Starting a DIY sous vide cooker project

[Scarpozzi]

I'm still not a fan of cooking in plastic...I don't care if it's bpa free or whatnot. When I take foodsaver bags out of the freezer, I typically will heat the outside of the bag just enough for the food to release from the inside of the bag...then I move it to a glass container (microwave) or stainless steel saucepan (stovetop) for defrosting.

It's a neat concept to use heated water, but I would think the heat would make the plastic more likely to render toxins to the food.

 

[Howard]

It's a neat concept to use heated water, but I would think the heat would make the plastic more likely to render toxins to the food.

Which toxins?

 

[Howard]

Huh, just realized I don't actually need to use an immersion heater to heat the water. I can use the hot plate, as long as whatever I want to cook fits inside a metal pot.

sweeeeeet

 

[Howard]

Question: does a device need to be plugged directly into a GFCI outlet to get leakage protection? Or can the outlet be possibly upstream and perhaps split, as well?

 

[oogabooga]

I'm not sure the answer to your question but just wanted to drop in and say good luck looking forward to more writeups.

 

[TastesLikeChicken]

Huh, just realized I don't actually need to use an immersion heater to heat the water. I can use the hot plate, as long as whatever I want to cook fits inside a metal pot.

sweeeeeet

It wouldn't work.

A hot plate would not provide the precise heat control required for sous vide. The variation of a degree or less can ruin a sous vide dish. Also you'd still need a water pump because true sous vide cookers have to circulate the water to ensure that the water maintains a uniform temp and doesn't have hot spots.

The one the OP is building has promise. I hope he keeps us updated.

 

[Howard]

It wouldn't work.

A hot plate would not provide the precise heat control required for sous vide. The variation of a degree or less can ruin a sous vide dish. Also you'd still need a water pump because true sous vide cookers have to circulate the water to ensure that the water maintains a uniform temp and doesn't have hot spots.

The one the OP is building has promise. I hope he keeps us updated.

The hot plate would be controlled by the PID. Nothing changes from the original plan except that the heat is not provided to the water directly, but through a layer of metal. EDIT: Why do I want to do this? I already have a hot plate, but I don't have an immersion heater.

And it's simple enough to drop in a small submersible pump. I already bought one.

Can anyone answer the question about the GFCI outlet?

 

[TastesLikeChicken]

The hot plate would be controlled by the PID. Nothing changes from the original plan except that the heat is not provided to the water directly, but through a layer of metal. EDIT: Why do I want to do this? I already have a hot plate, but I don't have an immersion heater.

And it's simple enough to drop in a small submersible pump. I already bought one.

Can anyone answer the question about the GFCI outlet?

That might work then but I'd be concerned that residual heat from the hot plate as well as a lag from the hot plate heating back up would not provide the precise control necessary for sous vide. Let us know how it works out for you though. If it does work I might give it a shot with an induction hot plate.

 

[Howard]

That might work then but I'd be concerned that residual heat from the hot plate as well as a lag from the hot plate heating back up would not provide the precise control necessary for sous vide. Let us know how it works out for you though. If it does work I might give it a shot with an induction hot plate.

I'm not at all worried about the thermal inertia of the hot plate. The current is what heats it up, and once the relay shuts it off, the element certainly won't continue to heat up. If you're talking about the element staying hotter than the water, I don't think that would be the case since at steady-state operation (when cooking will take place) the element and the water will be at very similar temperatures.

The water beneficially acts as a large buffer, too; even if the pot soaked up the heat from a hotter element, the water wouldn't change much at all.

 

[Howard]

need an answer on the GFCI question

Also, how can I make sure that the GFCI outlet doesn't trip during cooking?

 

[TastesLikeChicken]

I'm not at all worried about the thermal inertia of the hot plate. The current is what heats it up, and once the relay shuts it off, the element certainly won't continue to heat up. If you're talking about the element staying hotter than the water, I don't think that would be the case since at steady-state operation (when cooking will take place) the element and the water will be at very similar temperatures.

The water beneficially acts as a large buffer, too; even if the pot soaked up the heat from a hotter element, the water wouldn't change much at all.

I'm not sure the element and water will be at similar temps. For example, heat some oil up to 375F and then measure the heat of the metal in the pan transfering the heat to the oil. It will be much hotter, by 25F or more. At a lower temp the gradient won't be nearly as significant but there will still be a difference. A large amount of water will provide somewhat of a buffer to help, but by how much?

What I'm saying is that with a hotplate you may run into variations in temp as much as 2 or 3 degrees. For most cooking methods that would be inconsequential but for sous vide it could have a significant impact.

I hope it works for you but I have my doubts. imo, you'll be lucky to maintain temps within a 1 - 2 degree window insead of the tenths of a degree that sous vide cooking requires.

 

[Howard]

I'm not sure the element and water will be at similar temps. For example, heat some oil up to 375F and then measure the heat of the metal in the pan transfering the heat to the oil. It will be much hotter, by 25F or more. At a lower temp the gradient won't be nearly as significant but there will still be a difference. A large amount of water will provide somewhat of a buffer to help, but by how much?

I'll agree with you there - when heating up a mass from the outside, you can have significant temperature gradients. But we're talking steady-state; the PID kicks in when the water drops in temp by 0.5C at most... why would the element cause noticeable overshoot?

 

[SZLiao214]

Just looked up the price of a low end Sous Vide machine. $400+!

Good luck with your project!

 

[TastesLikeChicken]

I'll agree with you there - when heating up a mass from the outside, you can have significant temperature gradients. But we're talking steady-state; the PID kicks in when the water drops in temp by 0.5C at most... why would the element cause noticeable overshoot?

imo, it's because you have an extra layer of metal (the pot) between the element and the water. There will be a heat transfer lag between the element and the water because of that extra layer of metal so by the time the water reaches the target temp there is still quite a bit of additional thermal energy stored in the pan bottom that will cause the water temp to overshoot.

You may be able to mitigate that somewhat by your choice of metal in the pan and the type of pan bottom. A thin-bottomed, aluminum pan would probably be the better option since aluminum dissipates heat more rapidly than steel or stainless.

 

[Howard]

imo, it's because you have an extra layer of metal (the pot) between the element and the water. There will be a heat transfer lag between the element and the water because of that extra layer of metal so by the time the water reaches the target temp there is still quite a bit of additional thermal energy stored in the pan bottom that will cause the water temp to overshoot.

You may be able to mitigate that somewhat by your choice of metal in the pan and the type of pan bottom. A thin-bottomed, aluminum pan would probably be the better option since aluminum dissipates heat more rapidly than steel or stainless.

Water has the same (or higher) specific heat capacity (by volume) as stainless steel, and almost twice that of aluminum.

Thermal conductivity could be an issue, but with the thin-ness of the metal we're talking about here, I doubt it.

 

[TastesLikeChicken]

I had a thought earlier. What if we could remove the necessity of controlling the heat so precisely out of the process? So I came up with this idea:

Use a reservoir for heating the water. It could be a large sauce pan or a stock pot. A hot plate or even the stove could be used as the heat source.

The sous vide basin would be a seperate container.

A tube from the reservoir would run to an external pump with an exhaust tube going to the sous vide basin. Another tube (an overflow/return tube) would would exit at a few inches from the top of the sous vide basin back to the reservoir. Only the thermocouple would be placed in the sous vide basin.

The PID would be wired to the thermocouple and the external water pump.

Heat the water in the reservoir (monitored by a plain old kitchen thermometer) to a few degrees higher than the target temp of the sous vide basin. Initially it would probably be much higher to bring the water in the sous vide basin up to temp and then would be adjusted down to a constant temp only slightly higher than the sous vide target temp.

The thermocouple monitors the basin water temp and kicks on the pump when the temp is below the threshold. Hotter water is pumped from the reservoir into the basin to warm up the water until the target temp is reached. At the same time, water from the basin is returned to the reservoir through the overflow hose until a water-level equilibrium is reached after the pump shuts off. The force of the water coming from the pump creates its own circulation so there really isn't a need for a circulation device in the basin. Temps could be controlled very precisely.

imo, it simplifies things somewhat and has the advantage of a safety factor. Wiring is much simpler. There is no chance of heating elements burning outand requring replacement. The only electrical device touching water is the thermocouple so that reduces the potential electrocution factor.

Any critiques?

 

[Howard]

In theory it might work - but it's overcomplicating things, and probably isn't any better than using an immersed heating element. An immersed heating element almost immediately drops back in temp to that of the water, and so overshoot is of little concern if the circulation is good and the PID is adjusted properly.

Leaving out factors such as the inaccuracy of the probe and whatnot, the easiest way to increase the performance of an SV system is to increase the thermal inertia of the system (e.g. use more water) and ensure maximum temperature homogeneity (e.g. circulate the water faster).

 

[TastesLikeChicken]

In theory it might work - but it's overcomplicating things, and probably isn't any better than using an immersed heating element. An immersed heating element almost immediately drops back in temp to that of the water, and so overshoot is of little concern if the circulation is good and the PID is adjusted properly.

Leaving out factors such as the inaccuracy of the probe and whatnot, the easiest way to increase the performance of an SV system is to increase the thermal inertia of the system (e.g. use more water) and ensure maximum temperature homogeneity (e.g. circulate the water faster).

I agree that it's overly complicated by creating the reservoir. Eliminating what I saw as one problem created other issues. Fortunately, as I was looking at the various parts to build my design I ran across a relatively cheap little component that takes the need for a heated reservoir out of my design. I've redesigned it now so all of the electrical components, except for the thermocouple probe, are still separate from the sous vide basin and there will also be a constant circulation.

I've already ordered the parts so once it's finished I'll let you know how it works out.

 

[Howard]

I agree that it's overly complicated by creating the reservoir. Eliminating what I saw as one problem created other issues. Fortunately, as I was looking at the various parts to build my design I ran across a relatively cheap little component that takes the need for a heated reservoir out of my design. I've redesigned it now so all of the electrical components, except for the thermocouple probe, are still separate from the sous vide basin and there will also be a constant circulation.

I've already ordered the parts so once it's finished I'll let you know how it works out.

:wub:

 

[Fritzo]

Why.

 

[TastesLikeChicken]

Why.

Why not?

It's a potential business opportunity as well. The design I have in mind could potentially be made with less than $100 in parts, labor, and overhead. The cheapest competition right now sells theirs for $400. Even if only a $100 profit per unit could ultimately be realized, selling a 100 or 200 a month wouldn't be bad business. Its worth the initial investment.

It's pure food geek fun too.

 

[Howard]

gettin' closer

EDIT: And yes, a GFCI outlet will protect everything downstream

http://pics.bbzzdd.com/users/Yucky/SV_schematic_05-12_R2.png

 

[Howard]

It works! I was setting up a loop with a hair dryer, haha.

Now I just gotta find a hot plate...

http://pics.bbzzdd.com/users/Yucky/IMG00043-20110513-2336.jpg

 

[TastesLikeChicken]

Nice job.

I have almost all of the parts in for my design except for the circulation pump. I'm having a hard time finding a pump that is rated for constant duty, can handle the temps required, and doesn't cost an arm and a leg. We are in the process of buying a house too so it's put a crimp in my tinker time. Once we finally move I'll have some time to complete everything.

btw, what I'm using to heat the water is one of these:

http://www.amazon.com/Kats-13150-Alu.../dp/B000BQUUR6

 

[Howard]

can handle the temps required

Put the pump on the inlet side of the heater so that the max temp would be the highest SV setting - probably 165F or so.