hot air sous vide revisited

6 years ago

last modified: 6 years ago

Hi All, has anyone seriously revisited hot air sous vide? I saw one discussion here but it got derailed. I've made both a water and a hot air version, and getting the timing right for air has been interesting (every recipe is water based). I mean getting the food internals up to temp takes longer w/air, but once it is there, the systems are virtually identical.

The results w/the water one were perfect on the first try, but I'm still seeing what the air version can do, as it is more convenient (sans extra cooking time and lack of research on it). I even tried using the air one as a dehydrator (breakfast sausage @145 overnight) and it worked well, and any moisture proof conformal wrap will do for the "vide" part.

anyway here are some pics, the water one works perfectly fine with a bang-bang controller (lots of mass) but I had to make a pid controller to get the air version to behave.

Any thoughts on how to accommodate the timing differences?

more build details:

for water I just use an old plastic cooler, an $8 1000 watt immersion heater, a couple small $5 fish pumps, and an inkbird ITC-1000f that I got for $12. And even with the bang-bang control, it works really well. All that mass makes for a very stable temperature, no pid required (unless your heater is REALLY overpowered).

for hot air I wound up with a styrofoam cooler, a trimmed down space heater (mods: was 2x750watts, put them in series for 375 watts, shaded pole and metal fan ftw, and reworked the motor/fan setup to get it as quiet as possible and reversed the motor and spun the fan around so it blows hot air at the wall for circulation), and a thermistor out of a $1 thrift store thermometer, and a microcontroller with PID, and a 20A ssr (doesn't even get warm), duty cycle is the control variable with a period of 1 second.

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Comments (4)

Islay Corbel
6 years ago
Never heard of hot air versions. What are the benefits?
davebdave
Original Author
6 years ago
last modified: 6 years ago
Hi Islay, fwiw the first attempts at low temp were done w/hot air.

I will take a stab at enumerating the pros/cons from my limited experiments.

Air pros:

1. don't need to fill with water (more of a problem with a cooler than a pot) and shlep. Also better for water conservation if you are into that, or for making your operation more portable.

2. in water the bag is there to prevent exposure to water, as it will dilute the flavors. In air you are trying to retain moisture, so you can use saran wrap, oiled parchment, probably foil, etc. Air is more flexible there. You still want it to catch the juices though, so don't put the seams on the bottom.

3. preheat is nearly instantaneous since air is low mass, I don't even bother pre-heating, just drop the food in and plug it in, though heater power is a factor in pre-heat time. Less energy spent on the heat transfer medium only to pour it down the drain.

4. doubles as a precision dehydrator.

Air cons:

1. The specific heat of air is much less than water. For a given set temperature it takes longer for the heat to be absorbed by the food to attain that temperature.

2. needs more complicated control scheme as temperature can change quickly, though this is a solved (affordably) problem.

3. very few people experimenting with it, and almost every recipe is geared towards a water bath these days, so while the temperature recommendation holds, the timings are way off.

4. Longer cook times will mean more energy spent on heating the food product (lots of variables).

It is an interesting experiment in thermodynamics in any event. That is probably my main motivator :)

edit: this site describes the heating difference pretty well
https://amazingribs.com/more-technique-and-science/grill-and-smoker-setup-and-firing/thermodynamics-cooking-and-how

After all your hand is made out of meat.
"Put an empty frying pan and pot of water in a 200°F oven for an hour. Open the door and stick your hand in the warm air of the oven and count how long you can hold your hand in there. Perhaps a minute? Then plunge one hand in the 200°F water and place the other hand on the frying pan. After you get back from the hospital, ponder the fact that not everything that is 200°F transfers energy at the same rate."
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dcarch7 d c f l a s h 7 @ y a h o o . c o m
6 years ago
Very interesting setup and experiments.

Sous vide using air, it's about the same as modern convection oven with digital temperature control.

Yes, air sous vide's biggest problem is getting the food to set temperature, especially with larger piece of meat from the refrigerator. The way to overcome that is to microwave the food at very low power until the temperature gets to almost set temperature. Once it is there then the cook time will not be that much longer then sous vide with water.

I see your set up, using a controller similar to the Romeo-All in one Arduino Controller, driving a SSR to bang-bang control a stove heating element.

My suggestion: possible serious fire hazard. If one of the components fails, the heating element can quickly start a fire inside the plastic foam box. I would use a mechanical relay instead of a SSR. When the system fails, a SSR shorts and continues to feed full power to the heating element, the opposite to a mechanical relay.

2. For water sous vide system, you need a very low power heating element. Either series connect a diode ($2.00) to the heating element and cut the power in half or buy a 220v water heating element ($10). A 2000 watt element becomes 500 watts when voltage is cut in half.

dcarch
davebdave
Original Author
6 years ago
last modified: 6 years ago
Hi dc! microwave is a good tip, I will have to try that perhaps, don't have a proper convection oven, but if the two play well together I may re-evaluate.

re: 2 With a cooler half full of water, bang bang seems to work just fine with the 1000 watt heater, but yes if you reduce the water(mass) or increase the heater power you need better control, which is simple enough.

the air one is a custom script on an arduino with PID logic added (and a timer) in a script I slapped together, after sorting out the thermistor. the water one is just a bang-bang, and I went the pid route when I saw how much fluctuation there was using bang-bang with air. Though no reason not to retrofit the pid to the water one.

re 1: fire hazard, granted this isn't "consumer grade", one of reasons I put the elements in series is that even if it fails on the nichrome should only get to about 450F (but it is in a heated environment so it will feed back...). It gets close to the melting temp of polystyrene, but below the ignition temp, so it should buy you some time in any event. Like halving the voltage, putting two 750 watt elements in series results in 375 watts (and 1500 watts in parallel). I don't recommend a diode though, that is taking something with fairly perfect power factor and completely ruining it. Though an ssr isn't perfect either, it is distributed through both halves of the cycle, and adding a diode may ruin zero cross detection (i.e. it gets stuck ON).

Though I wouldn't use a relay for cycling at 1hz(edit, or for controlling duty cycle, as the ssr has 120 "power" levels per second), I may add one that controls the fan which can also feed the ssr and as long as the logic is still functioning it can take appropriate measures.

but it is still proof of concept so not going to go overboard on the safety refinements without more consistent results, the air unit just sits next to my desk so it is not unattended at the moment.

hot air sous vide revisited

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Islay Corbel

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hot air sous vide revisited

Comments (4)

Islay Corbel

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