MarkTilles

That’s odd, I haven’t used it live yet but when I have created a test curve and the oven isn’t tied in but the thermocouple is, the countdown timer in the GUI does pause when this occurs. It paused forever for me in this “test“ scenario, as stated in the config file. Were you monitoring the GUI during the process? @jbruce Jason, might you have any suggestions for Leof?

Yes thanks, I posed that question to Jason (in a conversation off-line), who manages the code for this branch, and he said the same thing. He has also just updated the codebase with my corrections for max31856, though not all were needed.

Yes without this new Pi-based control system I have to practically sit next to my large Chemetex kiln's controller (VERY old, analog with tons of rectangular LEDs indicating the current temp) and continually adjust its %-effect-dials to keep the heating rate in the range I want. Pain in the ass! And my small kiln controller, the Bentrup, well it works so-so but only has a pair of settings one can change.

Will do! Offset of Pi versus Bentrup at 960 was about 30C (990 pi versus 960 Bentrup). So the behavior of the Bentrup isn’t as consistent whereas the other devices are much more so with eachother. Will run a 1260 eventually and see where that lies.

Yes we are cooking up to 960 on the kiln Bentrup controller right now, those numbers were just during the early process, so I will have more numbers soon.

Bill, I don 't have acess to a known good and accurate multimeter just now, but does the information below lend favor to the Pi being more accurate, excepting the possibility the S-thermocouple or compensation cable is bad/incorrect? (I described the cable to you and you replied was positive), The cables are under 15 feet long. I have two different PID controllers I bought online from Banggood. One BERM, one REX. Both set to S-type. I checked the temperature of the kiln with all four devices within 2 minutes of eachother,: Kiln Bentrup: 491 C Pi w/max31856 551 C Berm 557 C Rex C100 560 C (but when temp was read its SC was set to -1, I forgot to set to 0 first) This gives me the feeling it's the OEM Bentrup that is not displaying correctly, and maybe we are overfiring just so much our work is not being obviously damaged?

I know I keep going around in circles and am not directly heeding your advice Bill, but please don’t get me wrong, that’s not out of disrespect; Yet even without the switch I have exactly the same values, so: between the new pi and its directly-coupled max31856, versus the Bentrup … what are the likelihood one versus the other is 50°+ off? What are the most likely causes large measurement discrepancies like this would show up in this case? The controller and very likely the oven as well was manufactured in the beginning of the 1990s.

@Bill Kielb@newps I am now running a 960C bisque firing using the Bentrup TC 40 controller connected to one of my kilns. That controller, although old,seems to be digital and work OK as we haven’t had any bad results with bisque or glaze firing to 960 and 1260 respectively (as displayed on the original Bentrup controller) I have now installed a secondary S-type thermocouple at the same height and location in the kiln as the original. I am comparing the temperatures noted by the raspberry pi max31856 system to the Bentrup. As you know I have a pair of contacts and a double-pole switch in the loop with my Pi controller, so I am also checking what happens when I wire directly and bypassing those additional junctions. When 271C was displayed on the Bentrup, the pi system was showing 319C, or 48°C warmer. When I quickly bypassed all my switches and wired the thermocouple directly into the thermocouple board, of course using only compensation cable, there was no change in the measurement value whatsoever. I will check this also later in the heating schedule at a much higher temperature to see if there’s any difference when going through my switches or not. Here are a few points in the firing schedule with temperature values shown by both systems: Time (min)/Kiln (C)/Pi (C)/deltaT (Pi-Kiln) 0/14/11/-3 75/90/103/+13 210/223/265/+42 285/300/350/+50 As you can see the comparative temperatures are widening the hotter it gets. I will continue monitoring this about every half an hour until the end. I have several new s-type thermocouples and will also check the top temp with one of them later, as with welding gloves I can easily remove one thermocouple and push in another one. I suppose I should also check these values by swapping the Bentrup and pi systems on the respective thermocouples to see if the thermocouple swap changes things. I will the post full numerical results when completed. My question is, down the road, which system should I trust?

That’s great Liam. Now if those functions are actually available, I’d just like to figure out where one configures the GPIO values for, for example, “door.” I got a hunch from yor text and did s google search for "picoreflow door switch wiring" and now I understand this project looks like a spinoff of one called picoreflow - and I found some docs on it here: https://projects-raspberry.com/picoreflow/

Specifically I’d like to know what these five different icons represent on the top right of the menu, and why the furthest one to the right is enabled on my iPhone Safari browser at all times, yet is never lit on my laptop chrome browser, whether or not the system is running a heating cycle. What I haven’t found is a manual or readme with details about these things.

I was just curious there bill, no urgency, the system looks to work great. I just like to tweak things now and then. Jbruce threw together a nice box.

Can any of you out there point me in the right direction to be able to look at (and maybe even tweak) the web interface? I can't figure out where it is coming from and how it interfaces with oven.py. First off I want to see what the other status symbols in the menu represent ... Thanks in advance

DPDT, not center-off. just the labels confused you maybe

Ok, For grins, I have now cabled things in for testing, and labeled the various items. Attaching photos below. The thermocouples and SSRs shown - only one of two sets, for kiln K-2. I have built a 3-phase SSR heating array, which will be housed with a cooling fan in a steel cabinet under my big kiln. The array will be controlled by the Pi system. In order to start things up, I will always need to manually push in and enable the magnetic relay shown in the photograph once, which will enable the main relay in the kiln. Because of the loopback connection, it will stay depressed as long as the solid-state relay connected to it is enabled, and will disengage when the high limit is exceeded or with any power fluctuation in the building. I have disabled the PID’s proportional, integral and derivative functions so it never cycles. This magnetic relay then supplies power through to the SSR array/heating elements, controlled by the Pi system. The red button on the side of the box is simply a “safe shutdown” button for the Pi operating system. In previous posts I mentioned REX, this particular PIM is a BERM. (China Berme clone).

Exactly!

I guess my thought is kind of complicated to explain by text, at some point I can draw as simple schematic showing what I’m doing. The two controllers are not interconnected, they are independent systems. However, the Rex will control the main relay that supplies power to the heating elements, independent of what the pi controller does with the solid-state relays. Once I enable the main power on the rex controller monitoring system, if the temperature exceeds a maximum I set in it, then it will drop off the main power to the heating circuit.

Bill the Pi system is not going to have anything to do with the overheat “safety back up”, rather it’s the separate Rex C-100 PID with the secondary thermocouple in the oven. It will be wired up in such a way that one tripped the heating elements cannot be re-enabled without a manual press of a button.

Well, I do have two Pi’s as well as two 31856’s so having two systems is a possibility down the road. But as far as how well PID values might work between the two different kilns, one could always shell-script replacing the needed PID values in the controller configuration file when needed, that’s not a problem, just a couple of lines of code in the right place, and this functionality can probably be added to the web interface as well. At this point I’m going to just do some experimentation. I have the switches, I have the compensation cable, I have a working Bentrup TC–40 controller on my small60 liter kiln, so I’ll just wire things up and compare temperatures to the Bentrup readings. I will report back with my findings.

What is involved in turning this into a “current loop”? Again, to clarify, I am only going to flip the switches before starting one of the two kilns. I’m never going to run them both at the same time. I want to make one controller that I don’t have to be unscrewing wires and screwing them back together again when I change kilns. The switches won’t be flipped mid process. Once my newsystem is complete, I can compare values with one of my stock kiln control systems which actually looks to be very good, it’s just very limited in program ability. I should be able to use this to calibrate my endpoint requirements for the new system.

@Bill Kielb Bill I have a question regarding a creative idea that popped up in my thermocouple newbee mind this morning. Because there is a potential difference between dissimilar metals, every junction along the way between the thermocouple and measuring device could cause a problem. OK. In my case I’m trying to simplify the wiring connections with multiple thermocouples and toggle switches so I can switch the one controller system between two different kilns Ok If I install a terminal block at the back of my controller box, there is no material dissimilarity between the pair of screws for each terminal. I am also going to assume with this idea for simplification purposes that the same would be true with the toggle switch, however there probably is a different material inside, maybe solid copper, in which case there would be two junctions inside the switch, one in each direction of material change. My idea is this: what if I switched the compensation cable polarity along the way, so as to reverse the potential difference created at each junction? Could this minimize the total error received at the end?

I ran a small test. I powered up my Raspberry max31856 system with my S-type thermocouple and put the thermocouple in through the door of my toaster oven. I then sat the oven at approx 200C. Then I used alligator clips and attached the Rex C100 PID to the same cable ... made no difference at all, I see no temperature measurement change with the second device attached. Then I unplugged the max31856 from the cable ... no change on the PID temperature measurement either. This is good I think?

What I’m actually thinking about trying is having one thermocouple split at the controller box to two different control systems. The raspberry will control the heat, and the Rex C 100 will shut everything down if it notices it getting way too warm (or if the Raspberry might have hung up or otherwise misbehaves). I want to have a failsafe for that, or if the SSRs might’ve gotten stuck. That’s the plan. If it doesn’t work with one thermocouple, then I have extras and additional compensation cable. It’s just a lot more wire to run! Will have about 10-20 feet of wire between the kilns and controller.

Good feedback, thanks.

Did someone say it's POSSIBLE to have two controllers connected to the SAME thermocouple? If so, this would greatly simplify my needs to connect both my REX 100 PID as well as my Raspberry Pi to the kilns (I an using the PID for overheat protection to shut down the main relay).

Here's a question: Reading about the max31856, it has built-in resistors ... is there any idea to add external ones? I don't see anyone using them in their wiring photos, and maybe this could generate problems as well. The data sheet talks about adding them if one might expect surges over 45V. It seems the max31855 didn't have these, not in the product description for them. ... MAX31855: This breakout does everything for you, and can be easily interfaced with any microcontroller, even one without an analog input. This breakout board has the chip itself, a 3.3V regulator and level shifting circuitry, all assembled and tested. Comes with a 2 pin terminal block (for connecting to the thermocouple) and pin header (to plug into any breadboard or perfboard). We even added inline resistors and a filter capacitor onboard for better stability, as recommended by Maxim. Goes great with our 1m K-type thermocouple or any other thermocouple, really!