Shaun, None taken...
On Feb 7, 8:14 pm, "Shaun Taylor" <[email protected]> wrote: > I got half way into the email and lost... interest... no offence. ;-) > > My shots taste good. Yay. > > ---------------------------------------------------------------------------- > -- > > Shaun Taylor > > http://shaundoreenevankeegan.blogspot.com/ > > -----Original Message----- > From: [email protected] [mailto:[email protected]] On Behalf > > Of cgfan > Sent: 07 February 2009 21:47 > To: Brewtus > Subject: A possible design flaw? I hope I'm wrong about this... > > A possible design flaw? I hope I'm wrong about this... > > In the course of trying to figure out the source of some recently sour > shots, in the past few days I've been through a tumult of long > sessions on the Brewtus running various experiments with brew temps, > offsets, thermocouples and thermometers, switching roasters and > espresso blends, posting to boards, formulating various hypotheses, > and even discussing with my fellow but non-java oriented engineering > colleagues. Through it all I've formulated a hypothesis that though > so far I've been able to confirm by experiment, I still hold out some > hope that I've overlooked something that invalidates my conclusion. > > Here's the conclusion that I've reached re. my Brewtus I, but should > also apply to the whole Brewtus series, and, in fact, most other > similarly designed dual boiler espresso machines. The conclusion that > I've reached: given a thermal drop of x deg C from the pump to the > portafilter (noting that x is negative), the maximum possible brewing > temperature at the portafilter is 100+x deg. C. > > So given an offset of, say, -6 deg C, this implies a maximum possible > brewing temperature at the portafilter of 100 + -6 = 94 deg. C. > > A corollary to this is that the thermal drop x, which is the offset > that we program into our temperature controller as C0, can easily be > mistaken/mismeasured to be greater than the "intrinsic" thermal drop > across the grouphead, given a key point in the hypothesis. According > to the hypothesis a meaningful thermal drop x can only be measured > under certain operating conditions. > > So here's the hypothesis: > > The water that will brew your espresso is the very first volume of > water which exits the pump. As we pump out the water for brewing our > espresso, it is first being pumped out into an empty (and therefore > non-pressurized) brew path whose volume I estimate to be around 1.135 > ounces. Thus barring any significant losses at this point the water > at the output of the pump will have a maximum temperature of 100 deg C > or Tbrew_boiler, wichever is less. This is because once the water > leaves the pressureized confines of the brew boiler it no longer can > reach temperatures above 100 deg C. Any water which was kept in the > boiler above 100 deg C will instantly lose its excess heat as steam > and drop to 100 deg C. > > Now this water, as it travels from the output of the pump to the > portafilter, will have dropped x degrees by the time it reaches the > puck of compressed coffee. > > Thus, noting that x is a negative quantity, the brew water is > determined by: > > Tpump_out = 100 deg C : for Tbrew_boiler > 100 deg C > = Tbrew_boiler : for Tbrew_boiler <= 100 deg C > > Tbrew_water = Tpump_out + x deg C > > Thus: > > Max(Tbrew_water) = Max(Tpump_out) + x deg C > = 100 + x deg C > > Ouch! Not a good situation when our desired brew temps are so close > to 100 deg C and our offsets, irrespective of any debate, is at least > (-) 5, the original factory default for the Brewtus I. > > So here's the corollary which follows the hypothesis: > > The "offset" that we program into our temperature controller x is only > a meaningful offset from the boiler temperature Tbrew_boiler when > > Tbrew_boiler <= 100 deg C > > This can be seen by the relation for Tpump_out above. Under any other > condition the offset x is only an offset off of a fixed 100 deg C. > > Thus if one tries to estimate their offset x via a measurement of the > brew boiler temperature, it is important to do so only when > Tbrew_boiler <= 100 deg C. > > (I suspect that the tendency for users to suggest offsets greater than > (-) 5 might be due to brew boiler and portafilter measurements taken > under conditions where the brew boiler exceeds 100 deg C. Under these > conditions every degree increase in brew boiler temperature will not > increase the brew water temperature at the portafilter and hence > result in a larger "apparent" offset.) > > I've verified the hypothesis via a simple experiment that's documented > here:http://groups.google.com/group/brewtus/msg/107c4e427e109637?hl=en > > Basically I take repeated measurements of the brew water under a fixed > target brew temperature but varying offsets. I found the brew water > temperature not to change for any condition where Ttarget_brew_temp - > x > 100 deg C (again noting that x is negative). As soon as > Ttarget_brew_temp - x <= 100 deg C, the output brew water temperature > started to change accordingly. > > Note that the hypothesis applies to only the initial volume of water > sufficient to fill the total volume of the brew path, which my > experiments estimate to be around 1.135 oz. As any water pumped > subsequent to this initial volume will be pressurized due to the > purging of any air in the brew path, subsequent flow can break the > temperature restrictions implied above. However the initial 1+ oz. > for even double espressos represents a sizeable portion of the total > brew volume, and in my case of super short ristrettos this initial > volume alone is enough to generate several shots worth of brew water > under these temperature-compromised conditions. > > Note that temperature flushes will not relieve us from these limits, > as the temperature flush still leaves the brew path empty of any brew > water. (However temperature flushes will help us get as high a > temperature of brew water possible while still being limited by these > constraints.) > > I wouldn't have gone as far as to post this if my experiments did not > support the hypothesis. Unfortunately so far they do just that. > Believe me this is a case in which I'd enjoy being wrong! > > Anyone else out there notice the same? Any findings or experiences > supportive or contradictory to these findings? Any alternate > hypotheses for the observed behavior? Any of you input or feedback > would be most welcome. --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Brewtus" group. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/brewtus?hl=en -~----------~----~----~----~------~----~------~--~---
