First I've got to say that this NOT an original idea. I first borrowed this (more like an improved version of this) from Vicroamer to measure the OPV pressure a few months back. Haven't thought about it anymore until the OPV thread from seedlings came and it got me thinking again. Below are everything you need to make your own PF gauge:
1. PF with spouts removed
2. Spanners - adjustable will do
3. Pressure gauge - rated to 1600 kPA, 1/8" (or 6ml) BSP Thread
4. Adapters to make the 1/8" fit onto the PF thread
5. Plumbers tape

I got the pressure gauge and adapters from Reece plumbing for a total cost of $21.50, $16 for the gaug and the rest for the 2 adapters. Plumbers tape 80 cents from Bunnings. If you bring your PF gauge along, the guys at Reece will be able to get the correct adapters which takes out the guesswork of what size and thread type to ask for.

Accuracy? Well, prior to borrowing Vicroamer's, I had measured the OPV using a real PF gauge and it measured 12 bar. Then I took vicroamers and it measured 12 bar on the dot. Adjusted the OPV so it read 9.5 bar. That was a few months ago. Measured it again yesterday using this one and it measured 9.8 bar. Don't know if it's the accuracy or perhaps over time, the OPV might hve tighted up a notch, but its no cause for concern. It does a decent job for less than $25.
BTW: 1 bar = 100 kPA

Great idea.  How deep is it from pf to gauge N2E?

I guess the calibration needs to be considered. You already had something to measure against .

Great idea! I was hunting some ideas on exactly this some time ago but something came up. I thought of a right-angle joiner to get around the depth problem of the gauge. Is your gauge fluid-filled? If not do you find it oscillates under load?

Greg

Nice one NtoE  
A cautionary note, water pressure gauges are generally not rated to espresso machine temperatures, just a matter of being on the safe side and checking pressure before water gets to hot.

I know it's the pump supplying the pressure but is there any significant difference in pressure at normal brewing temp?

Good job NTE.....

Now, if you want to get really finicky about this, what you need to do now is fit a suitable Needle Valve in between the gauge and the drain so that you can control the flow to equate to a normal espresso shot. This is the best way to set the OPV and removes the need to add a "fudge" factor to allow for the difference in pressure from a closed head to that with a 120ml/Sec flow.

If you want to be really picky about it,

Mal.

If you are referring to the OPV "flynnaus" this should be unaffected by the temperature of water in the Boiler, since the OPV is physically situated upstream from the Boiler and the water going through it is always cold.

On the other hand, if you are referring to the Gauge's accuracy as temperature is varied, then yes, there would be some variation but so long as you stay within the published operating parameters for the gauge you have, then the difference is of no real consequence. I guess if you wanted to find out exactly the differences you can expect, a message to the manufacturer may result in them supplying you with some application charts.

Usually though, what is normally done where the medium being measured exceeds the temperature rating of the best gauge you can afford, is the fitting of an intermediate diaphragm installed between the gauge and the medium being measured and any intervening space filled with a suitable buffer liquid... Quite often a viscous silicone oil. None of this would really be required to set up the OPV in an espresso machine though.

Cheers,
Mal.

This might be a silly question, but what is the benefit of running your machine at 9.5 instead of 12bar?

Thanks Banjo.