Here is a picture of the gauge with free flowing water. It is dialed in so that a portafilter with gauge (Scace like device) reads about 8.5 bar. So the reading is about a bar above that.

Could you post a pic of your portafilter with gauge? Is there a way you can cap off the flow from this device such that it acts like a blind filter? That way you could do a self-check on the gauges themselves. With a blind filter effect the gauges should be equal as they are measuring the same pressure.

The gicleur drops pressure at the puck by about 1-2 BAR, but the puck increases pressure at the gauge by about the same amount. The two cancel out, which means the pressure set on the gauge at free flow is the pressure you get at the puck.

Within fluid mechanics, there is no such thing as "cancelling out". The drop in pressure through this gicleur depends upon the flow - see below pic. It also, of course, depends on the exact size and configuration of the orifice.



As always, the MS Excel file is available for the asking.

There is a 2 bar discrepancy between the two manometers. Now for some elementary physics to figure out which one is more accurate. Perhaps calibrate with a sphygmomanometer?

Now for some elementary physics to figure out which one is more accurate.

Well, that requires a pressure gage calibration which is really beyond the scope of this thread or your (and my) pocketbook. Pressure gage testers are known as "dead weight testers". I have personally done all of this many, many, many, years ago and will search for that working time machine

Or a Fluke PV350 off of eBay?

Can't crappy thermometers and manometers ever be right??? Life was simpler with one crappy dial.

Why can't crappy show up in dimensions other than precision (e.g. very precise but damn ugly; or ultra precision but barely legible reticule; etc.)?

Okay - so I now have a Fluke PV350, which I hooked up to one or Eric's adapters to check the manometers. This was also verified using a Extech pressure gauge.

The after market pressure manometer is pretty damn accurate across the range.

The La Marzocco one is accurate to about 6 bar. At 6 bar it starts climbing up faster and shows 8 bars as 9 and 10 bars as about 12 bars.

Is it possible to calibrate a manometer?

Is it possible to calibrate a manometer?

Certainly . . . I have done it and thus know exactly how to do it but, the cost to have it done likely exceeds the cost of a new gage. A perfect gage will have a slope of 1.0 when comparing the true pressure to indicated pressure over the range of the dial. Gages that are "off" can have differing slopes and/or identical slopes that are simply shifted up or down.

It should be sufficient to know that the installed gage reads "x" and it should be reading "y" where there exists a very high confidence factor for "y".

In fact, turned out to be easier then I expected.

Searching online one realizes that unlike good quality manometers - Cheap ones are usually spec'ed between about a 1/3-2/3 of the vernier/dials (and even if they start off linear, corrosion of cheap untreated metals usually ends any precision pretty quickly). So between 5bar to 10 bar I should be close to linear.

As it turned out - easier than expected. All it needed was readjustment of the zero point, and I now have all three dials in (pretty good) agreement - and repeatably so.