always too bitter, now matter how slowly you brew it.