Crystal Chamber PID Tuning

Since I'm trying to use a PID controller to keep the chamber temperatures under control I need to tune the PID parameters. And since this is designed to run for hours at a time, and because I'm crap at PID tuning, getting it right it is very tedious. Also presumably I'll need to tune it differently for different chamber loads. A liter of water is going to behave differently from a 50mL of solvent, so I figure I may need to retune it several times for different uses.

So I set it up to dump the parameters so I can visualize the process. Nothing fancy, just CSV out the serial port to a log file, then into Excel so I can see what's going on:

I've run half a dozen trials while I learn how to do PID tuning. That image is from part of the process where I'm looking for a good Kp that doesn't overshoot. After playing with it for a while I decided that I might as well get the rest of the hardware and software done, then play with the fine tuning later.

Crystal Chamber Controller Interface

The code I've been using to run the temperature controller for the crystal chamber so far has been pretty bare, just the core PID loop. I don't have it tuned yet, but it's generally functional, so it's time to start getting it in shape to be an appliance.

A while back I picked up some old IBM modems that were being retired from the modem pool at work. I was hoping maybe I could convince one to speak Bell 202, but that didn't pan out. Anyway, they have nice boxes and the front panel has a set of buttons and an HD44780 LCD controller, so I held on to it:

The box has a nice power cord and switch built in, so I will be able to just stuff everything inside and put a connector on the back for the heater and temp sensor wires. Then I can use different chambers depending on what size I need. I could also run set it up to run multiple chambers, it only takes a couple more pins to run additional chambers, so I suppose I could set up to run about three. Maybe for version 2.

I wired it up for the LiquidCrystal library, plugged it in and away it went. The controller is only good for 8 characters, so I've got it set up to cycle the display through each setting one at a time. Right now, that's the setpoint temp, the current temp, and the number of seconds left in the ramp.

I also noticed that at some heater PWM frequencies the power supply squeals, so I added start and end cycle beepers. When I switch power supplies I'll probably need to add an actual beeper, but this was kinda fun in the meantime.

I also mounted the heater and temp sensor for real:

It's there at the bottom of the image. I used poly tube standoffs to keep the leads from contacting the aluminum tape and to provide a relatively stable mounting to the foam. The temp sensor is in the upper right-hand corner. Previously it was just laying on the floor of the chamber. Keeping it up in the warm air rising off the heater helps the response time, but also makes it clear that there's some pretty strong stratification going on in there. I'm pretty sure I'll need to add a tiny fan to keep the air mixed. I didn't want to do that because of the possibility for vibration to affect the crystals, but if it does end up being necessary I can probably isolation mount it somehow. Or maybe I should use an ionic wind type fan. Every project needs some nice high voltages, right?

The next step will be to scare up a new power supply that'll fit into the old modem box and come up with a connector for attaching the chamber.

Crystal Chamber Temperature Controller v0.1b

I cobbled together a simple temp controller to try to get a feel for what it'll take to make one that will work for my temp controlled crystal-growing chamber. For this first iteration I've just got a simple PID-controlled heater, LM75 I2C temp sensor, and a 25W heater element.

For now I'm just running on an Arduino and a crappy bench supply (with a way-too-bright power indicator). For the finished project I'm thinking I will use a Teensy 3.0 as I've got some samples of them that I'd like to do a write-up about.

To house the controller I'll be repurposing an old modem that has a front panel with a small LCD and a set of buttons that I can use for menu navigation. I can mount a power supply and all the electronics inside and put a connector on the back that plugs into different chambers. This big chamber will be nice for large beakers, but I'd like to have a smaller one for test tubes. I can put a resistor on the chamber side of the connector so the control box can identify which chamber is connected and select the appropriate PID tuning.

I ran this for a few hours to see how it behaves. The 0.5C resolution on the sensor is pretty bad, but overall the results were encouraging. I don't really care much about how it handles large rates of change, since the hot solution will go into a preheated chamber. The main concern will be whether it can hold a stable downward ramp with minor fluctuations outside the chamber. I haven't tested that yet.

Next step is to get the heating element mounted so it is stable and out of the way and then test with a beaker of hot water so I can see how well it do temperature ramping. I'm pretty sure I'll need to upgrade to a higher resolution sensor for really long ramps, but for a day or two this 0.5C should be ok if I screw with the PID tuning enough.

The PID library has some kind of auto-tune that looks interesting, but I haven't figured out how to use it yet.

Insulated Temperature Controlled Chamber

For my crystal-growing I'd like to have better control over temperature so that I can cool solutions very slowly. I've been using a heavy ceramic dish and a hot water bath for this, but I want to try cooling rates of days or weeks. To achieve that I think I'm going to want heavy insulation, radiant barrier, and probably a small computer-controlled heater element.

As a start I'm using 2" pink foam with aluminum tape as radiant barrier on the inside surface:

The foam is rated R-10. I'll put a liter of hot water in this and log the rate of change to get an idea of how much it reduces heat loss over open air. That should give me an idea of how much power I'll need the microcontroller temperature controller to handle.

I'd like to be able to keep it as low as 0.1degC per hour while minimizing fluctuations, but at this point I don't have any idea how challenging that will be.

Update:

As described above I put a 1 liter beaker of hot tap water into the chamber and stuck a thermometer through the top down into the water. I recorded the temperature every 10 minutes for a couple of hours, then wandered off for a while and then came back and got a final reading: 

So I figure 1000L dropping 0.578C in 10 minutes is about 578 calories per 10 minutes, or 4.03 joules/second, 4 watts.

Room temp was 24C, and I was testing around 62C. The box surface is about 0.25 square meters, so that would be 4.03W/0.25sq.m, 16.1W/sq.m. R-value is ΔT/W/sq.m, so 38/16.1 = 2.36. The 2" pink foam spec is is R-10. Even with my shoddy construction I'd expect better than R-2.36.

Maybe that value is in RSI units. Wikipedia says the conversion from RSI to R-US is RSI * 5.68, so R-13.4. That sounds pretty reasonable considering the radiant barrier.

I might want to ramp the crystallization down to very cold temps, and it's a lot easier to just put the box in the freezer than it is to make a box that can cool itself, so the temp outside the box might be down at -20C. So, if I want to hold the box at 100C in a -20C environment, how much power do I need? Flipping the equation around and using the RSI value of 2.36: W/sq.m = ΔT/R, ΔT is 120C, so: 120C/2.36 = 50.9W/sq.m, or 12.7W for the 0.25 square meters of the box. Within my ability to measure that's linear scaling, which seems pretty reasonable from eye-balling the graph, which is also pretty linear over this narrow temperature range.

I'll give it some headroom and call the target 15W. On a 12V power supply I'll need to switch 1.25A. I've got a pile of IRF540N that'll be fine for that. My power supply is 12V at 3A, so my heating element needs to be between 4 and 10 ohms. Too low and I'll send the PSU into protection, too high and I can't get enough power out of it. That's probably another junk-box item.

Next up is electronics. I've got some old LM77 temp sensors laying around. They're not very accurate, but they'll be fine for a first iteration. 

Update:

Started prototyping the controller:

The chip is an LM75 I had laying around. It's only 9 bit with the LSB reporting 0.5C increments. That's fairly awful, but it's good enough to get started. I can swap in a better sensor later.

I dug up an old nichrome toaster element to see if it would work as a heater:

I connected it to the desktop supply (which seems to be running 15V open-circuit, I may need to get in there and see what's going on with the regulation), and it runs at 13.7V and 2.05A, for 28.1W. That'll be perfect, I just need to put a MOSFET on to drive it and I should be able to hold the chamber at any temperature I like.

The insulated board it is wrapped around is 5 inches across which is just right to fit inside the chamber. I think that if I leave it on the board I can put it on one side of the chamber so that convection will prevent any stratification. I'll probably use one of the other toaster elements which is wrapped with all the turns on one side. I can face it toward the wall to create a chimney while avoiding radiant heating of the solution, That could definitely disturb crystal formation. 

Crystals

Recently I've been playing around with growing crystals. It was always a fun and easy thing to do when I was a kid, so I thought I'd try some that are more interesting than the usual salt, sugar, alum, epsom, etc.

This is recrystalized acetylsalicylic acid, asprin, from OTC tablets. 10g pulverized, washed with cold water, vacuum filtered, then dissolved in 250mL of 75C water and allowed to cool slowly. Crystals approximately 3-5 mm in length. I will chill this and allow crystal formation to finish, then wash the crystals and recrystallize at a slower rate.

This is benzocaine, extracted from Oragel tooth gel. 0.5g of gel stirred in 15mL cold water until dissolved. Benzoncaine will form white flakes. Centrifuge and draw off supernatant. Add 20mL water and heat in hot water bath. A bead of molten benzocaine will form. Add sufficient water to dissolve the bead. Allow to cool and crystallize. Vacuum filter and collect for recrystallization.

This is what I think is probably salicyclic acid. 1g of OTC tablets, pulverized, heated in 20mL water until dissolved. When cooled the entire 20mL formed one huge crystal block, which was amusing. Melted again, added 10mL water and heated until acetic acid formed. Cooling produced fine, hair-like crystals, which I suspect are salicyclic acid (more research to do). Washed and vacuum filtered, will recrystallize to see if more pure crystals look different.

Update:

After filtering and cleaning a bit more (there were still some components of the asprin tablets in the solution which affected crystalization), I got some recognizable salicylic acid crystals:

I was hoping to get some more acetylsalicylic acid crystals, but I accidentally hydrolyzed them. I'll have to give that try again later. For now I'm bored with salicylic acid, needle crystals are hard to photograph. I think I'll move on to something else.

Update:

I had a couple of 50mL beakers with more ASA and SA crystallizing by evaporation. One had mostly SA dissolved in a small amount of isopropanol, and the other was water. They've been sitting for about 5 days doing nothing, and apparently just reached the point where crystals could form. They went from nothing to quite a lot of crystals very quickly.

This one started out with what I think is SA making little needles at the bottom. I left it to evaporate and a few days later this big block formed overnight. I'm not sure if it's ASA or SA. The size suggests ASA, but I'm not sure how completely I hydrolyzed this solution, so I don't know how much of each compound is left. I'm going to let it keep evaporating and see what happens.

This one is the alcohol. It's evaporating faster than the water, and it wants to crawl up the sides a lot more, so it's probably not as likely to make crystals that are as nice. I will probably let this one go for a while until the alcohol is mostly gone and then rinse the crystals and redissolve them in alcohol and try again but with slower evaporation.