# Which Pressure Gauge to trust?



## urbanbumpkin (Jan 30, 2013)

I've just cleaned out the OPV on my QM Andreja. No more screech mid shot.

I'm adjusting the OPV to be effectively 9 bar. But I've noticed the bar pressure on my machine is greater than the one on my PF pressure gauge.

At the current setting

my machine gauge reads 10.25 bar

PF pressure gauge reads 9.5 bar which I'm guessing is static pressure so equates 8.5 (actual).










Which one is best to trust? Or do I get another gauge for the Pf and see what it reads?

Any suggestions?


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## mremanxx (Dec 30, 2014)

Go with the one on the PF, I just did the same test on my Isomac, machine gauge shows 10.5 - 11 bar when pushing coffee out but 10 bar spot on at the PF, my gauge was a calibrated unit b ut did expect the pressure gauge on the machine to be inaccurate.


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## malling (Dec 8, 2014)

mremanxx said:


> Go with the one on the PF, I just did the same test on my Isomac, machine gauge shows 10.5 - 11 bar when pushing coffee out but 10 bar spot on at the PF, my gauge was a calibrated unit b ut did expect the pressure gauge on the machine to be inaccurate.


It's not necessarily inaccurate, the gauge just measure the pressure at a point in the closed system where the pressure gets higher then above the coffee puck.

Unless it measure the pressure just after the pump and before the one way valve (in a rotary pump set up ) then it will not read a pressure that are very useful, and it's bound to show a too high pressure then what is expected above the coffee puck. on vibe pump, pump pressure gauges is as good as useless.


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## mremanxx (Dec 30, 2014)

Yes but pressure is measured when brewing not static, thought that was the point of doing OPV mods?


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## marcuswar (Aug 19, 2013)

If you're comparing the two gauges at the same time then surely both gauges are showing the same static pressure, so no need to subtract 1bar from the basket gauge. This would mean the difference is only .75bar between the two gauges. My understanding is that "static" means its pressure measured against a closed loop (blind basket), whereas when pulling a shot some water actually goes through the puck so it's pressure is reduced.


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## urbanbumpkin (Jan 30, 2013)

marcuswar said:


> If you're comparing the two gauges at the same time then surely both gauges are showing the same static pressure, so no need to subtract 1bar from the basket gauge. This would mean the difference is only .75bar between the two gauges. My understanding is that "static" means its pressure measured against a closed loop (blind basket), whereas when pulling a shot some water actually goes through the puck so it's pressure is reduced.


Cheers Marcus that makes more sense.

It also makes me feel happier that one of the gauges isn't way out.


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## urbanbumpkin (Jan 30, 2013)

Just to confirm I've also completely replaced the OPV for a new one that I had as a Spare from Bella Barista.


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## marcuswar (Aug 19, 2013)

Glad it makes sense Urban. 0.75bar difference seems acceptable, especially as the accuracy of the basket gauge is also unknown.


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## urbanbumpkin (Jan 30, 2013)

So effectively the gauges are reading somewhere in between 9.5-10.25 STATIC which equates to somewhere between 8.5 -9.25 actual pressure.


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## marcuswar (Aug 19, 2013)

Well that would be my guess Urban.

What does the built in gauge say when pulling a shot (i.e. through coffee puck) ?


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