Skip to content

Es(Pi)resso Part III: Measuring Pressure

Part III in the series was published before Part II as I was waiting for a number of components to complete the PID modifications to arrive whilst I wrote this. Part II will follow shortly…

With that out of the way, the next stage will be to fit both an analogue and digital pressure sensor, with the latter being controlled by the Pi. This Part will deal with the analogue sensor. A later Part will cover the digital.

This modification isn’t quite as useful as the PID, but it is certainly worthwhile and is fairly straightforward to achieve.

Pressure is an important part of brewing a perfect espresso and any aficionado will tell you that you need 9 bars. An optimal pressure configuration can be achieved with the Classic by tweaking the Over Pressure Valve (OPV). You don’t need to install a pressure gauge – which is a destructive modification – to be able to adjust the OPV. Most people use a modified portafilter which has a pressure gauge fitted to the bottom to the same end.

…But, an analogue gauge just looks cool.

It will enable us to adjust the OPV, and also monitor pressure across each shot.1We might want to do something with that later down the line in the way of pressure profiling. And of course we will hook a separate sensor into the Pi so we can get a digital pressure readout on to our display.

But, first the analogue gauge. These are widely available on eBay or on espresso machine spares websites. Some sellers have them pre-configured for direct insertion into the Gaggia (albeit at increased cost). All is required is to fit a standard tee connector to the end of the pressure gauge. A push-to-fit connector will make life much easier.

There are many different types of tee connectors, so you should ensure that whichever you choose it is rated for at least 15 bars of pressure and high temperatures.

I decided to purchase one on eBay that had everything already assembled.

As such it was a simple matter of cutting through the PTFE tubing exiting the pump with a Stanley knife. I chose to do so fairly close to the pump output.

Placing the connector too close to the pump was a mistake. 

I had to continually connect and disconnect the tee connector as I was adding/moving hardware components. Over a short period of time that caused micro surface abrasions to the PTFE tubing. Eventually, I ended up with a leaking tee connector.

Be extremely careful to watch for leaks as you are working with high voltages.

I then was forced to cut back the tubing further. As things stand . If the machine leaks again I will have to replace the entire PTFE tube as I have run out of tube to cut back in to! In hindsight, I would recommend adding the analogue pressure gauge to your machine last, or use a different type of connector rather than a push-fit. The push-fit connectors, owing to how they operate, seem to exhibit considerable pressure on the tubing.

You can then connect the ends to the tee connector. On my push-fit connector the pipe secured itself after about 8mm of travel.

Turn on your machine for an initial to test. I drew some water through the grouphead, as well as simulating a back flush to test the full range of the gauge. My gauge maxed out at about 14 bars when back flushing. The Classic’s pump, the Ulka EP5, is capable of putting out a theoretical maximum of 15 bars of pressure. So 14 bars, when you account for manufacturing tolerances and some loss in pressure before it reaches the gauge, makes a lot of sense. I’m quite certain the gauge is accurate. I wasn’t getting the magical 9 bars at the group head, so I will need to calibrate the OPV – but I will come to that later.

Now for the hard part. We need to drill a hole in the front of the machine.

You will need a 40mm hole to fit the gauge through. There are a number of ways of going about it: you can either use a 40mm holesaw, or you can use a smaller standard drill bit and drill a series of holes in a circular fashion to the same end. I opted use the holesaw with a central positioning drill bit, as I figured that would give me a cleaner finish.

40mm holesaw

In terms of positioning, I didn’t take a particularly scientific approach. I placed a metal ruler against the bottom edge of the ‘Gaggia Classic’ logo on the front of the machine to ensure it was level on that axis. I then opted to position it approximately half way between the left hand edge of the machine and the left of the logo. In reality, it ended up being slightly to the right of centre, but I am fairly pleased with the result.

Some tips for drilling

When stainless steel gets hot, it hardens. So you will need to take it very slowly, and you should use cutting fluid where appropriate. There is approximately 2mm of stainless steel to get through. It wasn’t particularly easy.

I started off by marking out where I intended the gauge to go in pencil. I then drilled a small pilot hole with a 3mm drill bit (i.e not using the holesaw) to get an idea what I was in for. That also assisted in keeping the holesaw central in the initial stages of drilling.

Then I attached the holesaw and began drilling into the pilot hole. It took me at least 20 minutes of slow drilling, lots of mess, and a tiny bit of blood (steel fragments are not kind to the skin, be careful! ).

I was quite pleased with the result. Once I had my hole, I used some 400 grit sandpaper to smooth off the edging, which worked a treat.

You can now fit the pressure gauge through the front (it should be a snug fit!) and connect it all back up. Be sure to run the machine and check for leaks!

And here is the final result…

Doesn’t it look cool?

We’ll get to the digital pressure sensor once it arrives from China…2Could be a while.

Published inEs(Pi)resso

One Comment

  1. […] than that of the outside: I had a much easier time getting through this time, than I did for the pressure sensor.The amount of space in the 80mm x 40mm x 20mm box is probably a bit overkill, though its of no real […]

Leave a Reply

Your email address will not be published. Required fields are marked *