Brew Pressure at Rest

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#1: Post by pShoe »

Is it normal for the brew pressure to be at 12 bars at rest? It reads 9 bars when running the pump with a blank basket. I ask because a lot of the videos I see, the brew pressure gauge reads 8-9 bars while at rest.

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#2: Post by another_jim »

For vibe pumps, blind pressure (not "at rest") is traditionally set to 11 bar. This will translate to 8 to 9 bar when the water is flowing at espresso making rates. On rotary pumps, there is very little drop off between a blind and flowing pressure reading, so it is standard to set the blind pressure between 8 and 9 bar.
Jim Schulman

pShoe (original poster)
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#3: Post by pShoe (original poster) replying to another_jim »

It is a rotary pump. The pressure when pulling an actual shot and a blind is relatively the same as you say at 8.5 to 9. At rest it can creep up to 12, which seems odd. Is it cause for concern?

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#4: Post by HB »

No. From my previous reply to Faulty pump pressure gauge?
HB wrote:The pressure reading when the machine is idle isn't important.

To elaborate, pulling a shot introduces cooler water into a closed system, but then you suddenly close off the main exit at the end of the extraction. As the water in the boiler warms up, it expands and the pressure rises. The expansion valve is typically set to open around 12 bar for rotary pump espresso machines since they have their own bypass valve to regulate brew pressure. For vibratory pump espresso machines, the expansion valve (traditionally called an over-pressure valve in this context) is set to open at the desired maximum brew pressure, usually around 9 bar. This valve vents water into the driptray to avoid subjecting the hydraulic system to excessive pressure. It only needs to let a few drops of water escape to reduce the pressure sufficiently.
If the brew reading rises above 12 bar when the machine is idle, you should adjust the expansion valve such that it allows water to escape.
Dan Kehn