On request of a co-worker I have made an article on my Sous vide experiments.

Last week I tried to prepare a piece of pork tenderloin. I cooked it sous vide between 55.7 and 56.1 degree. Just by using a thermometer and a pan on the stove. After some time I found out I needed a burst of 7 seconds to get the water 0.4 degrees warmer. The tenderloin was already vacuum formed @ the supermarket. After 45 minutes (I know it was too short) I removed it from the “sous vide” and I seared the outside to make it nicely brown and crisp. Then I wrapped it in aluminum foil to let is settle. WRONG, should not have done that! I was used to let the meat settle, but you should not do that with sous-vide cooking. I should have put it on the plate immediately and served it.

Being tired of keeping the temperature at a constant temperature, I will try to make a device which can do the trick automatically and within a 0.2 degree window. There should be no overshoot which I do see happening in other projects, but that is just clever software. The software should be able to calculate the demands in power of the used rice cooker. Every time it should be calibrated because the amount of water and the meat have a specific heating curve.

Some requirements:

  • Dirt cheap
  • It should be electrically safe, we do not want to get electrocuted
  • Easy of use, fire and forget
  • An alarm in case I do forget 😉
  • Control temperature within 0.2 degrees
  • Temperature measure device should not touch the cold meat
  • Keep the water moving so it heats uniform
  • Not a lot of mess with cables and stuff
  • The used rice cooker should also function as: a rice cooker
  • Maybe insulate it to preserve energy
  • Controllable with an app 🙂

I want a revenge making the perfect pork tenderloin. Also I would like to make the perfect egg (whites are solid, yellows are liquid). Other recipes will follow (in case of success of their predecessor).

Things I have ordered so far:
A temperature sensor DS18B20 (0.5 degree precision).
This is a component I already used and source code is widely available for it. Now I found a cheap one which also has the needed isolation.
A datasheet is available at http://datasheets.maximintegrated.com/en/ds/DS18B20.pdf
Precision is not that accurate, but I think it will do for the goal. Maybe calibration can overcome the lack of precision.

Then we need to switch the power. A SSR (Solid State Relay) is good for the job.
I found the Fotek SSR-25DA.
Euro 6,95: https://www.hackerstore.nl/Artikel/383
Euro 2,92: free shipment, picture shows plastic cover, 18 times a 5 stars rating: https://nl.aliexpress.com/item/1pcs-SSR-25DA-25A-Solid-State-Relay-Module-3-32V-DC-Input-24-380VAC/32699692440.html
Euro 1,98: Heatsink: https://nl.aliexpress.com/item/1pc-Black-Single-Phase-Solid-State-Relay-SSR-Aluminum-Heat-Sink-Dissipation-Radiator-Newest-without-relay/32668968586.html
Here is a little more info on the SSE-25DA.
The PCB looks like this:

According to the datasheet, the device contains a switching mechanism based on a Thyristor. The Thyristor is controlled by a optocoupler which has an internal “Zero cross cirquit”. This cirquit takes care that the thyristor is only switched at the zero crossing of the AC. Once a Thyristor is activated it keeps actiated until the next zero crossing. This means that you can only use the device with an AC load. Ofcourse you can switch on an DC load, but you can never switch-off the load.
When the Triac has to switch inductive loads the voltage and the load current are not in phase. This difference in phase makes the Triac tend to stay in conductance. To circumvent this there is a RC Snubber Network on the output side. This is composed of a RC (Resistor Capacitor) network which is placed in parallel with the Thyristor. It takes care of switching-off the Triac in a timely fashion.
Inside the device there is a LED to notify that the device is activated. An optocoupler makes it a galvanically isolated device. The quality of the isolation is determined by the gap distance between the low voltage side and the high voltage side. The quality of the sealing by a resin will determine how well the device works when exposed to a moist environment. I am curious if the device from China is the real deal and is capable of meeting the spec. I will test it to the best of my abilities to determine its safety/fitness.
In the datasheet there is no mention about any cooling needs for te device. I will do some measurements of the temperature under different loads.

Open questions

What about the weakening stuff inside the plastic? Does a vacuum machine suck it out? Which types of plastic to use? All questions I did not find a definitive answer on. It looks that PET is safe, they say. Also, do I need to have it vacuum sealed or can I use a zip bag . ..