It stands for "my domain is shorter than your domain"

Here are a few pictures of my sous-vide PID controller project. Inspiration came from the Seattle Food Geek. Being an engineer, I wanted to create a standalone system so I added sockets for the heater elements, pump and thermometer probe. I also didn’t see the benefit of hanging the mains-powered system above the bath, so I made the controller a separate tabletop unit.

This is the bath. I use two plastic boxes from Wilkinsons, one inside the other, to give a degree of insulation. The immersion heaters and the Pt100 thermometer probe are clamped between two pieces of plexiglass. I keep the lid on when the heater is on. This is a temporary arrangement until I get better heater elements. I will probably attach these directly to the lid.

My first pump got a bit hot and gave up the ghost, so it doesn’t feature in these photos. A replacement has been ordered.

Front view. The PID controller has just been switched on. It’s a Chinese REX-C700, apparently a non-pin-compatible fake of a Japanese controller of the same model number. The enclosure happens to be such a good fit that the controller doesn’t move at all between the box top/bottom and the enclosure’s screw posts. The power switch is at the top.

Rear panel. On the bottom left is the fused input. On the bottom-right is pump output, always on when the switch (on top of the controller) is on. Top-right is the heater output, controlled by the PID driving a 25A solid-state relay. Top-left is the thermometer input.

Building Instructions

These instructions are work in progress. I intend to amend and improve them soon, including adding pictures and circuit diagrams. Meanwhile, comments and questions are welcome.

This project should not be attempted by inexperienced DIYers. In fact, it should probably not be attempted by anyone. Done incorrectly, there is a very real risk of electrocution. These instructions give a very broad overview of the building process; do not expect to see every step described in detail (or correctly).

Right. With that out of the way, let’s crack on.

Bill of Materials

• 2 x REWIREABLE CEE22 CABLE PLUG, Rapid item 23-0120.
• SNAPFIT 4.8MM PLUG, Rapid item 23-2101.
• 2 WAY PUSH FIT IEC SOCKET, Rapid item 23-3107.
• LIGHT GREY ABS BOX 190 X 100 X 80, Rapid item 30-1498.
• SHEET OF 25 22.0 X 10 BLACK FEET, Rapid item 31-0465.
• DPST illuminated rocker switch, Maplin item GU55K.
• 3-pole locking chassis plug and line socket, Maplin items FK23A and FM51F.
• PID controller, eBay. I bought this one.
• Solid-state relay, eBay. I bought this one.
• Stainless steel immersion heater, 1 kW (search eBay for ‘tauchsieder’, German for the heater, in EU).
• Pt100-type thermometer probe, eBay. I bought these.
• Aquarium powerhead, eBay. I bought this one.
• Really Useful Box, 19XL (like 19l but with taller lid), eg. From Ryman.
• Fuse, 5A, fast blow, for the IEC inlet.
• Sheet of perspex, 3mm(?) thick.
• Spade crimp connectors, eg. Rapid item 33-1053; Eyelet crimp connectors.
• Cable ties.
• M3 screws and nuts.
• Bicycle inner tube.

Instructions

1. Start by modifying the dual-outlet IEC socket. We want to control each socket output separately, so they need separate live connections. Cut the ‘live’ metal strip into two and bend the strip so both sides can accept a spade connector. The ‘neutral’ strip can be left unmodified.
2. In the enclosure rear panel, cut suitable holes for the IEC sockets. The holes are probably easiest to cut in the edge of the panel as seen in the picture. Drill a hole for the 3-pole chassis plug. Cut two strips off the front panel to fit the PID controller. Cut a hole in the top for the switch.
3. Mount the IEC sockets and power switch in the enclosure. I used liberal amounts of epoxy glue to fix the IEC sockets in the panel. Because the panel is quite thick, I needed to trim the IEC sockets a bit to make them fit.
4. Using mains leads of suitable length with spade and eyelet crimp connectors, wire up the sockets, switches, PID controller and SSR. Wire both live and neutral from the power inlet through the switch. Be sure to connect the power switch the right way around; if it’s the wrong way, the neon light is on all the time. If it’s the right way, the light goes out when the switch is off.
5. Cut a suitable length (no longer than necessary) of the shielded thermometer probe cable off the probe assembly and use it to connect the 3-pole chassis plug into the PID controller. Choose some suitable wiring order; I wired the common lead in the middle pin. Attach the chassis plug into the enclosure.
6. Attach everything in the enclosure. I used some double-sided tape to fix the SSR to the side of the enclosure, as well as to keep the PID controller in place. The controller came with a couple of clamps which I used to tighten it against the front of the enclosure.
7. Close the enclosure. Attach rubber feet.
8. Cut a strip of perspex about 5 cm in height. I did this by first scoring the sheet several times on both sides using a ruler and a craft knife, then clamping it between the floor and a metal sheet and bending until the strip broke off. Done correctly, this leaves a neat clean edge.
9. Attach the thermometer probe and heater element to the perspex strip. I did this by first stretching a short piece of bicycle inner tube around the heater handle and the thermometer probe to provide an anti-slip surface, then drilling a couple of holes in the sheet and putting cable ties through the holes and around the probe and heater.
10. Attach the perspex strip to the box. I can think of a couple of ways of doing this. I decided to bend the ends of the perspex sheet into a 90-degree angle (using a clothes iron to warm the perspex and make a sharp bend) and then attaching the strip in the box with screws. Alternatively, you could just use cable ties.
11. Replace the pump and heater element power plugs with the IEC plugs. Solder the 3-pole line socket to the thermometer probe.

Done! (Kind of.)

Issues

• SAFETY: The device should never be left unattended.
• SAFETY: Combining mains power and water is always risky. A residual-current device adds a layer of safety and should be used at all times. Even if your home has RCDs, adding an extra in-line device can’t hurt. I bought one from B&Q for about £8.
• The immersion heaters are not designed to work for long periods of time. I have no idea how long they will last. Observe the minimum/maximum water level indicators and avoid splashing water above the high water level mark.
• Running the heater dry, even for a short time, would almost certainly break it. My heater has an internal overheating protection, a thermal fuse which cannot be changed. A good solution might be to add a float switch to make sure the box is filled with water. This could be added in the SSR control circuit.
• If the heater element is longer than the thermometer probe, there’s a risk of filling the box so that the heater is underwater but the thermometer isn’t. In this case, the heater would be on continuously, eventually boiling the water away. The best solution would be to get a longer thermometer probe.
• My first pump was a water fountain one, but it broke after a session at 85 degrees. I would not use the new pump at temperatures above about 68 degrees. Temperatures higher than that are normally used for things like fruit and vegetables where I suspect exact temperature control is less important, so you could just leave the pump out when cooking such things. There are some pumps designed to be submerged in caravan hot-water tanks that are rated for continuous operation at 70 degrees, but they require 12V and so may be less convenient.
• Due to the shape of the lid, condensed water on the lid drips down along the lid and out of the box. It would be better to add a seal of some sort on the lid to avoid this.
• At least in the UK, limescale is a big problem. In areas of hard water, a single 24-hour session can cause serious limescale buildup. I am currently experimenting with adding a small amount of washing soda in the water to prevent limescale buildup. If you notice a white powdery residue collecting on the heater and pump, descale!
• The insulation of the box is not great. I have measured that, when the box stands on a couple of cork trivets, it consumes around 150-180W to maintain the temperature at 60 degrees. This could be improved by insulating the box better. One option might be to put the whole thing in a soft picnic cooler bag or similar.