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Selecting the right resistor

Discussion in 'Electronic Basics' started by Jesse, May 7, 2011.

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  1. Jesse

    Jesse Guest

    I have a DIY cooking project with which I'd appreciate some
    help.

    I'm trying to hold several gallons of water at 130F with
    minimal variation. A digital thermostat regulates the
    temperature (to within .3C), an aquarium air pump provides
    circulation, and a small immersion heater (Norpro 559) heats
    the water. The latter draws 300W on a standard 110VAC line.

    So far so good.

    The problem is that even though the heater powers on for less
    than a minute (out of even ten), when it turns off, the
    residual heat overshoots the mark, causing twice the range in
    temperature (.6C) necessary with the thermostat.

    So I'd like to prevent the heater from getting so hot,
    reducing the wattage by approximately half.

    I'm pretty sure that wiring in a resistor into the hot lead to
    the element would do it, but I don't know the specifics.
    Resistors are available in a huge array of OHM values and
    watts and what is utterly bewildering to someone like me is
    probably mindlessly simple to many of the experts who frequent
    this group.

    Any suggestions?

    Jesse
     
  2. Rich Webb

    Rich Webb Guest

    Where is the sense point for the digital controller? Perhaps moving it
    to somewhere closer to the header might reduce the overshoot. AIUI, some
    home thermostats have a small local heat source (e.g., a hot resistor)
    near to their temperature sensor so that the sensor gets hot faster than
    the room air, so it turns off the heat just a little sooner and lets the
    residual heat "coast" to the final room temperature value.

    Ultimately, you'll probably need a more sophisticated control setup,
    with, say, a triac controlling the heater so that you're not running a
    bang-bang control (full on - full off) but one that can taper off as it
    gets closer to the set-point, or turn on just a little if just a little
    heat is needed.
     
  3. Ecnerwal

    Ecnerwal Guest

    Zeroth question - other than you fussing about it, is this really all
    that big of a deal? Few cooking or culturing processes care all that
    much about a lousy degree, F or C.
    Get another heater just like the first one. Run the two in series.
    That's exactly the size and power handling you'll need for a resistor,
    and it's already packaged/built. Each heater will only deliver 75 watts
    at half voltage, but the two together will deliver 150. If you find that
    150 is still too much (and it probably is if you're at 10% duty cycle
    with 300W), move one heater into a separate tank of water. Or just get a
    smaller heater to begin with - seems like 50W would be about right (you
    have 10% - one minute out of 10 - with 300W - so 30W would do, and 50 W
    gives you a bit of reserve for variations, along with a heater that will
    be on more than half the time, rather than 10% of the time.)

    If you really think you need super precision, a microcontroller and PWM
    control are the way to go. But it could be that you really don't need so
    much precision. I don't know what you're trying to culture at that
    temperature, but things like yogurt cultures really couldn't care less
    about a few degrees variation (and happily work down at 105-110F.)
     
  4. BluntChisel

    BluntChisel Guest

    I'm not an electronics expert, but I'm interested in what other more
    knowledgable people in the group think of the following suggestion:

    - If you want to reduce the power by half, would a series diode be a more
    efficient way of doing it? A series resistor would have to dissipate
    150watts, which is more than most can handle.
     
  5. Jamie

    Jamie Guest

    You move the sensor closer to the heater elements or get a digital
    heat controller that has a PID controller in it.

    With a PID ("Proportion-Integral-Derivative"), you can use the "D"
    parameter to slow down the heating cycle when it starts to approach
    the SP (Set point).

    Dropping the heating wattage may not be such a good idea, it's better
    to have some reserve.

    Of course, if you were using a RTD or simple PTC/NPC, one could whip
    up a basic voltage comparatar with some Lead (Derivative) in it to
    throadle it back when it gets close to the set point.

    Jamie
     
  6. Sjouke Burry

    Sjouke Burry Guest

    Making the physical bulk of the heater as small as possible,
    for a given power , will reduce overshoot.
    A long,thin, well insulated heater wire looses the remaining heat
    very quickly, and causes less overshoot.
    Increased turbulence will distribute heat faster to all parts, and
    will let your sensing circuit react faster.
    Last, by using a dimmer to reduce power in reverse proportion
    to the present temperature error, a balance may be achieved, and the
    power never goes completely off, eliminating overshoot.
     
  7. Rich Grise

    Rich Grise Guest

    Lamp dimmer?

    Good Luck!
    Rich
     
  8. Phil Allison

    Phil Allison Guest

    "BluntChisel"
    ** Absolutely.

    Long as the immersion heater has no wires exposed to the water - all is
    fine.

    Yes, there is a DC component created in the AC supply, but too small to
    matter.

    My 1500 watt " Black and Decker " hot air gun uses a single diode for the
    " Lo" setting.


    ..... Phil
     
  9. amdx

    amdx Guest

    Not sure I like the idea of pumping 72* F air bubbles through a system I'm
    trying to keep at constant 130*F.
    There is a better way to mix your water.
    Mikek
     
  10. Rich Grise

    Rich Grise Guest

    I wonder what he cooks at 130°F ± 0.3°F! =:-O

    Cheers!
    Rich
     
  11. amdx

    amdx Guest

    I don't care to have any drug research done on my computer.
    What's he cooking Rich?
    Mikek (-:
    PS. Guess I could look up acetone and alcohol boiling points.
    Hmm.... Acetone 133*F
    Ok, what dissolves in acetone?
     
  12. Jesse

    Jesse Guest

    <snip>

    Many thanks to all who troubled to share their thoughts. I
    thought I would be lucky to get even a single reply, but in
    only a few days there have been fifteen from almost as many
    individuals.

    Rather than reply to each individual message, I'll try to
    clarify matters with a single response. I hope everyone who
    replied will see this contribution to the thread.

    First, to respond to Ecnerwal
    And Rich Grise
    Over the past several years, within the general realm of
    "molecular gastronomy" exists that of "sous vide" (French for
    "under pressure") cookery.

    Misleading nomenclature to the contrary, what this is really
    all about is instead of cooking, for example, a beef roast at
    350 degrees and removing it when it reaches the desired
    internal temperature, it's wrapped and placed in a water bath
    of the desired temperature (130 for example) to begin with.
    It's then cooked not just for hours, but sometimes for days.

    Okay, what's the point?

    There are several advantages (google to acquaint yourself with
    them) but among the most important is that, as in pit BBQ,
    this "low and slow" cooking has a tenderizing effect, offering
    the chance to turn the lead of cheap chuck into the gold of
    rib eye and having it STILL come out medium rare.

    Being a somewhat new approach to cooking, however, it simply
    isn't known what effect temperature fluctuation has on the
    process. The general consensus is that less is better. I knew
    when I bought the thermostat that it only allowed a .3C
    variance and was content with that.

    Many have used a PID controller, as whit3rd noted, but I
    thought that even though it allowed much greater temperature
    stability, its complexity was beyond my capacity to properly
    configure.

    When I discovered that my STC-1000 thermostat exhibited a
    small but significant overshoot, I thought it was probably
    acceptable but I wondered if there was an easy way to reduce
    it.

    One obvious approach was to keep the heating element from
    getting so hot. I now realize that a simple resistor simply
    won't work, it would have to be massive to reduce the current
    by the required amount.

    I'd like to use a less massive heating element, as Sjouke
    Burry pointed out, but finding something ready made that can
    be immersed in water, isn't so easy. A home aquarium heater
    can be hacked to remove its thermostat, but this isn't a
    project I'm comfortable doing.

    Rich Grise saw a good solution
    It looks like $5 spent at Home Depot should do the trick.

    Mikek had another insight...
    And John Fields...
    To mix the water.

    Originally, I tried placing the stockpot on a hot plate
    (connected to the thermostat) without the bubbler, hoping that
    natural convection would do the trick, but the overshoot was
    pretty bad. Adding the bubbler helped, but using the immersion
    heater and bubbler was the best solution so far.

    Cooling the water by bubbling room temperature air through it,
    I knew, wasn't such a good idea, but the only economical
    alternative I could think of was something like a submersible
    tabletop fountain pump, like a Sunterra. The question was
    whether it would tolerate such hot water. I emailed the
    company and inquired, but received no response.

    Rather than delay any longer and worried about mixing
    electrical current and water, I thought the air pump, despite
    its disadvantages, was probably the better option.

    Perhaps some who have followed this thread may have a thought
    on this topic. Would one of these tiny submersible pumps
    function at 130F temperature? For very long? Would it simply
    fail and need to be replaced or result in some disaster?

    Anyway, for less than $50 - compared to $1400 for a Fisher
    laboratory circulator or even $400 for a retail Sous Vide
    Supreme - I'm cooking sous vide.

    And very happily too. Like pit BBQ, of which I am also a fan,
    my few attempts so far have revealed that it can create
    genuine culinary magic.

    Thanks again to all for so generously sharing their knowledge.

    Jesse
     
  13. Jesse

    Jesse Guest

    oups.com:

    Thanks for the reply.

    Bacterial growth is of great concern, but has been studied
    extensively - google Doug Baldwin, for example, for more info.
    From all available info, both theoretical and practical, the
    130 mark is accepted as safe.

    As for the concern over temperature fluctuation, you may well
    be right. I wouldn't call my concern "obsessing" and I'm
    content with what I already have, but if was easy to keep the
    range a little smaller, and it seems that it is with a dimmer,
    I'm willing to polish the setup to make it a little better.

    Lastly, please realize that heat transfer in liquids is MUCH
    greater than that in air, some 23 times as much, as I
    understand.

    Putting your hand in a 350 oven for ten seconds is no big
    deal. Doing so in the same temperature oil is rather
    different.

    Jesse
     
  14. Jesse

    Jesse Guest

    oups.com:
    Getting to the right temperature faster goes, at least in
    part, to the health question you raised initially.

    The idea is that you want to go from refrigerator temperature
    to "cooking" temperature (I agree, 130 is cooking?) as quickly
    as practical.

    Why?

    To minimize time at the temperatures at which problem
    organisms flourish.

    Which is why most limit the size of cuts to about 3" thick to
    make sure that internal temperature is reached soon enough to
    be safe.

    I think that you're right though, once it's at 130, a standard
    oven might work too. Although, since we've already troubled to
    build the water bath, it's probably just as easy to just use
    it and keep things both simple and within a narrower
    temperature range common with standard ovens.

    Jesse
     
  15. Jesse

    Jesse Guest

    I really don't want to debate the merits of this method, but
    was only seeking information on how to economically implement
    it.

    Since Phil already replied, I'll make a single exception and
    point out that food is not boiled, it's first packaged in a
    water tight pouch (eg FoodSaver) and then gently poached.
    Rather than searing before hand (to achieve the Maillard
    reaction) it is usually seared afterward.

    If you're interested in learning whether this cooking
    technique is for you, this newsgroup probably isn't the best
    forum for doing so. There's lots of information on the 'net
    about it, by those who have studied it at length.

    I'm not here to advocate anything one way or another. In fact,
    I too was skeptical at first, dismissing sous vide as yet
    another, soon to pass, fad. I decided to build this
    experimental DIY rig to settle the issue one way or the other
    and, to my surprise, am glad that I did.

    Jesse
     
  16. amdx

    amdx Guest

    Very interesting Jesse,
    Do you worry about some kind of bacteria growing at 130F? And how
    long do you have to let the meat cook? I was the lunch cook at a
    fairly nice restuarant for about a year. The first thing I did when I
    came in the morning (~5:30 AM) was to start the prime rib for the
    evening. Started at ~220F for a few hours and then dialed down to
    170F IIRC.

    I think you are obsessing excessively about a few degrees, of
    temperature fluctuation.

    Bon appetit,
    George H.

    I'd like to add a new wrinkle, "meat glue" butchers have started gluing
    small pieces of meat together to make full size pieces and it is
    difficult to tell.
    Besides possible problems with the chemical, any bacteria that was
    on the outside is now on the inside. If you cook it rare you won't kill
    the bacteria.
    Jesse's method may kill the bacteria, but usually the recommended temp is
    160*,
    his long cooking time at 130* may kill bacteria.
    Meat Glue video.

    My short search finds no ban in the US.
    Mikek
     
  17. Jesse

    Jesse Guest

    Thanks for the response, but please understand that I am no
    expert on this subject and it is hardly "my" method. What I
    know about it, I've learned from the 'net, which you can do at
    least as well as I have.

    Perhaps chief among my doubts about this approach was that of
    safety. In that, as both you and George have pointed out (as
    well as many before us) I was hardly alone.

    Still, the 130 threshold seems to be the magic number, one of
    the reasons why hitting it and holding it is so important. If
    it rises too high, you loose the medium-rare target you're
    trying to hit. Too low and you offer pathogens an increasingly
    hospitable habitat.

    Interestingly, this setpoint applies exclusively to beef. In
    cooking pork or poultry, little concern over the result being
    medium-rare is present, so these can and often are cooked at a
    higher temperature.

    What is actually of concern here, I think, is really just
    pasteurization. If you've ever done any camping, or maybe just
    watched Survivor, you probably know that rendering water
    potable is not just a function of temperature, but of time as
    well.

    Thus eggs can be made safe by heating them until the yolks are
    set (~160°F). But it's thought that they can also be cooked in
    the shell at 135°F in a sous vide water bath for at least 1
    hour and 15 minutes to achieve parallel safety.

    For a general introduction to sous vide cookery, please see...
    www.douglasbaldwin.com/sous-vide.html
    Of particular interest is the section on Safety (Part 1
    Section 1).

    For some background on the approach, please see Amanda
    Hesser's piece, Under Pressure,
    www.nytimes.com/2005/08/14/magazine/14CRYOVAC.html?_r=1
    &pagewanted=1&ei=5090&en=3d5db17005368139&ex=1281672000
    &partner=rssuserland&emc=rss

    For lots more information on sous vide, please search
    egullet.com.

    I'm afraid this is the best I can do in explaining the method.
    It was precisely because I wanted to avoid becoming the go to
    guy to learn all about this lunatic cooking approach was the
    reason why I left out the specific nature of my "DIY cooking
    project". It was only because some were growing uncomfortable
    with just what is was that I was, uh, "cooking" that I
    reluctantly mentioned it all.

    Again, there's tons of stuff on the 'net on all this. If
    anyone in this group wants to know more, google is your best
    guide.

    Jesse

    +

    All truth passes through three stages... &c &c &c.
    ~ Arthur Schopenhauer
     
  18. amdx

    amdx Guest

    "Jesse" wrote in message


    Thanks for the response, but please understand that I am no
    expert on this subject and it is hardly "my" method. What I
    know about it, I've learned from the 'net, which you can do at
    least as well as I have.

    Perhaps chief among my doubts about this approach was that of
    safety. In that, as both you and George have pointed out (as
    well as many before us) I was hardly alone.

    Still, the 130 threshold seems to be the magic number, one of
    the reasons why hitting it and holding it is so important. If
    it rises too high, you loose the medium-rare target you're
    trying to hit. Too low and you offer pathogens an increasingly
    hospitable habitat.

    Interestingly, this setpoint applies exclusively to beef. In
    cooking pork or poultry, little concern over the result being
    medium-rare is present, so these can and often are cooked at a
    higher temperature.

    What is actually of concern here, I think, is really just
    pasteurization. If you've ever done any camping, or maybe just
    watched Survivor, you probably know that rendering water
    potable is not just a function of temperature, but of time as
    well.

    Thus eggs can be made safe by heating them until the yolks are
    set (~160°F). But it's thought that they can also be cooked in
    the shell at 135°F in a sous vide water bath for at least 1
    hour and 15 minutes to achieve parallel safety.

    For a general introduction to sous vide cookery, please see...
    www.douglasbaldwin.com/sous-vide.html
    Of particular interest is the section on Safety (Part 1
    Section 1).

    For some background on the approach, please see Amanda
    Hesser's piece, Under Pressure,
    www.nytimes.com/2005/08/14/magazine/14CRYOVAC.html?_r=1
    &pagewanted=1&ei=5090&en=3d5db17005368139&ex=1281672000
    &partner=rssuserland&emc=rss

    For lots more information on sous vide, please search
    egullet.com.

    I'm afraid this is the best I can do in explaining the method.
    It was precisely because I wanted to avoid becoming the go to
    guy to learn all about this lunatic cooking approach was the
    reason why I left out the specific nature of my "DIY cooking
    project". It was only because some were growing uncomfortable
    with just what is was that I was, uh, "cooking" that I
    reluctantly mentioned it all.

    Again, there's tons of stuff on the 'net on all this. If
    anyone in this group wants to know more, google is your best
    guide.

    Jesse,
    But we all found it interesting, we wanted to hear about it.
    I just added the "meat glue" info because it was new to me.
    Mikek
     
  19. Rich Grise

    Rich Grise Guest

    How hot is the air that comes out of the car heater by your foot? I
    once heard a UL about some guy who was driving and felt drowsy, so
    he pulled over, but left the engine running, with the windows
    open a bit so he wouldn't gas himself, and the heater going so he
    wouldn't freeze.

    Anyway, the UL was that the guy slept for several hours and when
    he woke up, he found that his foot had been cooked. It sounds a
    little implausible; I wonder if that UL debunker has anything on
    that.

    What's the URL of the UL debunker site?

    Thanks,
    Rich
     
  20. Rich Grise

    Rich Grise Guest

    All the cooking shows that slow-cook beef say to first brown it all
    over in a searing-hot skillet first, _then_ slow-cook it, and also
    use the liquid from the sear in the stew or pot-roast recipe.

    BTW, it's nice to see such a long on-topic thread. ;-D

    Cheers!
    Rich
     
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