Jeff wrote:My experience is that above 8-9 bar in the basket, the flavors go "flat" and subtlety is lost.

Interesting. For someone who is just developing his palette for these things, I wonder if you could expand on what you mean by flatness vs. subtlety. What do you feel you can distinguish in a shot pulled at 8-9 bar that is lost at higher pressures? Do you have any idea physically why pressure might have this effect?

baldheadracing wrote:As you're in Calgary, you have some of the best roasters in Canada local to you - Phil&Sebastian, Monogram, Fratello, Rosso ... IIRC, Fratello roasts darker so they should have something ... but yeah, if I was in Calgary, I'd want the brew pressure set to 7.5 bar at the most, and enjoy the high quality medium and light roasts available locally.

Fantastic. I've seen portafilter pressure measurement kits, but these all seem to measure static pressure. Since the Gaggia has a more or less monolithic boiler/group assembly, I assume it wouldn't be possible to insert a gauge to measure brew pressure. Is this something that's usually estimated based on pump pressure?

AZRich wrote:Until you get your new opv spring kit, the simple way to reduce the pressure at the puck is to slightly open the steam valve when you pull a shot. That will bleed off some water (into a container) to get you down to a much more appropriate pressure.

That's an interesting idea. I might try adding graduation marks to the steam knob so I can make this a little more consistent. I wonder if it might be possible, by measuring the flow out of the steam wand at each of these marks, to infer the effect on brew pressure. Maybe a project for tomorrow.

crwper wrote:I recently bought my first home espresso equipment: a Gaggia Classic Pro and Eureka Mignon Specialita. I am a tinkerer, so I have big plans for the Gaggia. I've got an OPV spring kit on the way from Shades of Coffee, and a PID controller coming from Banggood. I design electronics for a living, so one of the things that drew me to the Gaggia was the active modding community.

In the meantime, though, I'm temperature surfing and pulling shots with the stock OPV and double basket (non-pressurized).

Typical advice for a 9 bar system seems to be to adjust grind so that the pull is 25-30 seconds with a 1.5 to 3 ratio of output to input. Lots of sources seem to say things will be different with 14 bars, but I haven't been able to find any specific advice.

While I'm waiting for mods to come in, I'd love to hear any tips and tricks you all have for pulling the best shots I can with the stock 14 bar OPV. How should I adjust conventional technique? On a technical level, how does the higher pressure affect extraction?

Thanks!

if you open the steam wand just a crack it will lower the pressure at the group but since there's no gauge it's a little difficult to know what's going on. you can run it without a PF and see the sort of impact it has on the flow, if you find a lower water flow position you can use this for preinfusion, then ramp it up to higher pressure. you would have to play with it to find the sweet spot. Lance Hendricks just did a video on How to hack the Gaggia Classic Pro it's a bit long but he has some useful tips in there without actually modding anything and just using your stock machine. He's going to also do a before/after video of a modded machine so you might want to keep an eye out for that one on his channel.

I thought I'd share my Sunday morning's work with you all, in case someone else finds it useful.

I had the idea to add a scale to my steam knob so that I could perform some quantitative tests. After a little quality time with CAD software, I developed this scale which divides the full rotation of the knob into 10 large steps with 10 smaller ticks between each:



Next, I devised a protocol to measure the flow rate at several different knob positions. With the valve closed, I would turn on the main power switch, steam switch, and brew switch, then quickly open the knob to the desired setting. With water flowing from the steam wand into a container on a scale, I would tare the scale and start a timer. When the timer reached 10 seconds, I would quickly close the knob. This gave the following data:



Finally, I wanted to work out a good fit for this data. I started with two basic ideas:

• The pump's pressure decreases linearly as flow rate increases, i.e., \delta p = m q + b
• The needle valve's flow is given by q = k C_v \sqrt{\delta p}

Combining these two, I got an expression with two constants derived from k, m, and b. I then used the two remaining constants to get the best fit to the data. Empirically, I found that I got the best fit if C_v is proportional to the steam knob position, which gives this result:



It's a great fit, I think. Originally I just used a linear fit to the first part of the data, but this fits the whole data set.

My plan is to use this data, and the graduated steam knob, to lower brew pressure in a more consistent way. Maybe this all seems a bit pointless given that I have an OPV spring kit on the way, but I love to characterize systems like this, and I think the experiment has helped me understand the way this machine works a little better. Maybe it will do the same for someone else.

Might as well finish off this little experiment.

Let's say we want 9 bars of pressure at the pump. If we assume the pump's datasheet is accurate (see the plot I posted earlier in this thread), then we need a flow of 4.33 g/s out of the pump to bring it down to 9 bars.

Let's assume we're using 16 grams in, 32 grams out, in 25 seconds. So we're going to have a flow of 1.28 g/s through the brew group. This means the rest of the total flow, i.e., 3.05 g/s, needs to go through the steam wand.

Earlier I said that the flow through the steam wand is determined by q = k C_v \sqrt{\delta p}. We can get k from the experiment we just did (it turns out to be about 4.99). We know \delta p is 9 bars and q is 3.05 g/s. So C_v is 0.20, which we said is the same as the steam knob position.

Since I've divided the knob into 10 units, a position of 0.20 means you're turning the knob 2% of one turn. As someone else said, you're really just cracking it open, but without the scale on the knob, it would be very difficult to make sure you're hitting 2%.

What happens if you're a little off? If you open the knob 1%, you'll wind up at a pressure of 10.9 bars. If you set open the knob 3%, you'll wind up at a pressure of 7.5 bars. So this technique doesn't seem very forgiving.

Nice - I like the data. A couple random thoughts: Be careful - their steam valve is imho a piece of crap - it is metal on metal and mine always started leaking no mater how careful I was to baby it, and it leaked again after replacing it. In contrast, I have an old very cheap used Hamilton Beach that has a much better valve - never drips despite being maybe 15 years old and it steams 5oz milk very nicely. The La Pavoni I used to have had a valve that could easily be fixed from dripping by a cheap mod. I like most of the upgrades the pro model made over the classic I had. (sold a few years ago) But, the old model had a 'real' opv that could be easily adjusted and I feel they went cheapo on this new one hanging off the pump. I saw a pressure test on youtube of a pro where the stock machine peaked out above 12 bars which I think is just terrible. Also, the boiler volume is really tiny, so bleeding off much water will likely lower the brew water temp pretty quickly. The Lance Hedrick video shows a brief toggle of the steam switch, and it could be that even leaving that on for the whole shot might not be a bad idea for lighter roasts. Good thing is the heating elements are in the boiler walls, not in the water itself so burning them out by running the boiler dry is not an issue.
regards, Rich

AZRich wrote:Also, the boiler volume is really tiny, so bleeding off much water will likely lower the brew water temp pretty quickly.

That's a great point. During my tests, a knob setting of 0.2 looked like just a trickle. But put into context, it is about 2.5 times as much water (3.05 g/s) as is going through the brew group (1.28 g/s). This would almost certainly have an effect on temperature during the shot.

This certainly underscores why changing the OPV valve and adding a PID controller are such highly recommended mods.

Nice work with the data mapping.

In your example of 32g in 25s, there is more at play.

First, figure the puck will absorb it's own weight in water in addition to the volume that lands in your cup.

Next, figure roughly 18ml that must flow through the group to fill the headspace, so your 32g now becomes 32 + 16 + 18 = 56g of water in 25s, so your total flow rate is much higher than you'd think at first glance.

Finally, bear in mind that this flow through the brew group is not constant. The initial flow that fills the headspace occurs at roughly whatever the free flow rate through the group is. This is traditionally called the "water debit" of the machine. Easy enough to measure this, and this typically happens at roughly zero pressure at the puck.

Once the headspace is filled, you begin to saturate the puck and the pressure begins to rise. This is where that next 16g of water comes into play. I bet you it hit 14 bar in this phase, because the flow rate drops as the puck fills with water. It reaches a local minimum right around the time the first drops fall from the basket, which is convenient, because you can measure this by seeing how much ends up in the cup over the first 5s or so once the first drips appear. This is typically around 0.5ml/s, which is only 30ml per minute, which is why I strongly suspect you are seeing 14 bar.

Once the pour starts going, (probably around 5-6s after turning on the pump) you will start to get a steadily increasing flow rate. The average flow rate is maybe 30g in 18-20s (1.5-1.7ml/s) but if you look more closely, you'll see that it accelerates from a minimum at the start of around 0.5ml/s to a maximum of around 3.5ml/s.

Thus, you will see the most bang for your buck by cracking the knob when the puck is becoming saturatedis and the flow through the group is very low and the pressure would ordinarily be very high. Once the pour starts in earnest, you can start closing the steam valve, because the pressure will already be lower once puck starts flowing. Since the chart estimates 10 bar at 200ml/min (3.33ml/s), it recommend rolling the steam valve closed as the shot progresses, aiming to have it completely closed around the end of the shot.

Bear in mind that this approach of softening the early pressure rise will make the shot behave as if you had preinfusion (because that's kind of what you are doing with it) and you may have to grind finer, even though you are reducing the peak brew pressure.

Cheers!

- Jake