Introduction: 1200°C Tube Furnace Using a CJD-9000P PID Controller
This is a guide on how to build a basic tube furnace, easily capable of reaching up to 1200°C.
Supplies
Quartz Tube - I chose a tube with dimensions: 30mm O/D x 1.5mm thick x 310mm length, but basically any size tube will work.
https://www.robsonscientific.co.uk/
Refractory Cement - Can get this from any DIY/trade shop.
https://www.screwfix.com/p/flomasta-fire-cement-2kg/9588v
Alumina Ceramic Wool - Any kind will do.
https://www.ebay.com/itm/315204021378?var=613676262865
Nichrome Wire - I used 22 gauge (0.6mm) nichrome wire with a resistance of 3.855Ω/m, use more resistive nichrome if you want to use a shorter length for your heating element.
https://www.ebay.com/itm/356972679165
Ramp/Soak Controller and Solid-State Relay - I used a CJD-9000P; make sure you use a ramp/soak controller, don't cheap out and use a PID-only controller as they wont have rate control and you will find it much harder to prevent the quartz tube heating up too fast. My controller came with a solid-state relay, any kind will do, just make sure yours is rated for the maximum heating element current.
Thermocouple - Do not buy an immersion thermocouple. Thinner is better as the thermocouple will respond faster.
https://www.amazon.co.uk/dp/B0FH9VH84K?ref=ppx_yo2ov_dt_b_fed_asin_title
Other Bits - For the mains power cord just use any old cable and cut the ends off. Use solid-core wire for signal leads. If you are in the UK, make sure you use a 13A fuse in your plug. Have standard stuff on hand: lugs, heat shrink, scrap wood, electrical tape, duct tape etc...
It goes without saying that you should make sure that all components that are going to get hot are rated for the maximum temperature you wish to run your furnace at.
Step 1: Creating the Heating Element
Begin by applying a layer of refractory cement to the quartz tube, it doesn't need to be precise. This is to insulate the tube from the hot nichrome wire. Once this is applied, bake the tube at around 100°C for at least 2 hours to set the cement in place.
Next, determine the length of nichrome wire required for the heating element. Start by choosing a desired power output for the furnace, then, by calculating the necessary resistance, determine the length of nichrome. For example, I wanted a power output of ~1kW with an input RMS voltage of 240V, hence I needed a heating element resistance of ~60Ω (and hence a length of nichrome around 15m long). Make sure that you leave plenty of spare wire either side of the heating element to make your connections with. Wrap the nichrome round the tube, keeping the spacings constant where possible - try to avoid bunching turns together as this will thermally stress the quartz tube.
Once the nichrome is wrapped, use some masking tape to hold the ends in place. Tuck the excess wire into the tube and apply another layer of refractory cement. Bake at the same temperature/duration again. You can also terminate the heating element with lugs at this point, this makes connecting the nichrome wire much easier. I would recommend you use less refractory cement than I did, mine went slightly dry and was harder to manipulate as a result.
To cure the cement, a final, hot bake (at a minimum of ~900°C) is required. A simple visual clue that the furnace is hot enough will be from the quartz tube, at around 700°C it will begin to glow dimly red, above 1000°C the tube will be almost white hot in the middle and red hot at the ends. The final bake can be implemented the first time you heat up the furnace. Alternatively, you can bake the tube by running the heating element with no controller (either powered from a variable high current DC source or from a stepped down variable AC source, like from a variac) and manually regulate the temperature with something like an IR temperature probe. The cement will turn grey/white when it is fully cured.
Step 2: Assembly
Cut the insulation to an appropriate width (slightly shorter than the tube) and wrap the tube up. I used around 3.5m of insulation and this kept the outside of the furnace cool while running (i.e. no warmer than 45°C at maximum operating temperature). Use something to hold the insulation in place, tape will work - I used a spare length of copper wire. I decided not to build a case for my furnace, instead I mounted everything to a spare piece of MDF. To prevent the 'furnace' part from rolling around, I mounted it on a spare bit of metal that connected to the board.
The wiring will depend on your controller, most will come with a sticker on the side that shows the terminal wiring diagram. Once again, using lugs or solid-core wire will make your life easier. The thermocouple connects straight into the controller, likewise with the mains power input. The solid-state relay should be connected on the low side to the heating element (i.e. one end of the heating element is connected to live, and the other end connected to the solid-state relay, with the other solid-state relay terminal connected to neutral).
Step 3: Programming the Controller
Quartz tube is very sensitive to thermal stresses, it should not be heated/cooled too quickly (a change of 15°C/min is a typical maximum, the specific value will vary with thickness). The controller can be programmed to manage temperature changes so that tube is not overstressed.
From the maximum temperate rise/fall rate of the quartz, an appropriate rise time for the furnace can be calculated. For example, to bring the temperature up to 1000°C, I used a rise time of 80 minutes (~12°C/min). The cooling rate will be the same; if you use sufficient insulation you will find that the tube will stay hot for hours after use and that the heating element will be off for the entirety of the cooldown.
For the CJD-9000P, the programming regime is as follows:
> Press the A/M/AT button (left arrow)
> Set C, the starting temperature
> Press SET
> Set t, the rise time
> Press SET
> Continue to set the next coordinates and so on so forth...
A typical set of values might look something like this (to implement a simple bake at 1000°C for 2 hours):
You can probably get away with leaving the PID values of your controller unchanged, if your furnace suffers from large steady-state error it might be worth increasing your integral term. If your values are causing oscillations you will need to tune your controller. PID tuning is beyond the scope of this guide, however one simple method is to start by setting K_i, K_d = 0, then increment K_p from zero until you choose a sensible value (i.e. sufficiently fast response with minimal oscillation), then set the remaining two parameters according to steady-state error, slew rate, settling time etc.
Lots of these generic PID controllers are very poorly documented, often the only way to use them is to play around with their settings (while the heating element is disconnected or with a dummy load) and work out what does what.