How to turn a Crock pot into a precision heat bath
Entry posted by Swede
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Thanks to some inspiration from Rosco Bodine at SMDB, among others, I have successfully hacked a $10 crock pot and turned it into a precision heating bath capable of holding +/- 1 degree C.
Background: I have been using expensive polypropylene tanks to do the bulk of the work, and an immersion heater provides the heat. The immersion heater is an industrial cartridge heater encased in copper pipe, soldered shut at the business end. It works... but the Cu is not always compatible with the bath, it corrodes, and worst of all, it occupies precious volume in the system. Difficult to fabricate, hard to mount so that it does not touch the plastic tank... the immersion heater concept leaves much to be desired. Enter the crock pot.
Note: To do this job well, you need a true heat controller, especially one with a PID algorithm. These are available NEW for about $40 to $60 on eBay and other places. I detailed how I turned a nice one into a full-up heat controller in a previous blog:
When you set up your heat controller, do so in a way that allows you to swap heating elements AND temperature probes easily. Mine uses "Dean's Ultra" plugs for the heating element(s). If you are using a thermocouple rather than an RTD (I recommend the latter), be aware than thermocouples require special compatible extension wires and connectors, whereas RTD's simply require decent, high-quality Cu wire.
First step. Buy a crock pot. At Wal Mart, they range in price from $10 to $75.
This specimen is a 2 quart heater, and costs a whopping ten bucks. They are very simply put together... a pressure washer on a shaft on the bottom keeps the whole thing together. It had to be freed with a dremel by grinding the individual fingers of the pressure washer off, but it popped free with little fuss. At that point, the entire stack came apart vertically.
And this is what is inside:
Free the aluminum central chamber from the rest of the device. The heating element is a spring-loaded band that is a snug fit around the aluminum pot. Aluminum conducts heat so well, when that band heats up, the entire aluminum pot also heats nicely, and the Pyrex or stoneware insert absorbs the heat.
Almost all cheap crock pots have two settings (besides off); low, and high. I discovered with an ohmmeter, and by analyzing the rotary switch , that there are two separate heating elements inside the band, one of a lower resistance, the other higher, with the latter, of course, being the "warming" element. When the unit is on low, only the high-resistance (low wattage) element is energized; when on high, BOTH of them are on.
Since we are using a good temp controller, there is no need to keep the elements separate electrically, so they were pig-tied together, and a crimp was used to secure. The three lines consisted of the "hot" AC line (which is smooth on a zip cord like this one), and the two heater element wires. The original fiber protective sheathe was reinstalled over the crimp. There will be no ON/OFF switch, since the controller takes care of that, although if you wanted to, one could be added at this stage, or you could make use of the original with a bit of rewiring.
At the other end of the heater band, there is only a single wire... somewhere internally, the two elements are tied together. This single wire is pig-tied (and crimped) to the neutral line. On an AC zip cord, the neutral line has a "ribbed" sheath and also corresponds to the larger prong on the plug. In the USA, on a 120VAC 2-line system cord:
Hot =
- Black
- small plug
- smooth wire
Neutral =
- White
- large plug
- ribbbed wire
I carefully (respecting the live voltages) plugged the unit in, and it began to warm. That's all I needed to know at that point. It was time to put it back together.
The original wiring had some useless wire loops that were supposed to keep the wiring off the heater band. I did a bit of re-engineering with stainless steel wire, and formed some supports to do a better job of it. The idea is to prevent the excess wiring from contacting the aluminum pot on the inside of the unit.
The heater band is an interesting setup... being spring-loaded, it can be moved on the aluminum pot, and if a more rapid heating is desired, buy a second $10 crock pot, and add the heater band to the first. This will also give you a spare stoneware crock.
$10 is an incredible deal for what you get... a heat-resistant crock, heating element (moveable) and lid. I was paying more than $10 for a simple cartridge heater. Coming n a variety of sizes, the lowly crock pot, combined with a heat controller, is an awesome combination.
To secure the crock pot back to its original form, I simply split the shafting at the bottom, bent the "petals" over, and drizzled epoxy all around.
Time to test! I rewired the plug to a female Dean's Ultra to match the output of the heater controller, added cold water, set the controller to 80 degrees C, and let rip. Heatup was quite slow but it was steady, and about an hour later, my two quarts of water stabilized at 80 degrees. The PID logic in the controller did what it had to do to keep it there. A killer plating bath (or a warming bath for a number of processes) for $10, minus the controller.
The controller is the "big part" of the system. The controllers coming out of China seem to work fine, and combined with a SSR (Solid State Relay) will do what you need. Here is an example:
$60 or so will get you going. It does not have to be fancy like the one I made... hot glue the components to a board, watch out for voltages, and functionally it will be the same!
This two quart unit will see duty plating small test anodes with lead dioxide.
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