a good place to fit a sensor in the outside of a boiler with submerged heater would be close to the water exit, ain't it?

It depends ... the boilers I am working with have a standpipe inside that takes the hottest water from the top of the boiler - and the standpipe gives up some of that heat as it is passing through the cooler water that is being pumped in at the bottom. Also on the boilers I'm working with, the group is actually the bottom half of the boiler so it is being heated (or cooled) by the water in the boiler and by heaters that make up the upper half of the boiler.

I picked my sensor location based on all of those things and what I wanted to measure..... you can pick your location based on what you know about your boiler (which is very different from mine) and whatever it is about your boiler that you want to measure.

Well, in the Gaggia Yahoo Group I found a schematic for the Gaggia Espresso Color & Pure. These are almost identical to mine. And I learn a lot of things only looking at it.

And Andy, you're absolutely right. My machine's boiler have the heater inside it, and the bigger part of the mass of the heater stays more on the bottom half. The cold water comes from the side of the boiler, close to the top. The two termostates are located very close to the cold water entry. This explains partially why the pump activation trigger the boiler on, and, after the activation, why it stays heating for at least 20 seconds (my estimate, don't measure it yet).

The brew termostate is temperature centered at 95° C, but I don't know what is the range it works yet.

I think the only way to obtain close to real life readings of the brew temperature is making a thermofilter. I never do a styrofoam cup test, but in the case of my machine (small boiler) this method will don't produce very useful data, because the bigger water intakes and their impact on the water temp that comes out.

GlennV wrote:I think what is happening is that you are driving the system a long way from equilibrium, so that the difference in temperature between the boiler water and the group and is a lot greater than if you left it to stabilize under PID control. So, very hot water is hitting a relatively cold group and it's almost cancelling out.

Actually I think it is the opposite happening, at least with my routine ... I think the group is quite hot since on the Gaggia the group is bolted directly to the heaters and that upper half of the boiler is very hot - conducting that heat through the metal to metal contact to the group. So my concept of what is going on is that the upper half of the boiler is very hot (about 227-232F) and the group is also quite hot (198-200F I suspect) and I believe the water inside the boiler is cooler (due to the previous shot or cooling flush) and absorbing heat from the boiler as it makes its way to the group. I suspect (though I haven't directly measured this) that the group is close to equilibrium with the temp the brew water has reached by the time the water gets to the group ... this would help account for how and why the temp doesn't drop much during the shot.

AndyPanda wrote:Actually I think it is the opposite happening, at least with my routine ... I think the group is quite hot since on the Gaggia the group is bolted directly to the heaters and that upper half of the boiler is very hot - conducting that heat through the metal to metal contact to the group.

You'd think so, wouldn't you, and this is indeed what happens without water in the boiler. But water changes everything. At equilibrium, the water in the lower brass part of the boiler, sitting on top of the group, is at a temperature about 8 degrees C lower than the bulk of the water in the upper part or the boiler, where the heaters are. It appears that this water doesn't participate in the convection, and this seems to keep the group cool. I have data which shows that the water temperature where I measure it (in the centre of the boiler) responds significantly more quickly than the group to switching on the steam switch. When flushing, the temperature of the group and the boiler water follow each other down. This means that it is very difficult (and probably counterproductive) to get the boiler into a state where the water is much colder than the group.

Just to be clear though - I like what you're doing (and it clearly works!). Have you thought about putting a second thermometer on the group?

I have thought about it - just haven't found a convenient spot and so far haven't really needed it. I'm not sure where to measure - I would guess the outside edge would be much cooler than the inside (say the dispersion disk above the showerscreen) and it would vary between no PF, Bottomless PF and spouted PF due to air circulation. Where to you measure your group temp?

If you notice the temp that the thermofilter is reading in my video. When I unlock the PF and dump it out, then put it back in, the thermofilter drops a lot (puck simulator and everything cools in the air) and then comes back up to about 190 before I start the shot. Then between shots, I don't unlock it -- the TC in the thermofilter is about 2mm below the showerhead and the puck simulator holds just a tiny bit of water and it drips out a bit as it sits between shots (if I were to unlock the thermofilter, I'd only be able to shake out 1-2 grams of water sitting in the thermofilter). But the temp continues reading in the 190-198F range as long as it is locked into the group - this is why I am thinking the group must be in that general range.

I haven't measured the water in the center of the boiler as you do. But my seat-of-the-pants guesstimate based on how it behaves ... I'm thinking the water in the boiler is probably around 212-215F (in my video - just before I start a shot) while the upper half of the boiler is around 228-230F and the group around 195F or so. I have no data to back that up - that's just how I imagine it and it is the premise I'm using with my routine. I'm also imagining the cool water pouring in at the bottom and swirling around (it comes in at an angle that looks like it would start a whirlpool motion at the bottom) and probably starts cooling the group and the bottom of the standpipe as soon as the brew switch is pressed.

Andy,

Filling the water reservoir with hot water (90° C, lets say) is safe to the machine (pump and etc.)? Because it's a easy solution to sustain intrashot temperature...

I believe hot water will ruin the ulka pump considering there is a mod to do to the Classic to ensure that hot water doesn't make it's way back.

I believe this is true. But I like to know why.

Another option in the way of obtaining intrashot temp stability could be to install a kind of dimmer in the circuit of the heater. I know one that is designed to be installed in the circuit of electric showers to control the water heating, and maybe could be adapted to the Gaggia's heater.

I observed if I hit the steam button during the whole shot, the coffee ends too hot. If means the temperatures rises intrashot. This way, if I could hit the steam button during the qhole shot, but control the heater power, I could obtain a more stable intrashot temperature. I guess...

Hey, Rodrigo.

I'm thinking again about your idea of PWM the heat element after you asked me about it on the Brasilian forum.

The approach Andy is using is like keeping some extra heat at the aluminum boiler so the water temp doesn't change much during the shot. I assume one who uses this method will have the group at a reasonable temperature, not overheated after the machine is idle for long time etc.

As the Saeco boilers has less metal and stainless steel has low thermal conductivity, you don't have this options. But, on the other side, I think the water volume is bigger (around 100ml inside the aluminum boiler, maybe 200ml inside the Saeco?). Maybe you could find a sweet spot for heating the water a little bit above the target point before pulling the shot and that would even things - the start temp would be not so hot as the group would heat sink and the end temp would be better as there was hotter water to mix with the cold water.

At the same time, maybe a PWM on the heat element could do a nice trick indeed. You'd have to find the nice point so the element can add some heat to the equation during the shot, but I think 30 seconds is very little time. Anyway, assuming the flow is around 3 ml/s (it starts high and can lower if the shot is ristretto etc), you have something around 3 grams of water entering at each second, and you have to heat it around 70-75°C from fresh to 95°C. That would be something like 900W (3 g x 4.2 J x 75°C = 945W). Indeed, if you try a very slow ristretto shot, it would require 600W or even less. BUT... There is inertia to account etc, so you would have to try and find the spot.

This problem is well known for every machine that has a dedicated boiler, even the commercial ones (think GS/3, Dalla Corte and others). Dalla Corte, for instance, has a similar setup (group under the boiler), but huge metal mass and 450ml of water. Other machines pre-heat the water, and I can't think a better example than the Breville Dual Boiler considering boiler size etc. GS/3 probably would work like a charm at home, on a slow pace, even without the pre-heat, but it helps to keep the machine stable for a high pace session.

Some Gaggia owners have made some pre-heat mod to improve that. That would help, and to use hot water on the tank is not an option, the Ulka pump is rated for 25°C operation - it counts on the fresh water to keep it safe.

Márcio.

The old style Gaggia boilers have two heaters. For 110v the two heaters are both connected in parallel and for 220v the heaters would be connected in series. Since I'm in the US with 110v I tested just disconnecting one of the two heaters to see if 50% power would hold the temp up during an extended shot. At the rate of flow for a double, there was still a bit too much heat coming in and the temp would rise over time.

I had thought about all the tricks of using preheaters --- when I finally built the thermofilter and started measuring what was going on during a shot I was really surprised to learn how consistent I could hold the temp and was really glad I hadn't invested a lot of time and effort into building a preheater.

I suggest you measure and find out what is actually happening during a shot that runs at a slow rate of flow that matches an actual espresso shot. You can't really tell when using a styrofoam cup test because the water rushes through the boiler so fast. If you can restrict the flow (or use my trick of putting a dimmer on the pump which gives you the advantage of being able to do low pressure/low flow shots for certain roasts that work better that way) - anyway, if you can restrict the flow while measuring the temps I suspect you will find that the temp doesn't drop nearly as much as you think it does. Or if it does drop, you can find the timing to cycle the heat on/off during the shot to hold the temp up. This is pretty easy to do on machines without the 3-way solenoid - just use the steam switch on/off at a certain rate.