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Domesticated Witch - Adding PID control to the Bezzera Strega

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  • Domesticated Witch - Adding PID control to the Bezzera Strega

    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!
    Attached Files

  • #2
    Awesome work.

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

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    • #3
      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.

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

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        • #5
          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
            Sure, a different thermostat would be an option, I'll take the PID personally - makes temperature adjustments a breeze.

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            • #7
              Nice work Rick! I've been waiting for someone to do his mo for years .

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              • #8
                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 !

                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; 19 August 2015, 01:12 PM.

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                • #9
                  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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                  • #10
                    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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                    • #11
                      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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                      • #12
                        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?

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                        • #13
                          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.

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                          • #14
                            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
                              Would love to PID mine, pity I'm down in Tas.

                              What was done to your steam wand?

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