HOMEBREW Digest #5395 Wed 13 August 2008

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  Re: Dumb HERMS Question ("Craig S. Cottingham")
  Re: Dumb HERMS Question ("Jeff Dieterle")
  RE: pumpkin ale ("Josh Knarr")
  PID Controller ("A.J deLange")
  HERMS: Liquid Return ("Dave Larsen")
  PID - Heat Output ("A.J deLange")

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---------------------------------------------------------------------- Date: Tue, 12 Aug 2008 22:53:22 -0500 From: "Craig S. Cottingham" <craig.cottingham at gmail.com> Subject: Re: Dumb HERMS Question On Aug 12, 2008, at 12:26, "Dave Larsen" <hunahpu at gmail.com> wrote: > What exactly does a PID do? I know it has to do with temperature > control, and has relays to turn things on and off. I assume that that > is to turn on and off the heat. But, doesn't say a Ranco temperature > controller do the same thing? Why would I need a PID? The short answer: A PID is, in this context, a fancy temperature controller. I don't know if the Ranco controller is a PID; assuming it isn't, then a PID does the exact same thing the Ranco does, only better. The long answer: "PID" stands for proportional/integral/derivative, which are the three inputs to the transfer function that turns the input (the signal from the temperature probe) into the output (the power to the heating coil). The proportional stage scales the output proportional to the difference between the measured value and the desired value. If the HLT is cold, and it's supposed to be hot, then the output will be large. If the temperature difference is small, then the output will also be small. Note that the old bimetallic-coil-with-a-mercury-bulb- switch thermostat most of us grew up with in our houses was a degenerate case of a proportional controller; there were only two output values -- on or off. The integral stage sums the difference between the measured value and the desired value over time, and produces an output proportional to the accumulated error. The longer the measured value is not equal to the desired value, the larger the output gets. This is important because proportional-only controllers suffer from a condition called offset; they eventually reach a point where the output is small enough that you're putting in the same amount of heat as is being lost through the walls of the HLT, the tubing, etc., and you *never* reach the desired temperature. Add an integral stage, and you can overcome the offset. (Integral controllers have their own problem, called windup. Imagine that you're heating water at ambient ground temperature, say 55 degF, to 155 degF. The difference between the measured value and the desired value is going to be big -- on the order of 50 to 100 degF -- for a long time, and the integral controller will build up a very large error value. This can lead to overshoot -- pouring enough heat into the HLT to bring the water to a boil.) The derivative stage tries to take into account how rapidly the measured value is changing, and damp out wild swings in the output value. Unfortunately, they tend to be sensitive to noise in the measured value, and try to damp out changes that aren't really happening. There's a nice article at <http://en.wikipedia.org/wiki/ PID_controller>. It doesn't seem too technical to me, but then again I understand this stuff somewhat already, so I may not be an good judge. The bottom line is, you don't *need* a PID. However, if you're the kind of person who is obsessed with precision and accuracy, you're probably going to want one. - -- Craig S. Cottingham BJCP Certified judge from Olathe, KS ([621, 251.1deg] Apparent Rennerian) craig.cottingham at gmail.com Return to table of contents
Date: Wed, 13 Aug 2008 08:27:52 -0500 From: "Jeff Dieterle" <djdieterle at hughes.net> Subject: Re: Dumb HERMS Question Date: Tue, 12 Aug 2008 12:26:46 -0700 From: "Dave Larsen" <hunahpu at gmail.com> What exactly does a PID do? I know it has to do with temperature control, and has relays to turn things on and off. I assume that that is to turn on and off the heat. But, doesn't say a Ranco temperature controller do the same thing? Why would I need a PID? Dave Tucson, AZ Dave, PID is the control algorithm used to match a match an output to a process variable, Proportional, Integral and Derivative are the 3 types of gains used. There are several control schemes this is simply one of them. For a pid to function the output must be variable, not to practical in most brewing rigs. However it is possible with pwm control of an electric element or a modulating burner. Consequently most pids used in brew rigs are doing an on/off type of control which is exactly what a Ranco does. So unless you're going to get fancy with buner or element control or get a pid style controller at a good price a Ranco will do the same thing at a lower cost. E-mail message checked by Spyware Doctor ( Database version: 5.10460 http://www.pctools.com/en/spyware-doctor-antivirus/ Return to table of contents
Date: Wed, 13 Aug 2008 10:22:26 -0400 From: "Josh Knarr" <josh.knarr at gmail.com> Subject: RE: pumpkin ale RE: Roasting Pumpking - Yes, I was planning on simply spreading it out on a cookie sheet. Then I don't have to deal with the skin of it contributing off flavors, etc. It seemed correct to me to bake it. One of the janitors also pointed me to the archives which suggested mashing it. You can also check the google archives but I figured I would start a fresh topic. http://www.google.com/search?q=pumpkin&domains=hbd.org&sitesearch=hbd.org But basically I was thinking that so long as the pumpkin was spread to pie-like thickness, it would do well for itself in the oven. If the edges got burnt, more power to it since we're going for roasty. RE: Raisins and Vanilla - I could see getting rid of vanilla, the extract stuff always tastes a bit funky to me. Maybe it's just ACME's fault. Raisins I think will add "dark sugary" flavors, probably on par with molasses or maple syrup. The idea here is to hopefully get some unfermentable flavor out of them. Return to table of contents
Date: Wed, 13 Aug 2008 11:54:55 -0400 From: "A.J deLange" <ajdel at cox.net> Subject: PID Controller "PID" stands for "Proportional, Integral, Differential (or derivative)" and means that the device sends a demand for heat (in a heating application) which is in part proportional to the difference between the process variable and the set point, in part a linear function of the integral of that difference and in part a linear function of the derivative of that difference with time. The relative amounts of the three components are controlled through "tuning" of the controller - a process which absolutely must be carried out in order for a PID controller to operate properly with it's given load and heat source. In proportional only control if the temperature of the load (the process variable - PV) is less than the set point (SP) there will be some heat output and the temperature of the load will rise until k1*(SP - PV), with k2 the gain or "band", results in a heat output which just equals the losses of the system and the PV will be regulated at whatever value meets this condition. Note that this is not where SP = PV but at some other value so regulation occurs at an offset. The loop is 0 order. If we integrate (SP - PV) it will grow indefinitely but if we scale it by another constant, k2, and subtract k2*(integral(SP-PV)) from the output the new equilibrium is reached (we have said nothing about over and undershoots at this point) is reached when (SP - PV) = 0 (i.e. SP = PV) and k2*(integral(SP-PV)) = the amount of heat required to just replace the losses. Not that SP = PV does not mean integral(SP - PV) = 0 unless SP has always been equal to PV. With this second component, the system regulates to SP = PV and will track changes in SP or load. If one wants a quick response to a change in the load (e.g. ice cubes thrown into mash tun) or to a change in the set point (step mash) the third component which adds heat equal to k3*d(SP - PV)/dt i.e. the rate of change of the difference between the set point and process variable is added. This boosts heat output at times when the temperature of the load is dropping rapidly The art of tuning a PID conrtoller is the art of getting it to respond to a step change in SP as quickly as possible without overshooting more than a tolerable amount (1 degree?). The setting of k1, k2 and k3 depends on the heat source, how linear its response is to the controller's output, how quickly it responds to the controller's output, the thermal mass of the load (how long it takes it to respond to heat input), heat losses from the load etc. It is problematical in brewing that the tuning for water in a half full HLT is not the same as the tuning a full HLT is not the same tuning for a 5 gallon mash which is not the same tuning for a 10 gallon mash. I thus find PID control useful only with close supervision and manual over-ride (or, to be strictly accurate wrt my brewery, pedal override i.e. I use a foot switch for this function). PID is fine for the HLT but for the mash tun I use the controller more as a temperature recording device than I do as a controller. A.J. Return to table of contents
Date: Wed, 13 Aug 2008 09:03:12 -0700 From: "Dave Larsen" <hunahpu at gmail.com> Subject: HERMS: Liquid Return I've been doing some research about how to return the liquid from the HLT to the mash tun. Some people use a manifold. Some people use something similar to a sparge arm. Whatever you use, it seems to me that you do not want to splash the liquid back into the mash tun, or risk HSA. The way that I liked the best was simply to lay the return hose on top of the mash. I guess there is the risk of channeling that way. What way is best? How do you get the liquid back to the mash tun without HSA or channeling? Dave Tucson, AZ http://hunahpu.blogspot.com/ Return to table of contents
Date: Wed, 13 Aug 2008 12:07:57 -0400 From: "A.J deLange" <ajdel at cox.net> Subject: PID - Heat Output I forgot to put anything in my last post about how the controller actually controls heat. Most of them put out a variety of electronic signals which are used to control heat (or cold or anything else which can be controlled electrically) in various ways. The device asks for heat as a percetage of maxium i.e between 0 and 100 percent. In proportional control it closes a circuit for x% of the time where x% is the commanded heat level. For example, in my brewery I use steam. If the controller wants 50% heat it closes a relay 50% of the time or 15 seconds as I have the cycle period (usually adjustable) set to 30 seconds. The relay is in a circuit with a 24V control transformer and a 24 V steam valve (the mash acts like a great big thermal capacitor and smooths out the resulting heat pulses). Depending on the controller and it's configuration it may have direct relay output or a logic level signal designed to control a solid state (SSR) or other relay. For direct control an analogue signal is set x% of the way between minimum and maxium i.e. between 0 and 5 volts for a voltage control or between 4 and 20 milliamperes for a 4-20 loop. Various devices such as SCR's, gas valves, steam valves and so on are capable of responding to these signals. A.J. Return to table of contents
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