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  • Question on PID in a HX

    There are several comments on here that a PID in a HX (eg Giotto PP v3) is simply a digital pressurestat, and that makes sense to me.

    But what I'm not sure of is how the PID actually works?

    On my non-PID giotto the pressurestat cycles from roughly 0.9-1.3 bar. Does a PID just keep it at a much narrower range?

    If it does, then wouldn't that lead to either reduced steam, or brew temp that's too hot?

    To explain, if the PID is set to keep it at say 1.3 bar then you'll always have great steam, but your brew water coming through the HX will be quite hot.

    And if the PID keeps the boiler at say 1.1 bar, them arguably your brew temp will be fine, but you're limiting your steam power.

    Or have I completely misunderstood what the PID in a HX does?

    Incidentally, I don't worry too much about flushing my brew water, but about half way through the shot I purge the steam for half a second, which drops the pressurestat below 0.9 bar and starts the boiler, so that when the shot has finished it's at 1.3 bar and I immediately start steaming my milk with maximum steam power.

    Cheers
    Jonathon
    Last edited by Jonathon; 23 September 2014, 12:46 PM.

  • #2
    Originally posted by Jonathon View Post
    There are several comments on here that a PID in a HX (eg Giotto PP v3) is simply a digital pressurestat, and that makes sense to me.

    But what I'm not sure of is how the PID actually works?

    On my non-PID giotto the pressurestat cycles from roughly 0.9-1.3 bar. Does a PID just keep it at a much narrower range?

    If it does, then wouldn't that lead to either reduced steam, or brew temp that's too hot?

    To explain, if the PID is set to keep it at say 1.3 bar then you'll always have great steam, but your brew water coming through the HX will be quite hot.

    And if the PID keeps the boiler at say 1.1 bar, them arguably your brew temp will be fine, but you're limiting your steam power.

    Or have I completely misunderstood what the PID in a HX does?

    Incidentally, I don't worry too much about flushing my brew water, but about half way through the shot I purge the steam for half a second, which drops the pressurestat below 0.9 bar and starts the boiler, so that when the shot has finished it's at 1.3 bar and I immediately start steaming my milk with maximum steam power.

    Cheers
    Jonathon
    interesting post. I was just having this conversation on another thread http://coffeesnobs.com.au/brewing-eq...estrale-2.html

    My opinion (and it is only mine - as coffee is quite subjective) is that a PID on HX machines gives you control that you don't otherwise have. Kinda like a cruise control in a car. It uses feedback loops to maintain the desired temp.

    I drink black coffees mostly but the rest of my family drink whites. I want the brew temp to be a 92-93°C but for steam that would be way to low. Since I don't want to fiddle with flushing the water all the time just to go between my coffees and theirs, a PID would eliminate a lot of those issues.

    However, as I said thats my opinion. Others will say that a PID on a HX machine is superfluous. My situation is unique, so most people who are only making white coffees and the occasional black probably don't need a PID.

    Comment


    • #3
      A PID and pressurestat would presumably be set to achieve the same pressure/temperature. The only difference would be that with the PID its easier to change the setpoint, and possibly you will have less deviation from the setpoint. The dynamic response will likely be different also.

      Comment


      • #4
        Originally posted by MrJack View Post
        A PID and pressurestat would presumably be set to achieve the same pressure/temperature. The only difference would be that with the PID its easier to change the setpoint, and possibly you will have less deviation from the setpoint. The dynamic response will likely be different also.
        It will be practically immeasurable...

        Also, with a HX machine, due to the thermal dynamics of the design, you will not be able to vary the brew water temperature in any meaningful way. Far better to ensure that the thermodynamics are stable (by design) and achieve a constant brew water temperature delivered to the top of the puck. A high quality p/stat coupled with an SSR is more than capable of achieving superlative temperature stability of the brew water temperature.

        The design of the Heat Exchanger coupled with the Boiler and Thermosyphon, are inextricably linked. If you want absolute temperature control of the brew water temperature, then grab a Single Boiler Dual Purpose Machine with PID Controller or a Dual Boiler Machine with a dedicated brew boiler that is controlled via a PID Controller. That is the only way you are going to achieve anything close to acceptable control.

        Mal.
        Last edited by Dimal; 24 September 2014, 06:52 PM.

        Comment


        • #5
          Perhaps I should have made it clear I was referring to boiler temperature, and for HX in general.

          The point I was trying to make is just that fitting a PID isn't going to suddenly compromise your ability to steam and brew (which the previous poster appeared to suggest). It's just another way to control to the same setpoint.

          Also remember that not all HX machines incorporate a thermosyphon, and I suspect even those which do are actually in unstable (or perhaps metastable) equilibrium.

          Comment


          • #6
            Originally posted by MrJack View Post
            Perhaps I should have made it clear I was referring to boiler temperature, and for HX in general.
            No worries...

            Originally posted by MrJack View Post
            The point I was trying to make is just that fitting a PID isn't going to suddenly compromise your ability to steam and brew (which the previous poster appeared to suggest). It's just another way to control to the same setpoint.
            Yep...

            Originally posted by MrJack View Post
            Also remember that not all HX machines incorporate a thermosyphon, and I suspect even those which do are actually in unstable (or perhaps metastable) equilibrium.
            These points are arguable but I don't feel like going over the same old ground again...

            Mal.

            Comment


            • #7
              Originally posted by MrJack View Post
              Perhaps I should have made it clear I was referring to boiler temperature, and for HX in general.

              The point I was trying to make is just that fitting a PID isn't going to suddenly compromise your ability to steam and brew (which the previous poster appeared to suggest). It's just another way to control to the same setpoint.

              Also remember that not all HX machines incorporate a thermosyphon, and I suspect even those which do are actually in unstable (or perhaps metastable) equilibrium.
              I'm not sure if you were referring to me. I was never suggesting that you must go out and buy a PID to put on your HX machine to achieve way better temp control. I also was not saying that an PID on an HX machine would be just as good as a DB or SB with PID. I was referring back to the other thread where Dimal said it was completely superfluous and I said that I disagreed. In this thread the OP was asking about what would a PID do on a HX machine and so I was merely responding to that.

              Absolutely, if you want good control get a DB. No doubt about that. In fact, go buy a Spirit.

              Comment


              • #8
                Originally posted by Ryadan View Post
                . In this thread the OP was asking about what would a PID do on a HX machine and so I was merely responding to that.
                Yep, to go back to my original question, my confusion is regarding exactly what the PID does in a HX.

                I know a PID is generally much smarter than a simple on/off switch. But in the case of a HX, particularly a Giotto v3 (I have a v2) I'm wondering if it merely controls the upper and lower pressure points based on temp, rather than the Siraj controlling those points based on pressure.

                And if it does do this, does it operate across a similarly wide range of temps that would be equivalent to say 0.9 to 1.2 bar or does it control the boiler to a much narrower range, perhaps even a set temp?

                Comment


                • #9
                  It's controls using temp rather than pressure across a narrower band. I suspect end users will notice little, if any difference in performance.

                  Comment


                  • #10
                    Originally posted by Jonathon View Post

                    Or have I completely misunderstood what the PID in a HX does?

                    Incidentally, I don't worry too much about flushing my brew water, but about half way through the shot I purge the steam for half a second, which drops the pressurestat below 0.9 bar and starts the boiler, so that when the shot has finished it's at 1.3 bar and I immediately start steaming my milk with maximum steam power.

                    Cheers
                    Jonathon
                    I believe a bit of research is needed to understand HX machines......

                    Opening the steam mid extraction is going to do zip......
                    Boiler and steam production is quite removed from brew extraction via Thermosyphon.

                    99% of HX machine need a cooling flush, length of which is determined by how long the machine has been idling since last extraction. The HX design is one that continues to absorb temperature the longer it is idling after initial warm up.

                    Comment


                    • #11
                      Originally posted by hotshod View Post
                      The HX design is one that continues to absorb temperature the longer it is idling after initial warm up.
                      Well, that only happens when the design/engineering is inadequate...

                      Mal.

                      Comment


                      • #12
                        Originally posted by hotshod View Post
                        I believe a bit of research is needed to understand HX machines......

                        Opening the steam mid extraction is going to do zip......
                        Boiler and steam production is quite removed from brew extraction via Thermosyphon.
                        You've misunderstood my point , understandably as I threw that comment into a post about something different.

                        I was saying that to maximise the steam output, I purge the steam during the brew just enough to drop the boiler pressure to trigger the pressurestat to kick in, so that when the brew has finished (say 8-10 seconds later) the boiler is at the maximum pressure and therefore the steam output is at its max power.

                        Comment


                        • #13
                          Originally posted by hotshod View Post
                          I believe a bit of research is needed to understand HX machines......

                          .....99% of HX machine need a cooling flush, length of which is determined by how long the machine has been idling since last extraction.
                          HX machines were designed for commercial use. My understanding is it doesnt actually matter if they need a cooling flush when brought "back on line" after idling for a while.... because the design works in continuous use which is what they are designed for. Additionally my understanding of the length of the cooling flush in any particular brand/model machine is it depends on the individual internal design of the model and how it is internally set up and not simply how long the machine is left to idle. They can be set up in different ways to suit particular markets or for particular uses, and this is done on the production line where it is easier to set up when manufacturing, than it is to retro modify some time later.

                          Originally posted by hotshod View Post
                          ....The HX design is one that continues to absorb temperature the longer it is idling after initial warm up.
                          My understanding is this can only happen up to a point...ie it doesnt continue to build ad infinitum (which is how it reads to me in the quote), and is determined by the internal set up mentioned above.

                          Good brand/model HX machines that are well set up for the market or use they are intended for, will need minimal cooling flushes after idling, but that doesnt present any problems to my knowledge, if that is what is meant in the quoted passages.
                          Last edited by TOK; 10 November 2014, 11:28 PM.

                          Comment


                          • #14
                            Generally speaking, a hx machine is just a tube full of brew water, passing through the steam boiler.

                            I'd just like to note, temperature is not transferred. There is a transfer of heat between two masses when they are at different temperatures (e.g. boiler and brew water) . The direction of transfer is always from the 'hot' to the 'cold' mass.

                            The brew water temperature cannot exceed the (max) boiler water temperature. If you sit the machine idle, the brew water will likely heat up to the boiler temp (or near to).

                            If the exchanger is designed to heat the brew water to the boiler temp and rely on cooling downstream to reach the desired brew temperature, then a cooling flush wouldn't have much effect.
                            Last edited by MrJack; 11 November 2014, 11:50 PM.

                            Comment


                            • #15
                              I had a good talk with a tech at Dimattina about my Rocket PID. The great advantage of an adjustable PID is that you can vary the brew temp without having to undo the machine to adjust a Pressurestat. Varying the PID allows the brew temp to be altered to get the best taste out of the beans. Most won't bother but it is there for adjustment. The Rocket Comes with chart that enables one to convert the boiler temp to the group temp. The tech said - be brave and play with it, see if it changes the taste and enjoy your weekend!

                              Now I come to think of it is the same reason I put a PID on my Silvia many years ago.
                              Last edited by dumiya; 14 November 2014, 06:51 PM.

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