Blue Star Coffee Portafilter Pressure Gauge   €23.99

Easy to install and use.
You'll need to tighten the guage and assemble to the portafilter so that it faces outward when used. I needed to remove the basket and inner piece under the basket to get a proper seal for the pressure to read on the gauge.
I set my OPV to 8.5 bar because on a taste test, that was least bitter and least acidic and had the best flavours.

A must have to check pressure.
Blue star are great to deal with.
Always great advice and service.
I have been dealing with them for years and it just gets better all the time.