Useful chart compiled by Jim Schulman.Important note: Every espresso machine I've ever worked on with built in boiler pressure gauges was improperly adjusted. That's because the Bourdan Tube gauges themselves were inaccurate; probably due to the espresso machine being bumped, dropped, and generally thrashed while being shipped. I either send the gauges out for calibration, or ignore them entirely and install a PID to manage & measure boiler temps.
The same is true of pump pressure gauges - they're always worthless unless checked for accuracy. I work around this problem by using a portafilter mounted gauge to set group pressure.
