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Old 07-27-2009, 09:19 PM   #21
eccsynd
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Using duty cycle control over the element isn't really want you want, it'll force the element to be active, say, 40% of the time regardless of the temperature. (which is why some PIDs label this function as Manual).

Setting the pulse width (or what we call Output Period in the BCS) isn't really applicable either. That'll set the minimum On time, not the maximum.

So what you're looking for is a way to limit the maximum On time of the output, thereby lowering the effective wattage of element? Cool idea. That feature doesn't exist in the BCS currently, but that's never stopped us before.


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Old 07-27-2009, 09:32 PM   #22
Bobby_M
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Keep in mind that I got C's in all my industrial control classes because I didn't care much back then. The tradeoff to short pulse on the SSR is usable life right? I suspect that it would be more practical to run the element at 120v. This should be doable with a DPDT switch where the Hot/Hot is replaced with Hot/Neutral prior to the SSR. I should probably draw it on paper before I write it but I know there are enough experts to correct my mistakes. Sorry/Lazy.


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Old 07-27-2009, 09:33 PM   #23
kladue
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Why not use one of the outputs to fire a contactor to switch one leg of the RIMS heater between neutral for low power, and 120 for 240 across element for high power, then the control parameters for cycle time should be in range.

 
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Old 07-27-2009, 09:54 PM   #24
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Sorry, all that talk about pulse width control and I didn't read that you already had a control unit.....

A good industrial SSR can switch a gigizilllionion times It's MTBF is not so much a function of the amount of switches, but more so a combo of heat, steady state and dynamic current, etc... They are cheap, so I probably wouldn't let that hold me back if I were the OP. (just a back seat forum poster on this one)

It is probably easier to use the benefits of 120/240 V operation... Just be careful about throwing switches at full current load...

 
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Old 07-27-2009, 11:42 PM   #25
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Quote:
Originally Posted by eccsynd View Post
Using duty cycle control over the element isn't really want you want, it'll force the element to be active, say, 40% of the time regardless of the temperature. (which is why some PIDs label this function as Manual).

Setting the pulse width (or what we call Output Period in the BCS) isn't really applicable either. That'll set the minimum On time, not the maximum.

So what you're looking for is a way to limit the maximum On time of the output, thereby lowering the effective wattage of element? Cool idea. That feature doesn't exist in the BCS currently, but that's never stopped us before.
I was hoping I could turn the PID parameters down so that it had a very slow dampened response. Wouldn't that effectively limit the wattage assuming the output period is fast? It's been over 10 years since I took a control systems class and my only real world experience with PIDs is from tuning servo motors.

Triggering a contactor to swap one of the hot legs to neutral is the best backup plan, but i'd rather solve the problem with software. Please let me know when that feature is available

 
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Old 07-28-2009, 01:17 AM   #26
kladue
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Swapping the voltage will reduce the response slope so tuning at low output demands would be a lot easier than trying to get a small percentage output at high input power level. Only other trick I can think of is 2 - 2K elements, one in each end of the RIMS chamber, controlled from 2 outputs. When one is fired for Rims use and both are fired for water heating. Build the chamber with a union fitting in middle so elements can be installed and positioned to pass by each other when assembled. Fire each element from seperate leg of the 220 so current flow through neutral is only when one element is fired, when both are in use the current flow is through the hot legs not the neutral.

 
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Old 07-29-2009, 12:02 AM   #27
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I tested the RIMS heater today and it will definitely meet my requirements as an inline water heater for sparging. I ran a hose to the rims and the output to a plastic measuring cup. I opened the valve to produce a flow rate of 1 quart of water in 50 seconds (1.2quarts per minute, which is more than recommended). The house water was 80F.







After I fixed some leaks, I turned the heater full on:



The rims output temp quickly shot up to 190 after 1 minute with little sign of deceleration so I shut it down, worrying about hitting the boiling point. The slightly flatter graph is the quart measuring cup I used to simulate a HLT. I had about 10feet of silicone hose going to the cup hence the lag. Not quite sure why that temp is higher than the RIMS temp. I did not calibrate the temp probes but they did read the same temp without heat. The three groups of oscillations was me testing the PID functionality set at 120F and 140F. The first test was with the default settings. The 2nd and 3rd was with a shorter cycle period. Wife was bugging me to cook dinner so I didn't get a chance to tune the PID. If I remember correctly, I believe oscillations are due to too much proportional gain, or was that overshoot? I'll need to read up on PID tuning. Once I get it tuned, I'll test the maximum flow rate. That would give me the efficiency of the system and allow me to calculate how it'll do in the winter. Overall, I'm very happy


 
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Old 07-29-2009, 12:18 AM   #28
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Nice man!
Tuning a pid can be a little bit of a pain.... Overshoot can be tuned with the derivative side of the controller.

 
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Old 07-29-2009, 02:25 AM   #29
eccsynd
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Wow, that is a wild system. The out-of-the box PID coefficients were chosen for systems with a high thermal mass, like most brew systems. Your system has ZERO thermal mass, and the big element makes it VERY responsive to heat. Keep the sample rate (output period) at a minimum.

I'd recommend using the Ziegler–Nichols method, set I and D to zero, and increase P until you get oscillation. Which shouldn't be a problem. Make sure to give it enough time to truly oscillate, not slowly converging.
PID controller - Wikipedia

Which way is the water flowing in your picture? If it was left to right, I could see that the measuring cup probe would be higher than the RIMS probe. If not, make sure to check that the RIMS probe is the one that's associated with the RIMS element output.
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Old 07-29-2009, 02:50 AM   #30
Sizz
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Thanks for the PID pointers. I'll definitely give that a try. The water is flowing right to left and the amplitude of the temp swings (higher swings on RIMS) makes believe that I had the probes wired correctly. But I'll definitely double check. I had some long (10') extensions on the 'psudo-hlt' probe. Would that matter? It was 14 gauge wire so the impedance should have been negligible.




 
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