Hi,

OPV Pressure drops as temperature rise ?

Has anyone here have a V3 Silvia with pressure gauge ? if yes, please confirm this.

this Silvia is a V3 2010. and i confirmed the pressure with a second gauge at the portafilter.

i notice that when the machine is cold, pressure is over 10 Bars when the pump is on, after 30 minutes at normal operating temperature, pressure drops to around 9 bars when the pump is on, then after 2.5+ hours, pressure drops to around 8 Bars, when the pump is on.

if i pass fresh water through the OPV and cool down the machine, pressure will rise up to 8.8 bars
is anyone else experience this ?

niro wrote:i notice that when the machine is cold, pressure is over 10 Bars, after 30 minutes at normal operating temperature, pressure drops to around 9 bars, then after 2.5+ hours, pressure drops to around 8 Bars.

As water heats, it expands. The boiler is a closed system, so the boiler pressure will rise as the water temperature rises. In this case, the over-pressure valve serves as an expansion valve to prevent the hydraulic system from being subjected to damaging pressure.

please re-read my post, pump is on every time.

Thanks for the clarification.

Perhaps the pump pushes more enthusiastically when it's warm. Or the OPV spring pushes with less vigor when it's warm. Are you measuring the pressure against a blind filter basket? Because venting all the pump's output through the OPV distorts the measured reading. Measuring with a portafilter pressure gauge/needle valve combo, while pulling a shot, or with a Scace II thermofilter will give you a more reliable reading.

PS: At some point I will ask, "So, how does the espresso taste?"

all pressure tests are done with no leaks. my assumption is:

the material of this spring in the OPV is becoming softer under temperature.

now, it could be,

1. all Silvia's new and old.
2. only the V3 with new OPV.
3. or unknown cause.

it could be a good reason to move the OPV location, + it will stop collecting calcium from the boiler.


hope to hear from a V3 owner soon.

niro wrote:all pressure tests are done with no leaks.

In that case, your observations may well have no bearing on actual use. See Brew pressure gauge reading vs actual brew pressure for details.

Dan,

my observation has nothing to do with actual brew pressure, as far as i concern, one can set it to 14 Bars or 5 bars.

i am trying to find out, how sensitive is the spring in the OPV to temperature.

i already can see, that on my test machine, working temperature have some big play.

best if we could get reports from other people with other machines at other places.

and i bet if the OPV was away from the boiler, the pressure reading was the same, in cold or hot machine,

niro wrote:the material of this spring in the OPV is becoming softer under temperature.

The melting point of steel is about 2500°F.

Many OPVs develop a screech after a few months. Once cleaned and relubed with a high quality food safe silicon grease, they remain forever silent.

My bet is sticky lube that softens with increased temperature.

Both the carbon steel spring and the brass housing of the OPV will expand with increasing temperatures. But the coefficient of expansion of brass is about 50% larger than carbon steel. So the spring should relax a little inside the housing as things heat up.

There may also be some spring relaxation caused by softening of the rubber gasket at the valve seat.

I wouldn't think these two effects alone would explain that big a change, but I can't think of a better explanation. You have to get carbon steel a whole lot hotter than Silvia boiler temperatures before you see much change in the elastic modulus.

Jim