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Is It Worth Putting A PID On A Rocket Giotto?

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  • 338
    replied
    Big thanks for the physics lesson, most appreciated. Amazing to think so little water equals 4.5kg of brass. Sorry to hear your builder didn't share your passion for chasing the ultimate solution, not uncommon to hear. I have a phrase I used to use on a few projects I did, 'the guy digging the hole shouldn't decide how deep it needs to be'.


    Apologies for this off topic digression.

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  • robusto
    replied
    Just 3 words....lag...lag....lag.
    There is no way a sudden 1 or 2 or 3 degree change in the temperature of a boiler containing from several litres to several gallons of near-boiling water is going to translate to an equal change of brew water temperature about to hit the puck.

    And that applies equally to whether the hx machine is controlled by a PID or the very efficient pressurestat.

    The heavy group with thermocycle has a few functions.

    After switch on it warms up so the portafilter will also be kept warm.

    Once warmed up, ambient air cools the group to well below the near-boiling temperature in the boiler.

    Thermal mass will also smooth fluctuations.

    How the group does this is inherent in the design. The invention itself. Fine tuning is also achieved by such things as the size of restrictors in the group and the length of the tubes going to and from the group to the heat exchanger.

    The deadband in the pressurestat is not going to affect what's happening in the hx chamber or the group head. Nor would a PID.

    The only advantage in my opinion of a PID over a pressurestat in a hx machine is that it tells you "a" temperature.

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  • Lyrebird
    replied
    In order:

    No, thermal mass (properly volumetric thermal capacity) is related to how much energy it takes to get the constituent molecules in a substance vibrating. It is thus mostly to do with the number of molecules per unit volume. Most solids have roughly the same number of molecules in a given volume, so they will have roughly the same volumetric thermal capacity (close enough to 3 MJ / m3 / K). This includes brass, so its thermal mass per unit weight (properly specific thermal capacity) is low because it is dense.

    Yes, K is just absolute temperature (Kelvins)

    Yes.

    Yes, water is anomolously high because the molecules can vibrate in more ways (more degrees of freedom) so it is an excellent heat trap. I wanted to run a whole bunch of water pipes under the polished concrete floors and connect them to an insulated tank but the builder couldn't cope so I had to drop it.

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  • 338
    replied
    Hi Lyrebird, that surprised me but assume it is because brass is far more conductive? In your equations does the 380 J and 4180 joules represent the energy needed to heat 1kg by 1 degree celcius?

    Does this work the same in reverse? ie will 400ml of water lose heat at the same rate as 4.5kg of brass to the surrounding air? Only ask as went through the first PassiveHaus in Sydney a couple of weeks ago and they had 8 or 9 floor to ceiling 500ml tubes filled with water to add thermal mass. I thought it was novel but based on your equation pretty effective as well.

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  • Lyrebird
    replied
    As I've said before, the E61 grouphead has about the same "thermal mass" as 400 ml of water.

    4.5 kg x 380 J / kg / K = 1710 J / K compared to 0.4 kg x 4180 J / kg /K = 1672 J / K.

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  • Caffeinator
    replied
    Originally posted by Lyrebird View Post
    Typical deadband on a pressurestat is about 20 kPa (~ 0.2 bar) , which corresponds to a temperature fluctuation of about +/- 1.5 oC at a typical setpoint.

    As an example if the pressurestat turns on at 100 kPa and off at 120 kPa, the temperature will vary from 120.4 oC to 123.4 oC (assuming the environment is at 101.3 kPa)
    Yes- all good and well. 5kg of brass has a much tighter deadband than that. Try measuring it with a Scace device

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  • Lyrebird
    replied
    Originally posted by CafeLotta View Post

    Others elsewhere seem to suggest there is a fair temp deadband on pressurestats but it sounds like that may not be the case?
    Typical deadband on a pressurestat is about 20 kPa (~ 0.2 bar) , which corresponds to a temperature fluctuation of about +/- 1.5 oC at a typical setpoint.

    As an example if the pressurestat turns on at 100 kPa and off at 120 kPa, the temperature will vary from 120.4 oC to 123.4 oC (assuming the environment is at 101.3 kPa)

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  • CafeLotta
    replied
    Originally posted by Dimal View Post
    Didn't read through this thread then?
    https://coffeesnobs.com.au/brewing-e...x-machine.html

    Mal.
    Interesting read re: PID on Hx from some CS heavyweights. Shame they're not all still on here.

    Seems like the main pro argument for PID on Hx is not accuracy but rather ease of adjustability and possibly longevity (maybe also quieter operation for sensitive ears).

    Others elsewhere seem to suggest there is a fair temp deadband on pressurestats but it sounds like that may not be the case?
    Last edited by CafeLotta; 23 September 2019, 02:57 PM. Reason: added "is not accuracy" to clarify

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  • Dimal
    replied
    Didn't read through this thread then?
    https://coffeesnobs.com.au/brewing-e...x-machine.html

    Mal.

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  • Caffeinator
    replied
    Originally posted by Shortie View Post
    Pulling back-to-back shots allows the machine to function as it is designed to. Temp will be somewhat stabilised by the workrate. Pulling randomly-timed shots throws in temp variations due to the wide band of temps allowed by the pressurestat. If Rocket felt it worthwhile to upgrade the V3 by installing PID then I want to know how to install this upgrage on my 2009 machine. I have successfully pidded a Silvia and loved the improvement in shot quality. Can anyone reccommend a PID kit for the older Rocket that is labelled 'The Rocket'? Thank you.
    Bring the zombies back to life!

    What you have not considered is that your Silvia had nothing much in the way of a group. The Rocket has almost 5kg of group which is pretty reluctant to change temperature.

    Whilst a PID will react more rapidly than a pressurestat, the group will not. Minimal, if any real world advantage.

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  • Shortie
    replied
    Back to Back shots

    Pulling back-to-back shots allows the machine to function as it is designed to. Temp will be somewhat stabilised by the workrate. Pulling randomly-timed shots throws in temp variations due to the wide band of temps allowed by the pressurestat. If Rocket felt it worthwhile to upgrade the V3 by installing PID then I want to know how to install this upgrage on my 2009 machine. I have successfully pidded a Silvia and loved the improvement in shot quality. Can anyone reccommend a PID kit for the older Rocket that is labelled 'The Rocket'? Thank you.

    Leave a comment:


  • MrJack
    replied
    Originally posted by Robbks View Post
    while you're water "leaving the boiler" may be 3-4' cooler, you need to take into account how much is still sitting between the boiler and the plunger in the group.
    All of which is being temperature controlled and hopefully stabilised to desired brew temperature by correct calibration of the thermo-syphon system.
    probably a lot more than 60mls.
    We're going at this from slightly different angles I think.

    All I was trying to point out was that a short, rapid drop in boiler temperature (such as during boiler fill) will have quite different dynamics to a slower, relatively prolonged cooling of the whole machine (as seen with a wide temperature deadband); so your results may not generalise to all fluctuations in boiler temperature, in all machines.

    I don't think the thermosyphon is the key in either scenario; what matters is the mass of the group.

    Why? My thoughts are as follows:

    The thermosyphon will maintain a particular group temperature (the equilibrium temperature) at which the rate of heat transferred to the group (which is ultimately governed by flowrate and boiler temperature) equals that leaving the group (governed by the relative ambient/group temperature).

    There is some capacity to compensate for fluctuating ambient conditions, but not for lower boiler temperatures. In fact, boiler cooling will counteract thermosython flow (and exaggerate the cooling at the group).

    The mass of the group however, buys you time; the greater the mass, the more heat needs to be lost for a given temperature change, and the smaller the temperature fluctuation before the element kicks back in.
    Last edited by MrJack; 20 August 2014, 02:20 AM.

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  • Robbks
    replied
    Originally posted by MrJack View Post
    In any case, its not really comparable to the temperature swing permitted by the pressurestat, which is much slower than following boiler fill (about 2 mins from element off to element on, on my machine). The water leaving the boiler will actually be 3-4°C cooler (as opposed to just appearing to be!).
    while your water "leaving the boiler" may be 3-4' cooler, you need to take into account how much is still sitting between the boiler and the plunger in the group.
    All of which is being temperature controlled and hopefully stabilised to desired brew temperature by correct calibration of the thermo-syphon system.
    probably a lot more than 60mls.

    Originally posted by MrJack View Post
    Really all this demonstrates is that there is a difference between the measured temperature and the actual temperature in your boiler (and that boiler temperature probe placement is important). If you had a similar 3-6°C drop in the temperature of the water leaving the boiler, I am sure you would see a more significant drop in the brew temperature and group temperature.
    My PID (Gicar, as used in the Giotto V3 and nearly everything else at the moment) shows you the "expected" brew water temp.
    That temp is the Boiler temp minus a few degrees. Which I have re-calibrated for my kitchen/ machine using my own version of a SCASE device (I have an old basket with a Fluke thermocouple fixed to it)
    Some versions of the PID do display the actual boiler temp instead of the output temp.

    but that's why I've tested and checked how my machine does operate and how the boiler temps affect the output temps.
    From all my testing and tuning of the PID, I can just keep pulling back to back shots until the cows come home with less than 1'c variance in temps through the puck.

    My Target brew temp is 93' as that's the ideal temp for the beans I source.
    Last edited by Robbks; 19 August 2014, 01:12 PM.

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  • MrJack
    replied
    Originally posted by Robbks View Post
    Just like to point out one thing after doing a lot of research and time spent calibrating my own PID'd DB machine,
    I can pull 60-90ml through my double shots with <1'c temp change at the bottom of the basket., (the temp does initially drop a few degrees as the puck soaks up some heat, due to not keeping ground beans at 93')

    BUT the PID shows 3-6 degree drop in boiler temps (due to cold water inflow) and proximity of the inlet pipe to the boiler temp probe.

    So as pointed out above, Boiler temp does not always directly relate to water temp through the puck.

    I think a few kg of brass and the correct amount of water syphoning through it will do a highly effective job of maintaining stable temps.
    moreso than a $30 piece of chinese circuitry..
    Really all this demonstrates is that there is a difference between the measured temperature and the actual temperature in your boiler (and that boiler temperature probe placement is important). If you had a similar 3-6°C drop in the temperature of the water leaving the boiler, I am sure you would see a more significant drop in the brew temperature and group temperature.

    In any case, its not really comparable to the temperature swing permitted by the pressurestat, which is much slower than following boiler fill (about 2 mins from element off to element on, on my machine). The water leaving the boiler will actually be 3-4°C cooler (as opposed to just appearing to be!).

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  • Dragunov21
    replied
    Originally posted by Hildy View Post
    The fluctuations between pstat firings, or a change in the boiler set point?

    with the former, very little (the group head is an inductor); with the latter, a lot.
    The former.

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