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Sous Vide: Recipes, Techniques & Equipment (Part 5)

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#331 nathanm

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Posted 25 April 2009 - 10:03 AM

This is an important point - the amount of time it takes to autotune can be very long because it must go through a couple cycles of heating and cooling to measure overshoot and time response.

If you change the set up (different heating element, different volume) you must retune.

That is another reason to prefer continuously tuning PID controllers....
Nathan

#332 Robert Jueneman

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Posted 25 April 2009 - 10:43 AM

Nathan, thank you for your always informative posts.

When I initially ran the autotune function on my SVM 1500A overnight with a 1500 watt, 10 liter commercial rice cooker, the resulting parameters were P=15, I=998, and D=499. Now, that means that it is going to put out full power until it is within 1.5F of the target temperature, and then integrate and differentiate the results to the max. Because the results were so far to the extreme, I simply discounted them as not realistic, and didn't actually try them.

I was about to re-enter those values and try them, but I can't! The maximum setting I can enter manually for the I value is 900, and the maximum setting for D is 300. Strange.

I'm about to go buy a USB to RS-232 adapter for my data logging thermometer -- the Belkin unit I have won't work with my Vista laptop because of driver issues(??), and running XP under VMWare on my Mac Pro is too much bother. But I should be able to automate some of this testing this weekend.

BTW, to the best of my knowledge, the Sous Vide Magic and the Auber units are identical except for the name, although there may be internal differences.

e_monster, when you calibrated your Auber unit, what type of rice cooker were you using, and how large was it?

#333 Robert Jueneman

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Posted 25 April 2009 - 10:58 AM

Cooking Vegetables

I think I have a reasonably good understanding of the science of meat cooking, the opposing forces at work, and why sous vide gets such great results.

But I really don't understand what happens when you cook a vegetable, and what the time/temperature relationship is.

Why do some vegetables require 185 or even 195F, while others are best at 165F (at least according to Thomas Keller).

How long would it take to "bake" a sous vide potato in a 131F bath? Would it work at all? I assume the same issues regarding pathogens would still apply, as they do in meat?

#334 tomdarch

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Posted 25 April 2009 - 11:51 AM

A quick, minimally informed response: cooking veggies is quite different than cooking meats. In meats (and eggs) a lot of the "cooking" is denaturing proteins, and otherwise breaking down connective tissues. In veggie cooking, you're breaking down very different components. I'm sure that proteins play some role, but to a much lesser degree. The vegetable tissue is also the "bones" of the plant, so they are very different than animal muscle/connective tissue. I remember that pectin is one of those components, but I can't remember if it's "cook temperature" is above or below 85C. The idea is that you are breaking down some components, but leaving others in tact. I suspect that "exploding" cell walls is also a factor, but I'm not sure specifically how. There are also a complex set of chemical reactions that happen when different vegetables are cooked, and sous vide can control which of these do or don't happen. Lastly, cooking in a bag prevents a lot of flavors and nutrients from being diluted out into the cooking water.

Because the "cooking" of vegetables is so complex, it makes sense that you would see different temperatures ranging from 165F/74C to 195F/90.5C. It's a question of what's in the particular veggie and what do you want to do to it? (I wish I knew exactly....)

Vacuum "compression" is a whole different, and interesting, set of issues.

So far, I have done potatoes and carrots (around 85C - roughly, because I was "tending" a pot on the stove) for about 45 minutes. The were roughly 1cm cubes, bagged with butter, salt and pepper. The result was definitely "cooked", and consistently cooked all the way through. But unlike fully boiled veggies, there was still a small amount of crunch. This was unlike a rapidly blanched carrot, which may be overcooked on the outside and still semi-raw in the middle. It was a good, interesting way to cook veggies, but not mind-blowing. I'm planning on trying parsnips next...

Here's an article by the Ideas in Food folks about sous vide veggies:
http://www.popsci.co...01/shades-green

#335 e_monster

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Posted 25 April 2009 - 03:03 PM

When I initially ran the autotune function on my SVM 1500A overnight with a 1500 watt, 10 liter commercial rice cooker, the resulting parameters were P=15, I=998, and D=499.  Now, that means that it is going to put out full power until it is within 1.5F of the target temperature, and then integrate and differentiate the results to the max.  Because the results were so far to the extreme, I simply discounted them as not realistic, and didn't actually try them.

I was about to re-enter those values and try them, but I can't!  The maximum setting I can enter manually for the I value is 900, and the maximum setting for D is 300.  Strange.
.......
BTW, to the best of my knowledge, the Sous Vide Magic and the Auber units are identical except for the name, although there may be internal differences.

e_monster, when you calibrated your Auber unit, what type of rice cooker were you using, and how large was it?

View Post


This was with a 22 quart Hamilton Beach tabletop roaster -- which is almost a worst case device for these PIDs because of the huge latency in heating due to the double-walled nature. So, I was pretty pleased that it came up with rock solid settings. (There is a small amount of initial overshoot that settles down quickly). Oh, for auto-tune I used room temperature tap water -- in normal use, I start with water that is already about 130F.

Even if the numbers looked weird to you -- it would be interesting to know if they worked. In my opinion, it is worth trying the settings because you might be pleasantly surprised.

The numbers I got were weird but they worked. They were similar to what you got (different P but the I and D values were the same -- which are the maximum allowed values). I suspect that there are different settings that may also work and look less weird -- but they worked for me so I was a happy camper.

The settings it came up with I couldn't re-enter by hand -- so I re auto-tuned and it came up with the same results. This was before I wrote to Auber to find out why I couldn't enter them manually. If I have to re-enter them again, I will take the advice offered by Auber which I provide below because others may also find it helpful.

Suyi is responding to a message in which I mentioned that auto-tune came up with settings that worked well but couldn't be entered by hand.

We limit the I to 900 because the controller can't display more than 999. In addition, when the I>600, the integration function is not really meaningful when we use a cycle rate of 2 second. The difference between 900 and 998 is very little. What you can try is set I=0. That will remove the I function. If that is not as good, reduce the P by 50% each time. We found using PD instead of PID also works better for large cookers. Based on the PID control theory (developed in 1940 but still the most commonly used), the D should normally about 1/4 of I. That is why the D is also limited



#336 blackp

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Posted 27 April 2009 - 08:42 PM

Oh dear. The more I read the more I know for sure that I don't know!

I'm wondering if I am being over cautious or am I correct in attempting to get the tuning of my SVM PID stable before cooking anything for a long time?

With a few hours to kill yesterday and still waiting for the data logger to arrive I filled the 12litre Tiger to about 9 litres - which is about 6cm or 2.5 inches below the top allowing for the addition of some food with a margin for error. (BTW anyone who says the Tiger JNO-(A/B)360 is 12l is correct, but you couldn't fill it with 12 litres of water and put any food in it as well).

The water was from the hot tap and was about 60C when added to the Tiger.

I allowed the Tiger to stabilise for about 45 minutes so that any error from the "cold" Tiger and the hot water would be minimalised.

When I powered up the SVM and Tiger the starting temp of the water was 49.6C. From that point I kept the lid closed through the whole sampling process. My intention was to wait until the temp stabilised and then to add a fake food load and watch the impact. Unfortunately I didn't get to that part of the experiment as getting to the target went beyond my bed time ;-)

After power-up I became a human data logger recording the temp from a thermocouple thermometer (not the SVM readout) every minute for the next 3 hours. (Memo to self: wait for data logger to arrive before attempting this experiment again!). After 3 hours I needed to get dinner happening so I set the kitchen timer for 5 minutes and recorded 5 minute intervals for the next 5 hours.

See Chart:
Posted Image

What I found that disturbs me was the massive initial overrun and the fact that the target temp was not achieved until about 7.5 hours after starting the experiment.

Also I cannot explain the oscillation between 58C and 56C before the temp finally reduced to near the target of 54C.

When I get some more time I'm going to try reducing the power to 50% as the Tiger is 1610W in it's 240V model.

I was going to cook some beef short ribs for 48 hours. Should I wait until I've stabilised the SVM or is it pretty safe to just give it a go?

Thanks,

Peter.

#337 blackp

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Posted 28 April 2009 - 06:57 AM

Given the "odd" outcome I wondered if anything was not right with my last experiment, so I ran a smaller one tonight.

Here are the results:
Posted Image

The only difference was that in experiment 1 I had attached the 2 temp sensors (one for the SVM and one for my thermometer) to a wire cake rack with rubber bands to keep them low in the water but off the surface. Maybe the cake rack itself gained temperature from the metal container of the rice cooker - who knows?

In test 2 I used a weight to hold the wires of the sensors down but did not let it touch the sensors themselves. The only other difference I can identify was that the ambient temp was 2C lower than the last attempt.

The difference is substantial - and a big improvement without changing any parameters.

When I get time I will play with settings to see what difference they make.

PB

#338 nathanm

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Posted 28 April 2009 - 07:17 AM

I am not sure why the difference between tests. Ambient temperature is ceratinly part of it.

In both tests you are getting a very strong overshoot, which lasts for a long time. This is not right and is something that proper tuning parameters should eliminate. You should not get this level of overshoot.

If you put an aquarium air pump and bubbler in to stir the water it will help. The air will be ambient temperature so that cools the bath down and will help reduce the overshoot. It will also stir the water around. This is only a partial solution because it looks like your PID parameters do need improving.
Nathan

#339 slkinsey

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Posted 28 April 2009 - 07:56 AM

The newer, and a bit more expensive self-tuning PID controllers will do automatic tuning at all times - instead of an autotune button, it just always watches what is going on and adjusts itself.  In general these are much better, and when I use a PID controller I try to get one of these.  However, I don't think that is the kind they use on SVM.

Any brand recommendations?
Samuel Lloyd Kinsey

#340 mtigges

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Posted 28 April 2009 - 08:39 AM

I have an equipment question, but at 80 plus pages, the discussion does not yield good enough resolution on searches. So, my apologies if this generates a little extra noise.

I'm going to step in to SV with one of the Auber units. My question though concerns the vessel that I'll be using for immersion. According to this link: aubers equipment recomendations my available option is not optimum.

I have one of these:

http://www.amazon.co...40932689&sr=1-1

I bought it because the pot can go on the stove top. I love it.

While it does have an integrated circuit board, it is nothing more than a timer and a digital readout of the time left, or heating setting. I'm quite certain the switch is mechanical. I would have absolutely no qualms about pulling the cord out while the unit is set to low or hi. It won't remember the time, but in this case I don't care.

Aubers pdf (linked above) says that if it has a digital readout it's not appropriate for use. I have a suspicion that that's just a CYA statement. So, I'm petitioning those here for whatever experience they have with similar units.

I would even consider a little surgery on the unit to install a plug to bypass ICB that I can use with the PID when used in sv. So any advice from people who have done this something similar very welcome. I'm reticent to get another tool (rice cooker) because I have enough clutter as it is.


All opinions welcome.

Thank you.

#341 slkinsey

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Posted 28 April 2009 - 08:54 AM

That's a pretty small vessel for cooking sous vide, especially with no circulator. 6 quarts is only 1.5 gallons (346 square inches) of volume. How much do you envision cooking in there?
Samuel Lloyd Kinsey

#342 dougal

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Posted 28 April 2009 - 08:54 AM

...
While it does have an integrated circuit board, it is nothing more than a timer and a digital readout of the time left, or heating setting.  I'm quite certain the switch is mechanical.  I would have absolutely no qualms about pulling the cord out while the unit is set to low or hi.  It won't remember the time, but in this case I don't care.
...

View Post

The absolute essential is that it remembers the heat setting - or defaults to something you can work with.
The external PID will be "pulling the plug" every couple of seconds or so ...

PS - also important is that powering on/off rapidly should NEVER confuse the "integrated circuit board" - like thinking 'time left is now zero, so I'm switching off'.

Simple is what's needed!

Edited by dougal, 28 April 2009 - 08:56 AM.

"If you wish to make an apple pie from scratch ... you must first invent the universe." - Carl Sagan

#343 e_monster

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Posted 28 April 2009 - 09:41 AM

Given the "odd" outcome I wondered if anything was not right with my last experiment, so I ran a smaller one tonight.


Have you tried running the auto-tune mode? At the risk of seeming like a broken record, I recommend trying it -- you have nothing to lose and it might find settings that work. It may take 6 to 12 hours for the tuning to be completed. The worst that will happen is that you will end up with settings that don't work -- but I think it is worth a try and I think it is likely to work (based on my experience).

#344 Robert Jueneman

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Posted 28 April 2009 - 09:56 AM

blackp, you now understand why I broke down and ordered the data logger accessory for my thermometer!

Using a 10 liter, 1500 watt commercial rice cooker that is similar to yours, for Celsius measurements with the SVM 1500B I'm using P=100, I=0, D=0. With those settings, beginning at ambient, I find that things stabilize very nicely within 20 minutes, with no more than about 1.5C overshoot.

With the SVM 1500A or 1500C, which offers 1 degree resolution, I use Fahrenheit for the improved precision. However, the SVM 1500B offers 0.1 degree resolution (Celsius only), so I've been using that for better control when cooking meat.

With the above settings, the 1500B stabilizes at 0.2C lower than the set point, and never budges from there. Both the SVM and the reference thermometer agree at that point.

Of course the easiest thing to do would be to simply change the set point, bu I thought I would try adding a little I value to see if I could tweak it a bit. I tried I=250, but after waiting for an hour noticed that the cooker was in the Warm position, rather than Cook. Stupid human!

As I understand it, the fact that you are showing oscillations is a strong indication that the I value is too high. I read somewhere that if that occurs, you should set the I value to one half of the cycle time in seconds. If I can read your time line correctly, the period is about 4.2 minutes, so I=125 might be a reasonable starting point to try.

But first, I would try I=0.

This assumes you are not likely to add more than 1 or 2 kg of meat to the pot, or not more than 20% of the mass of water, in which case the water itself is going to do a pretty good job of integrating over time. Now, if you were adding that much to a small 1.2 liter rice cooker, that would be a completely different story, and then you would probably need a D value, but I haven't played with that yet.

I've heard people say that the sensor should be touching the bottom of the unit, but I'm not sure I would agree. It would seem to make more sense to measure the water, rather than the bottom of the pot, which is almost surely going to be fluctuating every time it turns on or off and now you've got some crazy feedback going on.

As far as cooking short ribs and other meat such as brisket or chuck for 48 hours, I think you should wait until things stabilize, or add some cold water if necessary. The long time is to dissolve the collagen, but an hour or even a half-hour (depending on the thickness) will be enough to cook it to medium, rather than medium rare or rare.

Edited by Robert Jueneman, 28 April 2009 - 09:59 AM.


#345 mtigges

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Posted 28 April 2009 - 10:42 AM

...
While it does have an integrated circuit board, it is nothing more than a timer and a digital readout of the time left, or heating setting.  I'm quite certain the switch is mechanical.  I would have absolutely no qualms about pulling the cord out while the unit is set to low or hi.  It won't remember the time, but in this case I don't care.
...

View Post

The absolute essential is that it remembers the heat setting - or defaults to something you can work with.
The external PID will be "pulling the plug" every couple of seconds or so ...

PS - also important is that powering on/off rapidly should NEVER confuse the "integrated circuit board" - like thinking 'time left is now zero, so I'm switching off'.

Simple is what's needed!

View Post


There is a mechanical switch with four settings off, low, high, timer. For this application I would leave it on high. I'm fairly certain that the ICB is run off a relay to engage the display and the timer if necessary. If I unplug, and power back on while leaving that switch in either low or high it fires.

My only concern is if the cycling will damage the ICB.

#346 mtigges

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Posted 28 April 2009 - 10:51 AM

That's a pretty small vessel for cooking sous vide, especially with no circulator.  6 quarts is only 1.5 gallons (346 square inches) of volume.  How much do you envision cooking in there?

View Post



Note that the smaller the vessel the lower the need for circulation.

The pot has excellent insulation properties. Astounding actually. When I make stock, I bring it to a boil on the stove, and then place the pot in the slow cookers base. The liquid continuous to boil for at least 15 seconds after turning off the flame on the stove.

I have no doubt that this vessel will have very small temperature differentials throughout the liquid. The element in the base is below the unit allowing for natural convection and it has an excellent heavy lid.

Water to food ratio in my machine would be significantly higher than at the restaurant at which we ate on Friday. I had a very good view of their set up and it never wavered more than one degree and they're almost constantly fishing out packages, and adding new ones. Not that the transgressions of others should provide rationale for me.

To answer your question about quantities of food. Not very much. My true desire is 48 hour shortribs. Probably only cooking for myself as my wife finds the waste of plastic distasteful. So I see myself cooking duck breast, small pieces of fish, beef and pork & chicken, when she plays badminton and leaves me to my own devices.

#347 howsmatt

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Posted 28 April 2009 - 12:50 PM

That's a pretty small vessel for cooking sous vide, especially with no circulator.  6 quarts is only 1.5 gallons (346 square inches) of volume.  How much do you envision cooking in there?

View Post


Note that the smaller the vessel the lower the need for circulation.

The pot has excellent insulation properties. Astounding actually.

View Post


I started my SV cooking with a similar device. The problem is exactly what you stated, too much insulation-therefore harder for the PID to adjust quickly and lose temp when it overshoots. I suspect that like me you will soon want to do a few more items and certainly larger ones. If you have to wait 48 hours for short ribs, may as well cook 2 batches and freeze one for later. I don't think you could even fit one of the american cut short ribs that I buy.

I went out and bought this http://www.blackandd...roduct-304.html

Cost $30 Can at walmart (although I'm sure your wife won't like walmart shopping either).

The rice cooker works great with the PID settings. I also bought an 18 quart cooker which is good too but obviously slower up and down in temp.

Best of luck.

#348 mtigges

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Posted 28 April 2009 - 04:13 PM

That's a pretty small vessel for cooking sous vide, especially with no circulator.  6 quarts is only 1.5 gallons (346 square inches) of volume.  How much do you envision cooking in there?

View Post


Note that the smaller the vessel the lower the need for circulation.

The pot has excellent insulation properties. Astounding actually.

View Post


I started my SV cooking with a similar device. The problem is exactly what you stated, too much insulation-therefore harder for the PID to adjust quickly and lose temp when it overshoots. I suspect that like me you will soon want to do a few more items and certainly larger ones. If you have to wait 48 hours for short ribs, may as well cook 2 batches and freeze one for later. I don't think you could even fit one of the american cut short ribs that I buy.


I'm pretty sure you had your pid set incorrectly. The point of it (over a thermostat) is that it slows it's approach to target. Overshoot should not be an issue if you have it set correctly. Admittedly, it can be tricky to set well. Especially with a slow cooker whose electric elements will still contain and impart energy when switched off. This is an interesting consideration that hadn't occurred to me.

Did your unit have a printed circuit board relayed to a mechanical switch? If so did the cycling damage the electronics?

I went out and bought this  http://www.blackandd...roduct-304.html
The rice cooker works great with the PID settings.  I also bought an 18 quart cooker which is good too but obviously slower up and down in temp.

View Post


28 cups to fill it to the brim? That's only one quart more (15%) in volume than mine. It's not that much difference.

The last time I braised short ribs I think I put about dozen 3" english style in there. What the hell kind of cows do you have? :)

#349 slkinsey

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Posted 28 April 2009 - 04:59 PM

Surely you don't think they started out three inches long? :smile:

A full beef short rib is more like ten inches long. Let's say an inch wide and an inch and a half thick (both measurements being on the small side of normal) and you've got fifteen cubic inches per short rib. A dozen of those equals 180 cubic inches. 28 cups is around 400 cubic inches, so that leaves only 220 cubic inches (around 15 cups) of water to regulate the heat. And this is without any forced circulation, so there are hotspot issues of concern with this food-to-water ratio as well.
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#350 mtigges

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Posted 28 April 2009 - 06:38 PM

I don't think so. Conductively, the difference between the meat and the water is very small. The thermal sink should be nearly identical. It matters for when you're bringing the meat up to temperature for sure, but for 48 hours to melt gelatin? Irrelevant.

Regardless, as I said, I only plan to cook for myself.

#351 Robert Jueneman

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Posted 28 April 2009 - 07:15 PM

Back to the overshoot question. My laptop is at work, so I can't run the data logger, but I tried my 1500 watt 10 liter rice cooker with my SVM 1500B (in Celsius mode) using P=100, I=100, D=0.

The temperature climbed from 28C (well above ambient, but the water was still warm from last night) to the set point of 55.5C in about 10 minutes. Unfortunately, it kept climbing, and hit 59.4C 10 minutes later. 45 minutes after I started, it was still at 57.0.

That's not very user-friendly, so we'll write off that combination. I assume my previous suggestion of I=125 for a similar cooker would produce comparable results.

EDITED: After 1:20 minutes, it was back down to 55.3, or 0.2C below the set point, which is the problem I was trying to eliminate in the first place! EDITED #2 -- Now it's back up to 55.7, so it is oscillating, but not too badly -- unless it gets worse.

So... what do we know? Assuming the cookers are reasonably comparable, I=100 is too low (it gets too hot too fast), but I=400 doesn't work either -- it may produce oscillations, and takes forever to stabilize.

I guess I'll split the difference, and try I=250. If that still is ridiculous, I'll try I=1, and see what happens.

And yes, e_monster, I will also try the auto-tune settings (P=13, I=900, D=300 -- as close as I can get by re-entering them), just to see what they do as well, even though they seem outlandish.

Edited by Robert Jueneman, 28 April 2009 - 07:42 PM.


#352 blackp

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Posted 28 April 2009 - 07:51 PM

As per e-monster's suggestion my machine is now autotuning. As suggested I have nothing to lose.
It's been going for about 5 hours so far and I'll let it run until it stops. I'll provide more information as it comes to hand.

If the autotune doesn't give me better results my next attempt will be 50% power. There doesn't seem to be any problem with the unit heating up - even from room temperature it is quite quick, the main problem seems to be the latent heat in the unit after the element has been turned off, coupled with good insulation so that the heat doesn't go out of the unit quickly. After that increasing the "P" value will be my next step.

Peter.

#353 nickrey

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Posted 28 April 2009 - 09:11 PM

I have been through this exercise with the SVM as a consequence of overshoots very similar to those described in the posts above.

Using a smaller JNP-1800 10-cup cooker, I found the following settings work well.

The PID controlled advanced settings were used to set the unit power to 75% to moderate the speed at which the unit heated.

The PID settings that I arrived at after much experimentation (plus a rejection of the auto tune results which still resulted in overshoots) are P 100, I 60 and D 80. These seem to hold the temperature rock steady over the entire cooking period.

The other variable that seems to affect reaching temperature and temperature maintenance is initial water temperature. When filling the cooker, I run the tap water until it is at a temperature around 2C higher than my target temperature. On adding the cool sous vide packages, the temperature is typically near the target so the heating requirement is not as dramatic. The unit thus only has to maintain a constant temperature rather than heat to target.

Nick Reynolds, aka "nickrey"

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#354 blackp

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Posted 29 April 2009 - 01:41 AM

The autotune of the SVM finished just now - 11.5 hours after starting.

It came up with: P=19, I=998 and D=499.

Intuitively that sounds wrong.

If I get a chance tomorrow I'll give it a go to see what happens.

Cheers,

PB

#355 nickrey

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Posted 29 April 2009 - 03:09 AM

Sounds very similar to the values I got.

Nick Reynolds, aka "nickrey"

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Unless there are three other people." Orson Welles
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#356 blackp

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Posted 29 April 2009 - 07:09 PM

OK - here is what it looked like with the Autotune settings:
Posted Image

This would be a good result if only the set temp had been 57C and not 54C.

Next experiment I'll try Nickrey's settings.

Only thing on my mind is that my cooker is twice the size and has more wattage so the settings may not be appropriate.

I guess another alternative could be to re-calibrate the SVM with a 3 degree offset and keep the autotune settings, although those spikes don't look good.

One thing I observed is that the insulation is very effective - I turned it off last night when the temp was 56.8C, and 6.5 hours later when I got up this morning the temp was still 43C even though the ambient temp was 17C.

Cheers,

Peter.

#357 blackp

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Posted 30 April 2009 - 03:05 AM

More information:

Posted Image

After the Auto-tune experiment I tried nickrey's settings (P=100, I=60, D=80, Power 75%). The overrun was similar or worse than the default settings (P=100, I=700, D=40, Power 100%). I aborted that try, as it clearly didn't fix my large overrun problem.

Next I doubled the P setting to 200 (P=200, I=60, D=80, Power 75%) and the result was better, but the overrun was still too high.

The last experiment for today was to eliminate the I and D settings (P=200, I=0, D=0, Power=75%). While it took 20 minutes for the target temperature to be reached, once there it was very stable within a few 10ths of a degree.

Buoyed by this new stability I got an ice brick from the freezer and put it into the cooker at the 75 minute mark. The ice brick weighs 475g and the cooker was filled with 9 litres of water - that equates to the ice brick being only 5.3% of the total mass in the cooker.

After 5 minutes the temp had dropped 2.5C and stayed between there and a further 0.5C lower for 15 minutes. It then took a further 13 minutes to recover to the target of 54C.

I don't know if this is expected, but when time permits I'll repeat the same experiment with the "D" setting increased to see if the time spent hovering around a lower temp can be overcome.

More info as it comes to hand.

Cheers,

Peter.

#358 nathanm

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Posted 30 April 2009 - 08:04 AM

Buoyed by this new stability I got an ice brick from the freezer and put it into the cooker at the 75 minute mark.  The ice brick weighs 475g and the cooker was filled with 9 litres of water - that equates to the ice brick being only 5.3% of the total mass in the cooker.

After 5 minutes the temp had dropped 2.5C and stayed between there and a further 0.5C lower for 15 minutes.  It then took a further 13 minutes to recover to the target of 54C.

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The latent heat of fusion for ice is 334 kJ/kg

So, melting 475 grams of ice takes 158,000 joules of energy.

Taking 475 grams from -20C to 54C is 146,000 joules.

So melting the ice and bringing it up to temperature would require 305,000 joules.

It takes 4.18 joules to change the temperature of 1 gram (1 cc) of water by 1 degree C.

So if you just dropped the ice in without the heating element on, the temperature of a tank with 9 literes of water should drop 8 degrees C, by the time the ice was all melted.

But you did have a heating element on, and 1 watt = 1 joule/sec.

So, a 1500 watt heating element would take about 3.6 minutes to put 305,000 joules into the tank at 100% efficiency.

The efficiency is probably no more than 50%, and the heating element probably was not on high heat the whole time because the PID controller was gradually regulating the heat to avoid overshoot.

Remember that the PID only "saw" a 2C to 3C temperature drop, so it would not turn the element on to full power. The P parameter determines how the power goes proportionate to temperature difference.

So, your result on the dropping the ice into the tank is reasonbaly consistent with all of this. If you could record the PID % power output it could all be calculated very closely.

If you had dropped crushed ice in, you would have gotten a lower temperature spike on dropping it in. The ice brick took a while to melt, that is why the temperature was on a plateau for 15 minutes.
Nathan

#359 joesan

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Posted 30 April 2009 - 09:04 AM

Robert (Jueneman) - thanks for the interesting observations around the SVM controllers. I'm thinking about adding to my SV detritus by adding a SVM controller. I really want accuracy to the 0.1C level with this next one.

I can see on the SVM site that the 1500B model you mention has resolution of 0.1C but accuracy is only 1%. This is obviously a big difference.

1% doesn't seem much but it could have a non-negligible effect over prolonged periods of time I guess.

In your testing have you been able to confirm that this model is more accurate long term than 1%?

#360 e_monster

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Posted 30 April 2009 - 09:11 AM

With I and D off, I think you can probably lower the P value. As a test, run it in just P mode with the P value you got from Auto-Tune and if that doesn't give satisfactory results double the P. This should also increase the recovery rate.

I wonder if the weird behavior after Auto-Tune (stable but several degrees too low) could have anything to do with running the power at less than 100%. Maybe there is a software bug that causes problems for the the auto-tune if the power isn't 100%.

More information:

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After the Auto-tune experiment I tried nickrey's settings (P=100, I=60, D=80, Power 75%).  The overrun was similar or worse than the default settings (P=100, I=700, D=40, Power 100%).  I aborted that try, as it clearly didn't fix my large overrun problem.

Next I doubled the P setting to 200 (P=200, I=60, D=80, Power 75%) and the result was better, but the overrun was still too high.

The last experiment for today was to eliminate the I and D settings (P=200, I=0, D=0, Power=75%).  While it took 20 minutes for the target temperature to be reached, once there it was very stable within a few 10ths of a degree.

Buoyed by this new stability I got an ice brick from the freezer and put it into the cooker at the 75 minute mark.  The ice brick weighs 475g and the cooker was filled with 9 litres of water - that equates to the ice brick being only 5.3% of the total mass in the cooker.

After 5 minutes the temp had dropped 2.5C and stayed between there and a further 0.5C lower for 15 minutes.  It then took a further 13 minutes to recover to the target of 54C.

I don't know if this is expected, but when time permits I'll repeat the same experiment with the "D" setting increased to see if the time spent hovering around a lower temp can be overcome.

More info as it comes to hand.

Cheers,

Peter.

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