Regulating a DBS for cold smoke: A simple mod with no PID!

[mjdeez]

Ok, so my long term plan is to either make or buy a PID, but before I make one, I needed to know more about how my DBS worked (thermally, electrically), so I characterized the heck out of it.  I also wondered if I could trick my DBS SG into thinking the SG was sensing around 70F while setting it to some temperature in its settable range (above 120F).  For warmer days it isn't necessary, but I wanted a way to regulate the tower temp for cold smoking when it is cold outside.

As it turns out the Bradley digital smoker senses temperature with a thermistor type of temperature sensor; thanks to Brian from the head office for confirming this. The way it works is the resistance of this sensor changes as the temperature changes, but it is not linear (see the curve below). The SG has a 5V output with a 15k Ohms output resistance. Your short and thin 1 foot cable connects from the SG (with 5V and 15k R_out) to your tower (with your temp sensor).

Here is what the resistance of the sensor looks like vs temperature.  The red points are measured resistance of the thermistor (sensor) at various temperatures. I disconnected the temperature sense line between the SG and the tower for these measurements. Then I took just the SG (no tower at all), plugged different (known value) resistors into the sense line to mimic the temperature sensor in the tower, and there you have the red points.  The equation is just something excel came up with if you want to you use for an approximation.


So to get the tower to be at around 70F and the SG to "think" the tower is at least 120F (in settable range), there are two points of interest on here. 120F corresponds to about 50kOhms. 70F corresponds to 120 kOhms.  So I need to get the SG to physically sense 50kOhms and set it to regulate at 120F, and somehow change the total resistance so that 120kOhms of the sensor (sensing 70F) goes to 50k.  I'll skip the math, but suffice to say that an 85kOhm resistor in parallel with the sensor will do just this. Edit: NOTE -- this will not just subtract 50 deg F from any setpoint.  This setup is designed ONLY to work when programming the SG to 120F and expecting the tower to regulate at 70F.

First you need to tap into the sense line. I did it using some enercell products that were recommended by someone else on this forum. You'll need two 273-344 Type "M" tips and 1 cable 273-348.  These are available from RadioShack; they had them in stock at my local store. The polarity of these is very poorly marked but for this purpose it doesn't matter; you can't mess it up. The cable also has the advantage of being long enough for your cold smoke setup. You'll also need an 85k Ohm resistor, or a bunch of series / parallel combinations of resistors to get 85k. You can see my rats nest of resistors here. Wrap or solder your resistors to the two terminals as I have here (shown plugged into the cable in the third picture below). It doesn't matter which side gets plugged into the SG or tower; this 85k will effectively be put in parallel with the sensor. I chose to wrap the resistor terminals around the cable terminals (no solder) this way I can always just remove it. If you accidentally short these you'll hear a beeping from the SG.






Before I go on, here's a quick primer about on / off control, which is the DBS control mechanism: When the temperature rises and hits the set point, the SG will switch off the heating element. There will be some overshoot, as the heating element is still hot. The temperature will eventually fall, and when it hits the setpoint minus 10 degrees, the SG will switch the heating element back on. The 10 degrees is the hysteresis. Keep in mind that these 10 degrees are what the SG is sensing and displaying on the panel. If I have an 85k in parallel to the sensor, my math says its actually about 17 degrees of hysteresis, so the heat source will turn on at 53F.

So the other half of the problem is that depending on the outside temperature, using the stock 500W heating element may be more than overkill. I measured the temperature overshoot over 30 degrees, to almost 110F!  I decided to use a light bulb or heat lamp to cut the power down.  I only had 75W bulbs around the house, which was ok until it got down to around 30 overnight, then it was not powerful enough to keep the temperature high enough. I'll try a 150W heat lamp next time. The socket is a cheap $2 light socket screwed onto a couple blocks of wood (one for a back, one for stability) with 14 gauge romex wired up through the smoke vent.  I'm ordering some cheap connectors so this will all fit through the vent next time. I used wire nuts here and clipped them when I was done. Edit: Here is an extension cord you can use. It is the same type that is used between SG and tower for heating power, but has a male to bare wire connector. Not easy to find depending where you live. http://www.mcmaster.com/#extension-cords/=a1kma6, part number is 70355K22.

Here's are a couple of close-ups of it in action.





Thanks to rloethen for the mailbox idea. This was a lot easier than building a box for cold smoking.
http://forum.bradleysmoker.com/index.php?topic=16207.0

If this seems like a lot of work and doesn't buy you all that much, you're right.  For all the effort I put into this it would have been a lot easier to just buy an Auber. But I like the DIY aspect.

[squirtthecat]

Nice writeup and mod!

[TMB]

OK, I have to ask, since I'm a newbie at this.   Can't you just leave the smoke generator attached to the unit?  I mean if I'm looking at keeping the temp around 70 -80 degrees wouldn't my OBS stay around that temp with the vent open all the way and my unit being out in a cold garage?

Again, I'm new at this stuff just trying to get an understanding of why you used the set up you used. 

[mjdeez]

Nice writeup and mod!

Thanks!

Can't you just leave the smoke generator attached to the unit? I mean if I'm looking at keeping the temp around 70 -80 degrees wouldn't my OBS stay around that temp with the vent open all the way and my unit being out in a cold garage?

It may work but it is uncontrolled. If you have a fixed environment and you find that you can get those temperatures reliably then you're probably fine. For my setup I was doing the triple smoke, which is a 3 day process where you want to keep the meat at around 70F the whole time, but only smoke for 3 or so hours a day for those 3 days. With the SG hot plate off, I needed a different heat source. Since my setup is outside, my heating requirements will change based on weather conditions. A heat source such as the SG hot plate will only be able to heat the tower to outside temp + some delta.  If if the outside temp drops a lot, it may be too cold.

[mjdeez]

I almost forgot about this.  Here's some data that shows how long the Bradley takes to get up to temp. This is WITHOUT the 85k mod for cold smoke. It is dependent on a many things (ambient temp, wind, vent position, pre-heated thermal mass (hot foil-wrapped bricks or a cold hunk of meat), etc, but this will give you an idea.

I opened the door for 30 seconds at the 100 minute mark to see what the recovery time would be.  Temperatures for the SG panel (thermistor sensor) are shown in green and my DMM w/ thermocouple shown in blue. The sensor is attached to the back of the tower so it has high thermal mass and doesn't change fast (essentially filtered). The thermocouple was placed about an inch away from the sensor but measures air temperature.

[GusRobin]

OK, I have to ask, since I'm a newbie at this.   Can't you just leave the smoke generator attached to the unit?  I mean if I'm looking at keeping the temp around 70 -80 degrees wouldn't my OBS stay around that temp with the vent open all the way and my unit being out in a cold garage?

Again, I'm new at this stuff just trying to get an understanding of why you used the set up you used. 

Toomy, the basic answer is yes. Depending on how cold the ambient temp is, you may want to keep the door open a crack. I have also done it where the door was closed but I had a pan full of ice on the bottom rack. THe main thing is to monitor it and make adjustments if needed (add ice, open door, etc).

Or you can just build the cold smoke box out of wood and dryer duct. (I have an adapter if needed)

[TMB]

OK, I have to ask, since I'm a newbie at this.   Can't you just leave the smoke generator attached to the unit?  I mean if I'm looking at keeping the temp around 70 -80 degrees wouldn't my OBS stay around that temp with the vent open all the way and my unit being out in a cold garage?

Again, I'm new at this stuff just trying to get an understanding of why you used the set up you used. 

Toomy, the basic answer is yes. Depending on how cold the ambient temp is, you may want to keep the door open a crack. I have also done it where the door was closed but I had a pan full of ice on the bottom rack. THe main thing is to monitor it and make adjustments if needed (add ice, open door, etc).

Or you can just build the cold smoke box out of wood and dryer duct. (I have an adapter if needed)

I see where the cool box would help now. Gus I believe I can build a box but thanks for the offer , I'll work on it over the weekend I hope and will send pictures.  A lot of good info on this thread and forum about cold smoking I just need to find time to learn it all!   :D

[NePaSmoKer]

I dont know allot about resistance and kOhms and stuff so i opted for the easy way to cold smoke  :D