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Thread: Domesticated Witch - Adding PID control to the Bezzera Strega

  1. #1
    Site Sponsor coffee_machinist's Avatar
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    Domesticated Witch - Adding PID control to the Bezzera Strega

    Gene Cafe Coffee Roaster $850 - Free Beans Free Freight
    Thanks to Antony at Casa Espresso, I finally had a chance to indulge in some Strega tinkering. We have been talking about the idea of adding a PID for a while now, but when his latest stock arrived, we thought it was time to take the plunge! The Strega is already a very compelling machine, why not take it up another notch and see what can be done.

    The premise is thus: We know that Bezzera have been using cartridge heaters to heat groups in some of their machines for a while now, with a heat exchanger (non-thermosyphon - one way only) acting as a pre heater. It's an evolution from machines like the BZ99, which has the group physically attached to the boiler so that heat is conducted across. With cartridge heaters it is possible to heat the group mass directly, using minimal energy, without the need for conduction from a hot boiler, or a thermosyphon loop.

    That's all fine, but Bezzera, in a nod to simplicity (not saying that's a bad thing, mind - there are arguments why you would) chose to control the cartridge heaters with a good ol' bimetallic thermostat, of the same ilk that is found in Silvias, Gaggia Classics etc. with the inherent huuuuuuge deadband - often 15 - 20 degrees C.

    Group temperature, of course, has a big impact on brewing temperature. Especially in the case of the lever, where the brew water dwells on top of the coffee puck, surrounded by the group casting, for some time. It follows that a change in group idle temperature will have a corresponding change in extraction temperature.

    The nuts and bolts is simple enough, in place of the thermostat mounted to the group, we have a precision platinum RTD sensor, which is being monitored by the PID, controlling the cartridge heaters via a solid state relay. The PID sits externally mounted underneath the drip tray in it's own compact enclosure - for minimal disturbance to the front panel of the machine.

    I generated four graphs of the machine's performance, both stock out of the box (brand new machine) and with the PID retrofitted - one for heatup / idle behaviour, and one for shot temperature / group recovery. You can see the thermostat actually does quite a good job of keeping shot temperature consistent - in stock form the shots are relatively tightly grouped in temperature range, but they are all a bit toasty - the 95 degree rating of the thermostat sees to that, normally in levers I've found group temps are best kept in the mid-high 80's.

    The key thing for me was to enable control of shot temperature, and looking at the graph of shots with the PID fitted, I think I've achieved that. A correspondingly larger change to group temperature is required - for instance in order to lower the shot temperature by two degrees, the group temperature must be changed by a factor of 2, so minus 4, and vice versa to increase. It's clear that quite a bit of energy is coming from the heat exchanger. I've also wound back boiler pressure very slightly to help.

    All in all, this is now a fabulously controllable machine, and should be capable of extracting some simply stunning espresso - and most importantly, get the best out of different origins and blends for an adventurous owner.

    The modified machine will be for sale - Antony at Casa Espresso will be able to help with details, it will be a very reasonable premium to the normal Strega pricing!
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    Awesome work.

    So what does this mean for the intra shot temps, do you still get the declining temp profile ?

  3. #3
    Site Sponsor coffee_machinist's Avatar
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    Thanks Steve82.

    Yes absolutely. The green line represents the temperature logged in the scace basket, so over the 40 second or so extraction you can see that the temperature tails off approx. 4 degrees in the classic lever manner, before dropping sharply when the volume is exhausted and the handle removed.

  4. #4
    TC
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    Hope it's not leaving too soon Rick. Need. To. Play.

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    I've wanted to see this mod performed for many years since I read about the heaters.

    Well done! Engineering at its finest

    EDIT: Would a 'simple' mod to improve the toasty of this machine is to replace with a say 90deg thermostat? (ie. since the graph shows the temp is stable shot-to-shot, just too high overall?)
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  6. #6
    Site Sponsor coffee_machinist's Avatar
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    Sure, a different thermostat would be an option, I'll take the PID personally - makes temperature adjustments a breeze.
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    Nice work Rick! I've been waiting for someone to do his mo for years .

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    Quote Originally Posted by Enthusiast View Post
    Nice work Rick! I've been waiting for someone to do his mo for years .
    ^^ +1
    I could never really understand why Bezzera put a PID on the BZ07 boiler, but left the bimetal Thermostat on the group heaters !

    Quote Originally Posted by coffee-machinist
    .....for instance in order to lower the shot temperature by two degrees, the group temperature must be changed by a factor of 2, so minus 4, and vice versa to increase.
    ..Is that what you were doing between 8 mins and 16 mins , on the PID chart ?
    Last edited by blend52; 19th August 2015 at 01:12 PM.

  9. #9
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    Quote Originally Posted by blend52 View Post
    ..Is that what you were doing between 8 mins and 16 mins , on the PID chart ?
    Yes :-) 10chars

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    Seems eminently sensible.

    What's not sensible is Bezzera introducing a potentially massive temperature range variation on the final element that the water passes through
    Then again, the group will only shift the temperature of the water that exits the HX; not set the actual shot temperature

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    Quote Originally Posted by kaanage View Post
    ...Then again, the group will only shift the temperature of the water that exits the HX; not set the actual shot temperature
    What is the difference ?

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    Just curious as I'm about to go down this route myself (I post on Home-barista), do you flush to cool/heat the group, or is the strega turned into a walk up and pull machine with this mod?

  13. #13
    Senior Member Lukemc's Avatar
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    Domesticated Witch - Adding PID control to the Bezzera Strega

    I just had this mod completed on my Strega by Rick. 2 day turn around and the hot shots I was experiencing have been tamed.

    Rick made a change to my steam wand at my request as well and this has helped me generate silky micro foam which I was struggling with prior.

    Can't say enough about Ricks service, workmanship and communication.

    I was already a happy Strega owner, now I'm even happier

    Thanks again Rick.


  14. #14
    TC
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    Nice one Luke.

    A PID was what the Strega needed to morph from a fair machine to a very good one.

    Money well spent which should keep upgraditis to Aillio roasters for quite some time I'd reckon

    +1 also on Rick's work. Sheer brilliance and I'm very fortunate to have him just a couple of doors away.

    Cheers

    Chris
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  15. #15
    Roz
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    Would love to PID mine, pity I'm down in Tas.

    What was done to your steam wand?

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    Site Sponsor Casa Espresso's Avatar
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    Ditto for the quality of work from Rick. Second to none! As part of the development program with Rick we are offering full warranty coverage on new Strega machine with Ricks PIDs.

    We have a Strega on the bench now for any Snobers that want to have a look

    cheers

    Antony
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  17. #17
    Senior Member Lukemc's Avatar
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    Domesticated Witch - Adding PID control to the Bezzera Strega

    Quote Originally Posted by Roz View Post
    Would love to PID mine, pity I'm down in Tas.

    What was done to your steam wand?
    Hey Roz. Rick swapped the tip for me a while back and shaved a few threads off to make a smaller holed tip fit at my request. Whilst it worked well it wasn't perfect. I also felt the wand was too short and didn't allow me to rest the jug in the drip tray as I like to.

    When fitting the PID last week Rick also fitted a rocket steam wand and two hole tip for me which I find excellent (disclaimer I'm def no latte artist!!). I get silky milk with no effort now for my single piccolo latte but can still steam a big jug for my parents who like mega milky coffees. Can't be happier
    Last edited by Lukemc; 17th December 2015 at 12:12 PM.
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    Quote Originally Posted by Talk_Coffee View Post
    Nice one Luke.

    A PID was what the Strega needed to morph from a fair machine to a very good one.

    Money well spent which should keep upgraditis to Aillio roasters for quite some time I'd reckon

    +1 also on Rick's work. Sheer brilliance and I'm very fortunate to have him just a couple of doors away.

    Cheers

    Chris
    Really looking forward to that roaster your on the money as usual with the PID Chris. Very happy i got it done.

  19. #19
    Site Sponsor coffee_machinist's Avatar
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    Thanks Luke, Chris and Antony for your kind words and support. I'd happily do work like this all week, I get a real kick out of helping folks get the best from their equipment.

    Happy shot pulling!
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  20. #20
    kbc
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    Domesticated Witch - Adding PID control to the Bezzera Strega

    Beautiful work Rick. I dropped by at Casa Espresso last week. The Strega is a great looking machine. Your PID makes it a great performer as well

  21. #21
    Senior Member magnafunk's Avatar
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    Quote Originally Posted by Lukemc View Post
    can still steam a big hug for my parents
    You're never too old to steam your old man a big hug and they appreciate it too
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    Quote Originally Posted by magnafunk View Post
    You're never too old to steam your old man a big hug and they appreciate it too
    As well as a steamed hug, the folks get "silly milk", too. Couldn't ask for more

  23. #23
    nmk
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    @coffee_machinist: Is 4C drop under a shot not too much? Could a cold flush maybe help on it?

    @Lukemc:what was the problem with the steam wand. Had it too much holes?

  24. #24
    bmb
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    "The nuts and bolts is simple enough, in place of the thermostat mounted to the group, we have a precision platinum RTD sensor, which is being monitored by the PID, controlling the cartridge heaters via a solid state relay. The PID sits externally mounted underneat" ...

    Could you please explain: where exactly is the sensor attached and how is the connection done ?

  25. #25
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    Quote Originally Posted by bmb View Post
    "The nuts and bolts is simple enough, in place of the thermostat mounted to the group, we have a precision platinum RTD sensor, which is being monitored by the PID, controlling the cartridge heaters via a solid state relay. The PID sits externally mounted underneat" ...

    Could you please explain: where exactly is the sensor attached and how is the connection done ?
    Reckon that would be a clue...

    Mal.

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    Quote Originally Posted by nmk View Post
    @coffee_machinist: Is 4C drop under a shot not too much? Could a cold flush maybe help on it?

    @Lukemc:what was the problem with the steam wand. Had it too much holes?
    I wasn't happy with the tip nor the length of the wand. I like to steam whilst resting the jug on the drop tray and couldn't prior to the swap. Personal preference though.
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    Hi Luke & coffee_machinist - just wanted to know what the real-world improvement from PID'ing the group was, and what the bang-for-buck ratio is ?price of surgery. I'm in pre-contemplation phase, generally happy with the Strega but just wondering. Cheers, Dave

  28. #28
    TC
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    Quote Originally Posted by habahabanero View Post
    Hi Luke & coffee_machinist - just wanted to know what the real-world improvement from PID'ing the group was, and what the bang-for-buck ratio is ?price of surgery. I'm in pre-contemplation phase, generally happy with the Strega but just wondering. Cheers, Dave
    Hi Dave. Unless I'm mistaken, Rick's original post includes a pretty good report of the real world improvement.

    $$$? Call or otherwise make contact and I'm sure he'll be happy to quote you.

    For mine, it turns a Strega from what it is into what it should have been at the outset.

  29. #29
    bmb
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    Domesticated Witch - Adding PID control to the Bezzera Strega

    I have a digital thermometer attached to the Strega. The sensor is just like a normal thin wire and I have it "tied" to the group with a plastic strip.
    Once it's fully warm it shows around 78 and 82C , to an eventual extreme of 76 and 84.
    Even a 2C difference produces a big difference, and I also find the Strega somewhat too hot for darker roasts (it's really good for lighter ones, that require more punch and heat for proper extraction).
    As I read about this I asked a friend and he ordered a PID, SSR and sensor from China, that just arrived ...
    The sensor's end is like a screw, and I believe that the intended installation would be by drilling a hole in the back of the group, making the female screw with a tool (sorry as I live in Brazil, and not very handy, I don't know the correct name of these less mainstream tools), where the sensor should screw in.
    Evidently I would prefer to avoid drilling the group, therefore my question to the coffee_machinist on where he connected the sensor ?

  30. #30
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    The original thermostat is attached to the group via a M4 threaded hole, which is exactly where the temperature sensor goes. It's a direct replacement
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  31. #31
    bmb
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    Great ! Thank you very much !!

  32. #32
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    Wonderful thread here, and beautiful sounding mod. I'd almost sell my 3 year old Strega and purchase Rick's modded one, including the steam arm mod, but Nashville TN is a LONG way away, and I'm not sure the wife would understand what I'm up to!
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    Quote Originally Posted by Talk_Coffee View Post
    Hi Dave. Unless I'm mistaken, Rick's original post includes a pretty good report of the real world improvement.

    $$$? Call or otherwise make contact and I'm sure he'll be happy to quote you.

    For mine, it turns a Strega from what it is into what it should have been at the outset.
    Thanks for the reply, Chris. I did look at the numbers, which evidently look good. I was after a more subjective bang-for-your-buck equation in this case.

    With my Strega I find I have to pull water through the hot water wand and wait for the pump to finish filling the boiler, then pull the shot - if I don't follow this cooling procedure I get a slightly burnt shot. Would PID'ing solve this issue?
    Cheers, Dave

  34. #34
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    Hi Dave,

    If you prefer a flatter temperature profile, which in effect is what you're getting with your boiler fill routine, the PID'd group will not be of much use. The one-way heat exchanger will still dump overheated water into the group, all the PID does is keep the group stable, and allow it to run cooler or hotter to effect a change in the overall temperature range for the shot - without really influencing the shape of said curve. All this can be seen in the green plots on the graphs, a peak at the start then a gradual taper.

    This is classic lever stuff. Temperature curves can be changed, but for that you need to control water temp effectively at the heat exchanger. I tweak it down a bit, but there is not much room for adjustment before steam pressure suffers.

    In my modded San Marco for instance, I can control group temperature and feed water temperature through a second boiler before it hits the group. Then I can start to see a flatter temp profile.

    Sounds like your strategy is giving you the result you want, but to do so you're fighting the characteristics of the machine.
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    Quote Originally Posted by coffee_machinist View Post
    Hi Dave,

    If you prefer a flatter temperature profile, which in effect is what you're getting with your boiler fill routine, the PID'd group will not be of much use. The one-way heat exchanger will still dump overheated water into the group, all the PID does is keep the group stable, and allow it to run cooler or hotter to effect a change in the overall temperature range for the shot - without really influencing the shape of said curve. All this can be seen in the green plots on the graphs, a peak at the start then a gradual taper.

    This is classic lever stuff. Temperature curves can be changed, but for that you need to control water temp effectively at the heat exchanger. I tweak it down a bit, but there is not much room for adjustment before steam pressure suffers.

    In my modded San Marco for instance, I can control group temperature and feed water temperature through a second boiler before it hits the group. Then I can start to see a flatter temp profile.

    Sounds like your strategy is giving you the result you want, but to do so you're fighting the characteristics of the machine.
    Thanks Rick. I like the idea of reliably keeping the shot temp sub-95C - I suspect the slightly burnt taste from the >95C start temp works better with traditional Italian espresso blends than SO's. I'll have to give you a call before I next head down your way - it's a bit of a drive, but there's no-one around here that will service the Strega anyway so I'll have to pack her in the boot for the trip.

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    Quote Originally Posted by coffee_machinist View Post
    The key thing for me was to enable control of shot temperature, and looking at the graph of shots with the PID fitted, I think I've achieved that. A correspondingly larger change to group temperature is required - for instance in order to lower the shot temperature by two degrees, the group temperature must be changed by a factor of 2, so minus 4, and vice versa to increase. It's clear that quite a bit of energy is coming from the heat exchanger. I've also wound back boiler pressure very slightly to help.
    Could the same desired brew temperature and consistency be achieved by dropping the PID set point even lower and raising the boiler pressure, giving faster warmup and better steam?
    Last edited by simonko; 15th June 2016 at 09:36 AM.

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    Quote Originally Posted by bostonbuzz View Post
    Just curious as I'm about to go down this route myself (I post on Home-barista), do you flush to cool/heat the group, or is the strega turned into a walk up and pull machine with this mod?
    I was just reading through this thread and was interested to hear some feedback on this post which went unanswered. The last post in the thread also went unaswered and I might give it a bump for some feedback hopefully from Coffee Machinist or any Strega gurus.

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    Quote Originally Posted by simonko View Post
    Could the same desired brew temperature and consistency be achieved by dropping the PID set point even lower and raising the boiler pressure, giving faster warmup and better steam?
    Bump... can anyone provide some feedback on this question as well?

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    On Strega (and commercial levers in general), the final brew temp seen by the coffee puck is a product of group head temperature and boiler temperature (whether it is fed from HX or from the boiler itself). As such, you could have 120C water going into the 75-90C group head to produce an excellent shot with the right temperature.

    On a stock Strega, there are two sources of temperature fluctuation. The first is obviously the pstat-controlled boiler temperature but that typically stays within 3C (assuming 0.2 bar deadband). The biggest fluctuation however, comes from the thermostat-controlled cartridge heaters in the group head. These have a deadband of 10C(?) as Rick mentioned above. Because the group head has such a significant mass (compared to incoming water), its temperature then is even more crucial.

    Another reason (probably even more important than above) is the thermostat is non-adjustable and the setting/dead band they come from factory is way hot for the brew temperature we prefer. Thus the 'cooling' flush are attempts to bring down the incoming water temp so that the final combination of group head+boiler temp will be in the right range. However, how much to flush to cool into the right range can be quite tricky. Shall the grouphead comes with a lower-setting thermostat, the cooling flush could likely be eliminated (there's still the dead band issue).

    PIDing the cartridge heater eliminates the biggest temperature uncertainty. If it's well tuned, there is no need to flush (like many levers out there). You can certainly adjust the temperature by lowering or increasing the group head temperature - as mentioned above, the final brew temp is the product of group head+boiler temp so you can adjust from either end to achieve the same result (it's easier to keep one end constant and adjust another though).



    About raising boiler pressure and lowering PID, yes, technically you can. However, the trade-off is the recovery time (the time the group head needed to come back down on PID temperature). This is because by doing so, you're bringing more heat (hotter water) into the group head and the head has to work harder to cool down more.
    Last edited by samuellaw178; 12th September 2016 at 09:21 PM.



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