Sous Vide temp increase causes runaway

I made a very distressing discovery over the weekend.

I had a half dozen eggs to poach. My procedure is to sous-vide at 145F for 45 minutes, cool in a cold water bath, then crack a “window” in the shell and transfer the contents via a small bowl (for a nice round shape) into boiling water for 1 minute to set the white. Usually I use the circulator, but sometimes I use the oven, set to sous vide mode and 0% steam - rear element only.

Unfortunately this time I accidentally set the oven for 140F and didn’t notice until it had reached temp and I’d put the eggs in (nicely spaced out on a wire rack). So I bumped the temp up to 145 via the control on the door and went about my business. 20 minutes later I was horrified to check the app and see that the oven was reading 163. I immediately took the eggs out and chilled but of course they were already hard-boiled. I made a replacement batch in the circulator.

But I had planned to use the circulator to make beets, which take 50 minutes at 185F. So I set the oven (by now oscillating wildly between 135F and 165F) to 185F, crossed my fingers, and waited. The oven got to 185 and seemed to stabilize, so I put the beets in. The temp dropped to about 172 from opening the door to add the food, but oddly the door continued displaying 186 - I could only see this on my phone. I reset the temp to 184 via the phone and the display on the oven started updating again. I set the temp back to 185 and went about my business. 10 minutes later the oven temperature was 213. Uh, what?

I pulled the beets out and finished them in the circulator. We used the oven to toast some bread slices at 400F, no sous-vide mode, and it worked fine. The problem seems to be specific to sous vide mode and changing the temperature while cooking.

I contacted Anova support who said to unplug the oven for 30 minutes and replug. OK, did that today. Then I set it (empty) for 145F, 0% steam, sous vide. It came up to temp fine and held between 144.5 and 145.2 for 20 minutes without a hitch. So, from the app, I set the temperature to 185F, 0% steam, sous vide.

15 minutes later the oven temperate was 229F and rising. Boggle.

Anova Support’s next recommendation was to reset the oven. I am not eager to deal with re-pairing the balky Anova with my wifi network, but OK, I will try that. I do not expect it will change a thing, however.

The firmware is up to date with 1.2.10. Has anyone else seen this, or can anyone else confirm their oven does not do this if they increase the temperature during a dry sous vide cook?

Only one data point, but I have only recently begun to use the SV mode, and it seems to be rock solid. Takes a lot longer to come up to temperature than regular mode, but I can change temps and it seems to behave.

One thing I have begun doing is turning off the light using the app, because it creates a hot spot in the oven, especially at lower temps. But I don’t think that’s what’s going on here. So, I’m afraid I’m no help.

Short answer: avoid sous vide mode at 0% humidity.

Long explanation:

The oven can do two things: add dry heat (with the heating elements) or add wet heat (steam) with the boiler. It doesn’t have a compressor, so it can’t remove heat or humidity. It also doesn’t have an exhaust fan to remove it via venting.

Sous vide mode just means to target the wet bulb temperature, that is the temperature something wet experiences, after evaporation. The dry bulb temperature (what we normally measure with a thermometer) is warmer, unless it’s achieving 100% humidity.

The empty oven is relatively dry. It’s not 0%, but pretty low (around 6% at 140F, if your room is around 50% at 70F). Because the air is so dry, you get a lot of evaporative cooling. So much that the dry bulb temperature is nearly 300F.

Then you add water to the oven. That water very quickly starts evaporating. That very rapidly increases the humidity in the oven, leading to the huge overshoot (of wet bulb temperature) you saw.

If set it it at or above the humidity it’s going to steady-state at (which will be pretty high if you’ve got a bunch of water in there), you won’t have this problem.

I’m not sure to what extent this is a weakness in Anova’s control algorithms (e.g., does it see the faster dry bulb probes plunging in temperature and start adding heat for a bit? No idea) or if it’s a physics problem that’d require dehumidification hardware to fix.