PID questions

[StirCrazy]

Ok for all you guys out there that play with PIDs I have a couple questions. I am looking for one that can accept the input from two different K probes.

I want one input to be used for measuring temp and switching on/off an element to maintain this temp. the other probe input is used to measure a different temp and then make changes to the first one.

so lets use a oven as an example.. I want the one probe to measue the oven temp and then maintain that temp. I want the other probe to measure the temp of the roast in the oven and then be able to set off an alarm when a preset is reached (would be nice if I could program more than one preset) and also the ability to change the oven temp set point if another condition is reached.

another nice feature would be able to control the first probe so that I could say hold at one temp for two hours then raise the temp 10 degrees every hour.

anyways if some one knows how I can do this for an afordable price let me know..

Thanks
Steve

[ScubaSteve]

Doing dual input control systems is a PITA. Actually building it isn't really an issue but tuning and making it stable is. The simplest set-up is have it set-up such that you are essentially using the temperature difference between the two probes but that only works for certain applications.

Can you give me a pretty good decription of what you are trying to accomplish? I need to know what is part of the system that you are trying to control (like size and number of tanks, where your heating elements are located, how you are moving the flow about, etc). I'll try me best to draw up the most stable configuration for you.

[banditpowdercoat]

YA, I don;t know of Any PID controller that has RTD inputs. But I do know Honewells UDC 5000 has a 4-20mA input. That can be used to set setpoint alarm points etc.

But, that said, Siemens LOGO Mini PLC now has PID capability, and you can add more than one RTD card so with that it may be possible.
Any other full blown PLC it will be capable, but your looking at alot of $$$. The LOGO's are only a couple hundred, and about $100/temp card. I have a couple of them on my 150g, and use them at work alot too. Great little relay box for sure.


What is it your attempting?

[StirCrazy]

Originally Posted by ScubaSteve

Doing dual input control systems is a PITA. Actually building it isn't really an issue but tuning and making it stable is. The simplest set-up is have it set-up such that you are essentially using the temperature difference between the two probes but that only works for certain applications.

Can you give me a pretty good decription of what you are trying to accomplish? I need to know what is part of the system that you are trying to control (like size and number of tanks, where your heating elements are located, how you are moving the flow about, etc). I'll try me best to draw up the most stable configuration for you.

ok so what I am doing is an electric smoker. so I need a PID to maintain a setpoing temp. I found some with rise also so I can program it to sit at 120 degrees for 2 hours then 130 for an hour then 140 for an hour so going up 10 degrees every hour till it hits 180 then hold. this would only be one type of smoke, the rest would be a set temp till I turn it off.

the other probe I was hoping I could stick it into a hunk of meat and have it display the meat temp and then set off an alarm at a specific temp and then when another temp is reached I was hoping I could have it adjust the main temp. so for an example when the meat reaches 180, drop the cabnet temp from 225 to 180 for a holding tempature.

I can do everything buy buying to cheep PID's except for the hold temp thing which isn't a hugh deal would just be nice, but I was hoping to be able to get away with just one if I can.

this is the one I was thinking of using for the main control if I do a two PID setup.
http://www.auberins.com/index.php?ma...&products_id=5

Steve

[DiverDude]

Ok....as someone who loves to smoke a brisket (or whatever) on the weekend, you've got my curiosity.

Are you making your own smoker or modifying an existing one ?

I've seen systems like this for charcoal smokers -but they controla fan which blows more or less air into the combustion chamber to control heat.

Also, PID my be overkill for something this simple. Usually the way these things go is that you set your chamber temp and then monitor internal meat temp until done. Then you either turn off the heat and/or remove the meat (or fish !) mmmmm....mesquite smoked Regal Tang !

Kiddding !!! Juuuust kindding 'bout the Tang !

[banditpowdercoat]

Those controllers will work nicely and you most likely won't find what your looking for in 1 controller. If you do, expect it to cost thousands!

I have a few of those 1/16th DIN controlers in use too, on my powder coat oven's etc. They work great.

One thing you could do is run the main temp, through the "meat temp" controllers second alarm output. In a N/C setup, so the main controller control's heat, but if the meat get's to the second alarm level, the alarm controller interrupts the main heat control circuit

[StirCrazy]

Originally Posted by DiverDude

Ok....as someone who loves to smoke a brisket (or whatever) on the weekend, you've got my curiosity.

Are you making your own smoker or modifying an existing one ?

I've seen systems like this for charcoal smokers -but they controla fan which blows more or less air into the combustion chamber to control heat.

Also, PID my be overkill for something this simple. Usually the way these things go is that you set your chamber temp and then monitor internal meat temp until done. Then you either turn off the heat and/or remove the meat (or fish !) mmmmm....mesquite smoked Regal Tang !

Kiddding !!! Juuuust kindding 'bout the Tang !

building my own, I tried to buy a MES at canuck rubber but they sold out befor I got there and the reg price on the one I want is about 450 so I said screw it i'll build my own. I can get the sheet metal for around 150.00 and that is with them bending up the two boxes for me so all I have to do is weld them up and put isulation between them. be about 20 bucks for an element, 75 bucks for the one PID then some racks and paint. figure I'll be done for about 250 to 300, but instead of getting a 18x16x40 smoker, I'll get one that is 24x24x48 with a lot more function and automation. I was looking at it and I am thinking of making an external chip burner with a 120watt element in it or something like that.. I can hook that to the one alarm output of the PID and the main burner to the 10 amp output so it directly controls the main burner to maintain the heat in the smoker, but I can put a varable in so that if the temp gets over 130 then energize alarm 1 and that will start the smoke generator.

Bandit, you can't realy control the smoker temp through the meat as the idea of smoking is long and low heat.. so you want the smoker to be at 235 degrees for regular smoking, and the meat will slowly cook, in the case of a brisket it could take 12 to 16 hours. if you use a difarential type thing then it will crank the heat to start and slowly lower it which will not give you good BBQ. what I was thinking is a brisket is done when it is 195-205 degrees (varries with different people) so I thought it would be handy to have a hold feature so when the one probe read a meat temp of 205 it would tell the controler to change the smoker temp from 235 to say 175 degrees as a holding temp to prevent over cooking. but that is going to be to much work.. I found one that can do it but I am looking at 600 bucks instead of just over 100 for a couple single input ones.

the only time the cooking process changes is if I want to cold smoke, then I want to run the smoke generator but keep my cabnet temp under 90 degrees, or when making Keilbassa. for this you want to hold at 120 degrees for 2 hours to dry it out, then jump 10 degrees every hour and apply heavy smoke till you hit 180 degrees then hold at that temp till internal temp of the meat is what ever. so I could use the ramp functions to do 120 for two hours then each step would be 10 degrees higher for 1 hour. and again I could set the alarm 1 relay to 125 degrees so it starts the smoke after the first step is complete.

I guess the simplest way is just to get the one PID for 77 bucks as it has a 20amp relay built in and the rise function and just use a wireless theromomitor to monitor the meat as I already have one of thoes..

Steve

[StirCrazy]

Dan I just re-read your sugestion and I think I misunderstood, do you mean to use the meat controler PID alarm output to energize the smoker temp controler? so that as long as the meat is under the set value it will allow the main controler to do its thing, but ones the meat reaches the set temp it cuts the power off to the main controler? the only issue with that would be for the first 2 or 3 hours you don't put the probe into the briskit as you want the outside to be cooked a bit befor you insert it to eliminate the possibility of draging bacteria into the inside of the meat where it is going to be at a lower temp for longer.. mind you that is just a set point change.. start it out at 250, then when I stab it readjust it to the temp I want the meat to be.. not a big deal..

so more or less using the meat temp as a limit switch for the cabnet temp.

Steve

[banditpowdercoat]

Originally Posted by StirCrazy

Dan I just re-read your sugestion and I think I misunderstood, do you mean to use the meat controler PID alarm output to energize the smoker temp controler? so that as long as the meat is under the set value it will allow the main controler to do its thing, but ones the meat reaches the set temp it cuts the power off to the main controler? the only issue with that would be for the first 2 or 3 hours you don't put the probe into the briskit as you want the outside to be cooked a bit befor you insert it to eliminate the possibility of draging bacteria into the inside of the meat where it is going to be at a lower temp for longer.. mind you that is just a set point change.. start it out at 250, then when I stab it readjust it to the temp I want the meat to be.. not a big deal..

so more or less using the meat temp as a limit switch for the cabnet temp.

Steve

Yes, thats what I was meaning. I'm not familiar with smoking meat but ya. Or, what if the meat temp probe stays out of smoker while you cook for the first couple hrs, then it will remain cooler, then put in meat?

Also, I know the PID says it has a 10A capable contacts on it, but do yourself a favor and wire a relay with it. A relay is cheaper, and WHEN it fails(not If, WHEN), it can be replaced easier. Usually the whole PID control has to be changed when the relay goes. and I have had them go. It's not only current, it's current over amount of cycles. And when the cooker is up to temp, the PID will be Pulse Modulating the element on off on off. Doesn't take long to fry contacts that way. In my experience Ive changed a few of them. Companies hate it when you tell them their $5000 PID controller is toast because someone used the internal contacts to switch load.

[StirCrazy]

SO I should get the SSR output one then instead. now the alarm outputs are 3 amp, should I use external SSRs on them also for 1 amp loads? these ones won't be on/off cycling they will be on for the duration of the smoke.

hmm that leaves a spar alarm output.. I could rig up a time based sprayer to bast the meat every 30 min... hmmmm

Steve