The electrical cost of doing sous vide

[dmg]

Hi everybody,

Having bought a Sous-vide Professional (svP), I was immediately curious of how much it would cost me to run it.

I used an aluminium, 8l pot (nothing fancy), filled it with running water, and set the water temperature for 56 degrees and

cooked a steak for 12hrs.

The svP started heating the water at approximately 1kVA. Once the water reached the desired temperature, it dropped to approximately 100VA. It took 1.62 kWH to cook the steak (12 hrs--my first).

The measurements were made with a killAwatt.

The cost of electricity where I live (British Columbia) is a variable rate, with 8.27 cents/kWH at the upper tier. Hence, the electrical cost of cooking my steak was C$0.13. Significantly less than the bag where it was cooked (Foodsaver).

Of course, the numbers will vary significantly depending on weather (the current temperature in my kitchen is 18 degrees) and amount

of food. But after this experiment I know I don't have to worry about the cost of electricity any more

--dmg

[rotuts]

you will do even better if you use an insulated container

I use a coleman cooler 38 qts or so with the SV magic and cover the top which is not insulated with multi layered blanket.

it holds easily 15 + lbs. so i do a couple of turkeys ( boned out ) or a lot of beef etc.

even better my kitchen stays cool in the summer !

[Chris Hennes]

During the summer I actually run my SV rig (based on a Coleman cooler) out on the back patio. It rarely rains here at this time of year, so this works even for very long cooks. As a bonus, the starting temp is over 100°F... turning the SV machine on does not even show up as a blip on my energy usage, at least according to our power monitor. It's a far cry more efficient than the stove or oven (plus it doesn't heat the house).

[Lisa Shock]

You can always get a device like a Kill A Watt electricity usage monitor to find out exactly what's going on.

[dmg]

You can always get a device like a Kill A Watt electricity usage monitor to find out exactly what's going on.

I used a killAwatt to do the measurements. The other advantage of the killAwatt is that it has a timer. You not only know how much electricity is used, but also how long it has been on.

--dmg

[rotuts]

the next time I SV two turkeys Ill plug in my Kill-a-watt.

[PedroG]

@rotuts:

I wonder whether your Kill-a-watt will be able to measure the output of your SousVideMagic correctly. During ramping up at full power it may be OK, but at steady state with very short "blinks" it might fail as my Brennenstuhl PM230 energy cost monitor did, see my post in the old sous vide topic.

@dmg:

Your 8L pot with 100W at steady state compares with a FreshMealsMagic in its uninsulated 18L polycarbonate container, see ibid.

A customer review of the Kill-a-watt says:

The LCD display on this unit does not display the real-time power draw--but rather a snapshot about every second. So, it is impossible to know exactly what the peak draw is when my flashes fire.

So I wonder if the Kill-a-watt would give correct measurements with a PID controller, but I don't know if immersion circulators like the SVP have sophisticated controllers with solid state relays like the SVM, or rather bang-bang-control.

In fact, energy consumption of sous vide cooking is not of much concern, but if you do mind, you might insulate your pot, e.g. with bubble wrap, cutting steady state consumption by a factor 2-4.

[Paul Kierstead]

The SVP appears to use PWM to control power just like the SVM. It is pretty standard for industrial controllers.

[rotuts]

thanks PedroG

always glad to hear from you, as you are on top of things.

thats an excellent point re K-a-W and SVM

[dougal]

The SVP appears to use PWM to control power just like the SVM. It is pretty standard for industrial controllers.

I wouldn't strictly call the way that most of the little PID temperature controllers work "PWM", since the 'pulse' generally consists of multiple AC mains cycles.

They work on a time window of something like 2 seconds, and power on for a variable percentage of that window. (Many SSRs will only switch on the zero crossing, thus quantising the available power output in 1/100th of a second increments {1/120th second for the US with 60 Hz mains}, hence a 2 second window gives my system a 200 {US 240} step output control.)

Some PID controllers can display the output percentage - its an undocumented feature (in the good sense) of my N2006P, obtained by a long press on the Set button.

Watch it when the bath is 'stable', and decide what the typical value is.

My bath (with an 1800 watt element) runs about 8% at 55°C, so its consuming power like a 144 watt constant heater.

Insulating a waterbath will indeed reduce its energy consumption, BUT, by reducing temperature extremes it also reduces the vigour of the (natural) convection currents - and so an insulated bath will particularly benefit from assisted circulation.

A newly insulated bath will heat faster and cool slower, so your PID will probably want different settings for optimum control - retuning should be worthwhile.

But if your heater is much more powerful than the insulated bath needs (so that the %-on-time would become very small), then even with forced circulation, control becomes a very difficult task.

[rotuts]

Ive noticed in my SVM system while it gets to temp. ( and I add hot water to get it close ) it does oscillate

over under by a few degrees then settles down

so i need to change my PID settings?

PedroG is an expert at this and then is SVM folks themselves.

cheers

[blackp]

Ive noticed in my SVM system while it gets to temp. ( and I add hot water to get it close ) it does oscillate

over under by a few degrees then settles down

so i need to change my PID settings?

PedroG is an expert at this and then is SVM folks themselves.

cheers

On the surface of it, it does seem that your set up needs some tuning. It sounds like you have too much Integral ("I") action happening. When "I" is too strong oscillation is the usual outcome as the reaction to being off temp is being corrected to quickly. Try increasing the "I" value (yes this is counter intuitive, but the parameter is for the number of seconds to take before reacting so a larger number results in a smaller effect).

Have you read the PID Tuning Giude on the FMS site?

Keep your eyes peeled there's a version 2 which should be published very soon.

Cheers,

Peter.

[PedroG]

rotuts:

Before changing I (Integral) and D (Derivative) values, you should rule out a too narrow P (Proportional band), so you might set I=0 and D=0 and check whether you have oscillation, see here. If there is no oscillation in P-mode, too strong I and D may cause oscillation. Before changing I, try reducing Ar (Anti reset windup) from the factory-set 50% to maybe 25% or 30%, this often helps.

In the new version of the PID-tuning guide (which will be published soon, hopefully) there will be a chapter on Ar:

Ar: Integral Limit (0-100%) aka Anti - reset windup

“Integral” is used synonymously with “reset”.

What is Integral windup (= reset windup) ?

The integral function adds up (integrates) temperature deviation from SP as the area under the curve. With increasing integral, the integral function adds more output to the output allowed by the proportional function to help in counteracting negative offset. As long as the temperature is below the proportional band (e.g. during ramping up from ambient temperature to SP), the proportional function allows 100% output, and the integral function increases output to more than 100%, but more than 100% output is physically not possible and temperature rise cannot get faster as intended by the integral function. So the integral term gets larger and larger (it winds up), and when SP has been reached and the output by the proportional term would be zero, the integral term still creates output, leading to temperature overshoot, until the integral term has been unwound.

How is Integral windup avoided?

Modern PID-controllers shut off integral action as long as the temperature is outside the proportional band. This is called “Ar” or “Anti - reset windup” or “Anti - integral windup”.

Despite this Ar function, integral windup may happen even at temperatures within the proportional band if integral time is set very short (strong integral action) and/or if derivative action adds additional output during disturbance (e.g. by adding a pouch with cold food).

The SousVideMagic 1500D has an advanced Ar function which allows to restrict integral action to the upper range of the proportional band, thus obviating the need to make otherwise adequate integral and/or derivative weaker.

For example:

SP=55°C

P=10°C

I=900sec

D=100sec

So the proportional band would be 45-55°C.

With Ar=100%, integral action would be active between 45-55°C.

With Ar=30%, integral action would be allowed only in the upper 30% of the P-band, i.e. between 52-55°C.

What is the advantage of reducing Ar below 100% ?

The ex-factory default setting is Ar=50% . This allows for strong integral and derivative settings for fast disturbance recovery without having to accept overshoot after ramping up or after a disturbance load (adding cold food).

Summary

Sometimes with an “I” value that adequately compensates for negative offset, “I” action may be too strong especially in response to a disturbance (temperature drop) and lead to overshoot and eventually oscillation. Instead of making “I” weaker (increasing integral time), “Ar” can be set to a value lower than 100%, thus restricting the integration of negative offset to the upper range of the proportional band, e.g. setting “Ar” = 30% with a P-band of 10°C and SP=55°C would allow integration of negative offset only between 52-55°C instead of 45-55°C, i.e. the upper 30% of the P-band. In the lower range of the P-Band, output power allowed by the proportional function may be sufficient to counteract disturbance without the help of the integral function, but in the upper range of the P-band, the proportional function reduces output so much that additional output power must be contributed by the integral function.

[Shalmanese]

It doesn't take much energy at all to keep a pot of water at temp. I can keep a pot at a low simmer on a gas stove with a flame that isn't much larger than a cigarette lighter.

[dougal]

It doesn't take much energy at all to keep a pot of water at temp. I can keep a pot at a low simmer on a gas stove with a flame that isn't much larger than a cigarette lighter.

Indeed, but the question of just how much (or how little) is actually used becomes relevant because of the much longer cooking times that are often involved.

[Paul Kierstead]

I wouldn't strictly call the way that most of the little PID temperature controllers work "PWM", since the 'pulse' generally consists of multiple AC mains cycles.

The pulse is the controlling signal to the SSR.

They work on a time window of something like 2 seconds, and power on for a variable percentage of that window. (Many SSRs will only switch on the zero crossing, thus quantising the available power output in 1/100th of a second increments {1/120th second for the US with 60 Hz mains}, hence a 2 second window gives my system a 200 {US 240} step output control.)

That still describes PWM pretty much perfectly. The fact that the pulse is turning on and off an AC relay and has limited resolution (and accuracy) is really here nor there. PWM is used to control a very wide range of things; we don't consider it not PWM just because it cascades to something which isn't DC. And, of course, the heating element basically integrates (in essence) the (absolute value of the ) signal anyway, so you if you want to look down system, you don't have to stop at the SSR.

[PedroG]

To answer the initial question of the electrical cost of SV cooking, actual power measurements are not necessary, it can be calculated easiliy:

Multiply the steady state consumption by the hours of cooking, and add the energy to bring the water from ambient to cooking temperature.

Examples:

• FreshMealsMagic uninsulated, about 100W for 48 hours makes 4.8kWh. Heating 15L water from 20°C to 55°C costs 15kg * 35°C * 1.163 Wh/kg/°C = 0.593 kWh. Total about 5.4kWh.
  
• Tiger rice cooker 12L (10.5L water), about 26W * 48h = 1.25kWh, plus 10.5kg * 35°C * 1.163 Wh/kg/°C = 0.43kWh, totalling to about 1.7kWh.
  

For the price difference between a rice cooker and an FMM (or a stock pot BTW), you can cook many thousand hours with an FMM or a stock pot.

The table is from my earlier post.