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DIY PID controller for sous-vide

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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.

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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.

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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.

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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.

19 Comments

  1. That enclosure looks ideal, is it custom made? Any chance we could get a few more details?

    Many thanks,

  2. I’ve been meaning to put together a more comprehensive post on the controller, but here are a few pointers.

    The perfect fit of the enclosure was a bit of a fluke — I ordered it before the controller I bought from eBay came from Hong Kong. It’s the larger of these from Rapid Electronics.

    I have a Dremel-like tool but in this instance I cut all the holes for the switches and connectors with a craft knife. It took perhaps a couple of hours; in retrospect a power tool would’ve been a better choice. For the front panel, the fit of the controller was perfect in the vertical direction, so I just cut two narrow strips of the panel to insert on the the left and right sides. Tightening the enclosure screws clamps the controller and the panel into place snugly.

    The original immersion heaters were from eBay and shipped from Belarus. They were positively dangerous as they put a voltage in the water that would trip up the RCD protectors in my house. I replaced the box with a taller Really Useful Box (from Staples) and got a German-made, stainless steel 1kW Rommelsbacher immersion heater.

    I’m away from home until next week but once I get back, I intend to do a new, more detailed project description with photos, shopping list and other information.

  3. That’s fantastic, thank you so much for the info! there’s plenty there to get me started.

    Sounds like you got lucky with the case. I fully intend to shamelessly copy the design, assuming you don’t mind?

    What you’ve done looks fantastic, I am quite jealous.

  4. I’ve added the beginnings of instructions to this post. As I’m several thousand miles from home at the moment, I can’t add pictures yet. Once I get home, I’ll take the controller apart and snap a few photos that should clarify the instructions. I’ll also photograph the newer, neater box design.

  5. That’s wonderful, thank you! Still at the early stages so the shopping list is incredibly useful :) I’m planning on using my slow cooker to handle the heating, at least initially, but may add the heating elements later if I find I need a larger container.

  6. I am going to build one of these contraptions..
    Has anyone found a submersible pump that will work at ~185F?

  7. The best submersible pump I’ve found is a 12V pump on eBay (search for “P-38K”). It’s rated up to 80 degrees Celsius (176 Fahrenheit) but would probably take the 85. It’s used for solar water circulation so it’s rated for continuous operation. I’ve had mine for a few months now and it’s going strong. All other (aquarium, water feature) pumps have failed. The same pump manufacturer also makes a non-submersible pump that can go up to 120 degrees Celsius (248 Fahrenheit) which you could install on the surface and run a hose into the water. The only problem with these pumps is they need 12 volts, so you’ll need a power supply.

    I find that the temperature control only needs to be exact at lower temperatures, ie. when cooking fish or meat. If I were to cook anything at much higher than 80, I would probably just leave the pump out.

    I’ve built a second one of these controllers that I think turned out much better than the first. I’ve given it away but will try to take some photos later.

  8. OK thanks MPJ,

    I will give these a try and see how conservative their ratings are..
    I am doing my rig a bit different, I am using the Camco 1KW heating element from an RV water heater and am going to build a shroud for it and have the pump push water across it.
    I am also going to have successive baths at lower temperatures so you can cook meat and vegatables at different temps and have them finish at the same time..

  9. It sounds like maybe the P-38I model pump might be better for you. It’s not submersible but it doesn’t sound like too much of a problem in your implementation.

    Your project sounds intriguing. Do send pictures! You made me wonder if I could turn a power shower into a SV cooker…

  10. MPJ,
    The more I dig into this, the more I can appreciate the Poly Science design.. The fewer components to setup and store the better. 1 in their case. http://www.polyscience.com/lab/immerc.html
    You can see their coil of heating wires wrapped around their custom made centrifugal pump/circulator, all in nice 316 SS… The beauty of purpose built hardware and the volume to support the costs of it :)
    I am just messing around with the components now trying to find the best bits for this project. I am lucky in that I have a fairly complete machine shop to fab any metal parts I need. I will send some pics when I have something worth looking at. BTW HNY!

  11. hello. how did you go programming the REX-C700 pid with the Pt100?
    Did ur unit read accurate temp on the pv wen first used or did you have to program it?
    thanks.

  12. It’s been more than a year… and I started this DIY sous-vide.
    As Hamish I have an REX-C700 and I have the same question…
    Any Wiring diagram for the REX-C700 ?
    Thanks
    Carlo

  13. Did yours not come with a manual? Mine did. The problem is that eBay is full of products sold as REX-C700 and they may have nothing in common.

    With those warnings out of the way, I’ve scanned the user’s guide of my C700 which explains the programming procedure. It’s available here.

  14. Should the pt100 sensor have 2 ore 3 wires?

    Best regards
    Mathias Slyngborg

  15. I think mine had 3 wires; I’ll check tonight. Basically you’ll want whatever your controller supports.

    In all honesty, Pt100 is an overkill. I’ve stopped using the pump (because it was the only unreliable part of the system) and so there’s a temperature gradient of a couple of degrees in the box anyway. A normal thermocouple works just fine.

  16. Okay, I hope a 2 wire will work also.. I have just bought this:

    http://www.ebay.com/itm/290470730824?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

    And the same controller you have:

    http://www.ebay.com/itm/251071743637?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

    Do you have a wire diagram or is it easy to located the output/input and where the sensor should be added?

  17. The controller you linked to says “Input: K”. That may or may not mean that it only works with K-type thermocouples. With the Chinese controllers, you can never be sure until they turn up. Some, like mine, have firmware that allows you to change the input type. Also, don’t be surprised if it looks a bit different from the picture.

    The controller you ordered has a relay output. Depending on its current rating, you may be able to use it directly instead of an SSR. Previously I have modded a relay controller to work with an SSR instead (effectively by removing the relay from the controller board.)

    The inputs should be documented with the leaflet that comes with the controller, and/or on the sticker on the back of the controller. My leaflet is here but, again, don’t assume that your controller has the same pinout!

  18. You are right.. Maybe I was a bit to fast when I bought it!

    It could be nice if it will work with out the SSR, but I still hope it has output for it so I can use the one I also have bought :-) I will use a 1kw heater

    Damn China products…

    But thx for your help

  19. hello,

    Could you add a picture showing the inside of the unit to see how you realize connections?

    thank you

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