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Temperature Variations

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    Temperature Variations

    I am still loving my second owner KBQ, but wondering if the temperature tolerances can be tightened. I have a new digital thermometer that is showing the wide swings, that are fairly rapid up and down.

    Ambient air temp is now 63 degrees, started the cook at 5am and temp was 37 degrees and the pattern was the same.

    Would it be slower with a welding blanket wrapped around it? Do I need a new control box? Any Other suggestions?
    Attached Files

    #2
    Make sure the spring sensor (under one of the impeller blades on the underside of the comtrol unit) is clean. The dirtier it gets the larger the temperature range will be. KBQ recommends cleaning it every 100 hours or as needed.

    Comment


      #3
      Make sure the fire box and control box are correctly mounted to the cook box

      Comment


        #4
        Firebox and control box are both on correctly, spring was steam cleaned two cooks ago. I feel like I have had these swings the entire time I have owned it, but able to document it better with the new probe/app. Could the spring have had damage prior to me owning it?

        Comment


          #5
          I... don't believe your thermometer.

          Why? Because looking at the time scale it looks like temps are going from 200 to 270 and back every few minutes and, unless you're opening and closing the doors all the time, I can't see any smoker doing that. It's just not going to happen with a hot box without leaks. Even if you stopped feeding it wood, it's not going to drop 80 degrees in 5-10 minutes.
          Last edited by rickgregory; October 16, 2021, 03:32 PM.

          Comment


            #6
            Originally posted by rickgregory View Post
            I... don't believe your thermometer.

            Why? Because looking at the time scale it looks like temps are going from 200 to 270 and back every few minutes and, unless you're opening and closing the doors all the time, I can't see any smoker doing that. It's just not going to happen with a hot box without leaks. Even if you stopped feeding it wood, it's not going to drop 80 degrees in 5-10 minutes.
            I thought the same thing, but it is spot on for the other probes. Also, the fan kicks on and off at those peaks. Probe was set off of the grate about 3 inches away from the pork butt in the bottom third of the smoker. Water pan underneath. It almost seemed like it was drawing in cold air and dropping the temp in the cooking chamber rather than recycling it.


            Thanks for the suggestion about contacting Bill. I will give that a go. I appreciate all the thoughtful suggestions!

            Comment


            • Dr. Pepper
              Dr. Pepper commented
              Editing a comment
              Keep us up to date. Good luck

            #7
            I saw large variations yesterday on my KBQ in cool, windy conditions. Please post any additional info you find. TIA.

            Comment


              #8
              In real life, I do a lot of work with thermocouples and temperature changes. Swings of that magnitude and speed (in a controlled process) just don't happen unless there's something wrong.

              I always suspect the temp measurement, (as you and rickgregory point out). My first test is always to validate the reading. Get a second measurement in the same place with a tool you trust. I can't say how many times we've chased equipment problems, only to realize the thermocouple wire was broken, it was reading the wrong location, or had a valve open/closed that wasn't supposed to be. It sounds like you already did that, and you think the temp indication is correct.

              Fire won't change temp that fast, so it's almost got to be air temp swings connected with your fan, if they match so closely to fan coming on/off.
              ​Is there a damper loose? Something that swings when the fan comes on, and perhaps swings shut when it kicks off? A vent that should cause recirc, but missing or shifted? Leaky joints that suck cold air when the fan kicks on or made worse from windy conditions?

              Very curious what you find the problem to be - good luck.

              Comment


              • Caffeine88
                Caffeine88 commented
                Editing a comment
                Thanks for the link ComfortablyNumb. The two fan set up is pretty clear, and damn clever.

                Still the same questions though: do you trust the temp measurement? If so, there almost has to be a cold air opening that allows extra draw when the fan kicks on.

                It's possible the issue has been there a while, but became more noticeable when the ambient temps dropped out of summer weather. TheCattleman mentions 37F - 63F - could be enough.

                I don't know this one rig, but it looks superb

              • Caffeine88
                Caffeine88 commented
                Editing a comment
                Without a doubt ComfortablyNumb, your suggestion to call the guy who built it is the right one! 😁

              • Dr. Pepper
                Dr. Pepper commented
                Editing a comment
                But, whatever the outcome, we will need a full report!
                Also, ComfortablyNumb , in spite of his protestations, is a very smart and well read, hard working truck driver, farmer, orchardist, chicken growing, meat smoking hombre. He's just modest. You know, all cattle, no hat.

              #9
              Just to confirm what others have said, something is wrong with smoker or your thermometer. I bought a new KBQ a few weeks ago and have done both a pork butt and brisket. My temperature variation is only 30-35 degrees total.

              Comment


                #10
                Post pictures of your rig detailing where all the pieces connect. Ensure the bottom poppet is open. And there is no damage to the door and jam. And the manifold inside the cookbox has not been adjusted with the louvers closed.

                How long is the cycle where it draws gasses in to the cookbox, roughly?

                How long does it sit in between cycles?
                Last edited by lostclusters; October 17, 2021, 08:07 PM.

                Comment


                  #11
                  The observed dead-band depends on many factors, including the thermal mass and placement of the sensor. Comparisons make sense only if those are consistent, i.e., the Thermoworks RT-610B thermometer in the Controlbox port.

                  If you buy a tiny bead probe thermocouple and shove it into the upper mixing tube louver, you can measure a >500F swing as it cycles between recirculating flow of 250F (draft fan off) and a >750F blast right from the Firebox tailpiece (draft fan on). Such is the nature of ON/OFF control, which is suitable for an oven because the thermal mass of your butt is about 10,000X the thermal mass of your sensor. Embed a probe 1mm into your butt and you will see no variation through this ON/OFF cycle. FWIW, my high-end kitchen oven has a dead-band of 95F, measured in the center of volume with a nice probe.

                  Many years ago I built a fancy PID control for the C-60 that kept temps within a 2F band. The barbecue was no better. Why would it be?
                  Last edited by KBQ; October 19, 2021, 12:05 PM.

                  Comment


                    #12
                    In a somewhat related comment to this thread I took my KBQ over to a friends yesterday who recently purchased his first real smoker in the form of an Oklahoma Joe offset. He ran a brisket and I did a couple of pork butts. This was his 8th cook on it. The only true smoker I have owned is the KBQ. The intention of the visit was to help him along in all the basics of trimming, rub application, visual ques, temps, wrapping and doneness.

                    I was also intrigued as to fire management on an offset. The day started out calm and then became windy followed by rain and then back to calm with humidity. As much as I struggle at times to keep a good, thick coal bed in the KBQ, I learned an additional form of appreciation for that of the lack of need to worry that much about temperature control. The smoke quality we were getting was pretty good overall. Plenty of attempts at different wood sizes and stacking methods to try and keep things consistent. The temperature swings were were seeing were that of 50F which he considered good.

                    Overall, it was an enjoyable day. The reason I came his way in my CUV was that he could not have loaded his smoker into his truck nearly as easily.

                    Comment

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