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Some visual data on single boiler temp surfing

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  • Some visual data on single boiler temp surfing

    So I've been messing around with Artisan roast monitor recently, after I finally got my thermometer talking to my mac. Just for kicks, I decided to do some temperature datalogging on a Gaggia Classic I have been working on as well - Artisan works equally well for this purpose, as the monitor window is highly configurable.

    So, my test was to set up one thermocouple monitoring the temperature at the top of the boiler, and compare that data with data from the Scace thermofilter when pouring water from the group. Given the popularity of the temp surfing technique on this style of machine, I was interested to see how stable I could get the shot temps by taking control of the boiler away from the thermostat.

    The four graphs below show the results, two are with no intervention to the temps, and two show the results of aiming for a specific start temp. via mashing the steam switch.

    A couple of conclusions, specific to this particular Gaggia Classic:

    The shot temp. is generally around 20 degrees C. below the boiler temp, which is measured at the hottest point on the aluminium block. The thermal transfer from the aluminium boiler to brass block is not particularly efficient, requiring a higher thermostat rating.

    All shots via thermostat control end well below 90 degrees C, and only a few start above 90. Bear in mind that the scace has a higher flow rate than a good espresso pour, so this would be worst case scenario.

    Manual control enables relatively stable, controllable extraction temps. The more stable shot temps from the thermostat controlled tests are when the extraction starts just prior to the thermostat engaging - but the temps are still on the low side.

    The Gaggia Classic steam switch kills power to the brew solenoid when activated, so it's impossible to make the boiler heat manually during a shot without modifications.

    Pointless mucking around to make a cheap machine work well? Perhaps, but it's a good example of cause and effect, and I like data

    Notes on the graphs - time is in increments of 0.5 of a minute, and all extractions were 30 seconds. Boiler temp. is the top line, shot temp. the bottom one.
    Attached Files

  • #2
    It's interesting how the boiler temperature can increase from its minimum to maximum while the shot temperature remains relatively constant (particularly the second shot in Gaggia classic thermostat 2.png). In fact many of these show that the rate of change of temperature is different for the boiler and the group. I guess this emphasises that the boiler temperature and the extraction temperature are not just different, but that its a non-trivial relationship between them (more than just subtracting 20 degrees). It's to be expected though, because pumping cold water into a hot boiler and simultaneously taking water out of it is different to just draining water from a boiler that's reached some equilibrium.

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    • #3
      Hmmm?
      my experiments on a couple of different SB m/cs ( Silvia & Ascaso) made me realise that the precise location of the boiler TC made a huge difference to both the indicated boiler temp and the rate of change.
      Location relative to both the heater element and the cold water inlet, and i was also unsure if any steam "pockets" in the boiler may influence the temperature readings.

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      • #4
        Papagoose, agreed. This data would not readily apply to machines with brass boilers of bigger volume, eg. the Silvia. My conclusion is that the thermal characteristics of the Gaggia Classic are a result of the way it's designed, specifically in terms of materials (aluminium) and external elements. In this case the boiler is acting as a thermoblock, with the mass of metal heating the water as opposed to an immersion element heating the water, then the water heating the boiler body via conduction.

        Thus, there would be a significant buffer between the measured temp. in the aluminium and the actual boiler water temp, then an even bigger one to the temperature of the brass group, which is what we actually want to control for a nice stable extraction. So this measurement can only help to correlate the measured temperature of the aluminium to an extrapolated extraction temp - aided somewhat by seeing how the two sets of measurements interact.

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