egg boiler using electric kettle

[()blivion]

Yea, you could do the whole thing by eye too. But I thought we we talking about doing it all automagically?

 

[ccurtis]

True. How can this be used? Heat is added and the temperature rises until a plateau is reached, then you know it's boiling. It could be a method for determining the boiling point. But if I had a temperature sensor on my egg-cooking kettle I'd turn on the heat anyway as soon as cooling is noticed.

Without having to know what the actual temperature is, some sort of circuit (a differentiator circuit, perhaps) to monitor the rate of change in temperature of the water can be used to turn the heat on and off. If the rate of change (either increase or decrease in temperature) is below a certain (small) threshold level, the heat is turned off, otherwise heat on. I think what will happen, however, is that the water will be kept just at the boiling point, which is where small bubbles start to form at the bottom of the kettle and there will no rolling boil. But, does that matter anyway?

 

[Reloadron]

Though the boiling temperature of the water can vary significantly, what we do know is that the temperature of the water no longer increases when boiling is reached. That can be used as an indicator for boiling.

The same is true of cold. Water freezes at 0C or 32F and that is as cold as it gets, assuming 1 atmosphere at sea level and pure water. I was talking about this with my wife last night based on this thread. Been snowing so no shortage of snow piled up outside. Took a little hand held thermometer (digital temperature indicator) outside. The air temperature was about 13 degrees F. I placed the probe in a snow drift and the temperature was.... 32 degrees F. If I place that same probe in water on the stove and bring it to a boil I should see about 212 degrees F. and that is as hot as it gets. Matters not if the flame temperature below the pot is over 1,000 degrees F. or not, 212 F is as hot as it gets. Kathy was surprised when I placed the probe into the snow drift and the indicated temperature began to rise till it reached 32 degrees F.

So other than using a calibrated temperature sensor for this egg cooker can anyone suggest another economical method to detect boiling water? I am sticking with temperature, rest assured, the eggs will cook.

Ron

 

[ccurtis]

So other than using a calibrated temperature sensor for this egg cooker can anyone suggest another economical method to detect boiling water? I am sticking with temperature, rest assured, the eggs will cook.

Heck, they will cook at 90 degrees C so I think your method is the simplest and most economical. Why all the hang-up on boiling point, anyway? If you adjust the trip point at anywhere close to highest temperature (boiling temp) the eggs will cook and not waste any water and energy in the form of making steam.

 

[()blivion]

Water freezes at 0C or 32F and that is as cold as it gets...

I'm fairly sure that ice, when frozen, will continue to drop lower than 0c. The only reason that water boiling won't get past 100c is because at 101c it can only exist as steam, which being a gas, is more buoyant than the water and is carried away. Ice however, being a solid, is not going anywhere quickly. So heat can still be taken from it and it's temperature will continue to drop.

You can prove this by sticking a thermometer directly on ice in a freezer, and letting it stay for a while. The ice then will surely be colder than 0c, about -23c if I remember the recommended freezer temp correctly.

Snow on the other hand, may in fact end up no lower than 0c(32F) because that is the point that water will begin to precipitate out of the air. This is also why if the temperature drops below a certain point, it won't snow. Because all the water has already long since frozen and fallen from the sky.

RE, cooking eggs.
I don't see a reason that it can't float at 99~101C and not cook eggs. I also agree that it may even be more efficient.

 

[elfcurry]

At standard atmospheric pressure (sea-level) ....

Water in the presence of ice will be at 0°C so ice in your drink in a warm room keeps it at zero until the ice melts. Likewise Arctic ice keeps nearby seawater at zero* (-ish). Any additional heat will go to melting ice and the temperature can only rise further when all the ice has melted. This is why the Arctic is not getting warmer even though there's lots more heat around. When it's all melted, THEN it'll start to get a lot warmer.
So of course ice can be as cold as you want but if there's liquid water and ice it's at 0°C.

In a heated pot or kettle of water, the presence of liquid water keeps it from going above 100°C. My kettle will get hotter and plateau at 100°C until it's boiled dry.

*for fresh water, saltwater will be a bit different

RE, cooking eggs.
I don't see a reason that it can't float at 99~101C and not cook eggs. I also agree that it may even be more efficient.

I think this is right except it can't go over 100°C.

Maybe I should abandon the idea of keeping it boiling at 100°C and adopt the easier task of keeping it close to boiling. The eggs will cook - they'll never know the difference!

 

[()blivion]

"99~101°C" was my funny little way of taking into account atmospheric variation. You could theoretically be under higher than normal barometric pressure, or you could happen to be in some rare, lower than sea level place like death valley. Both of which would raise the boiling point of water ever so slightly above true 100°C. Which is ironic because 100°C is exactly defined as the boiling point of water... [SUB](at sea level)[/SUB].

 

[ronv]

I hate to admit it but I'm just happy to know you should start with room temperature water. Will this make them easier to peel?

 

[elfcurry]

"99~101°C" was my funny little way of taking into account atmospheric variation. You could theoretically be under higher than normal barometric pressure, or you could happen to be in some rare, lower than sea level place like death valley. Both of which would raise the boiling point of water ever so slightly above true 100°C. Which is ironic because 100°C is exactly defined as the boiling point of water... [SUB](at sea level)[/SUB].

Ah, sorry, I see that now. I live at sea level (well, 160ft ~50m) so any variation will be due to weather and probably too small to make a difference here. Not many live below sea level and many live above where boiling point (BP) will be less than 100°C.

My aim was to keep the water at BP (whatever the local value is) while the eggs are cooked for a specified time and then remove and eat them hot with a runny yolk. No peeling. But since the temperature won't change once BP is reached, (so using it to control the heating won't work*) maybe the aim of keeping it at BP while minimising energy wastage is too hard to achieve.

Perhaps I should change my original aim and try to keep it at, say 95°C with a temp sensor, adjust the cooking time slightly if necessary and carry on otherwise as planned.

*I think - please correct me if this is wrong

 

[()blivion]

Maybe the aim of keeping it at BP is hard to achieve?

Nonsense, ccurtis's method is perfect. It will be 99.999°C, but as was said, the eggs would never know the difference since water can't get any hotter than this anyway.

The circuit is simple, just use a thermistor as the temperature sensor. No need to worry about calibration or nonlinearities as we are not trying to get specific temperature from it, just rate of change. Though we do need to add a simple lockout to keep the differentiator from tripping the timer before we have even started to heat the water. Anyway, the thermistor output (possibly scaled) is input to a differentiator with quite a large C, the rate of change in thermistor output is going to be quite low. Then we invert the differentiators output and feed it to a timer. When the thermistor output becomes steady, it means the water has stopped changing temperature. This then activates the timer, which counts down and eventually turns off the heat source. And your eggs are cooked.

The above will work for almost all low power water heating tasks. If, however, the heating element is really powerful, or we are especially interested in saving water/energy. Then we can add a bit more complexity to the circuit to make it a changing duty cycle at very near the boiling point.

Edit: Just realized that the amplified output of the differentiator itself directly driving the heating element would work to keep the temp right at boiling, the rate of change stops, the water is at 100C, the output goes low, the heat is removed, the water cools, the rate of change goes up again, the differentiator output goes up again, the heat element is turned on again... Rinse and repeat.

 

[MrAl]

Hello Ron and others,


When i first read this thread i was going to suggest using derivative feedback also as ccurtis suggested. I think it would be good enough that way.

The idea is to measure the temperature with a micro controller and log it as the container is being heated and use the changes in temperature to detect the maximum temperature and then use that as the maximum temperature (minus a couple degrees C perhaps). That would discover the max temperature and that could be used as the set point. Of course periods where the container is cooling is also of use, where we could detect some level under the max and turn the heater element back on again, and again testing for max temperature using the same technique. That would update the measured data for max temperature so we'd be in constant calibration from one cycle to the next.

There's also the possibility of analog control rather than bang bang control of the heater element. That provides for smoother control but requires a triac phase angle control circuit for the element driver rather than a simple SSR or relay or just turning a triac on and off.

Actually max would not be too hard to measure. Given that the temperature is constantly increasing before the max, when the temperature reaches a given temperature and stays at that temperature for a period that is say 10 seconds (and during every other 10 second period the temperature always increased) then that must be at least near max.

Just for reference, traditional "hot pots" seem to use a mechanical thermal switch that turns off at a given temperature. They must be using a temperature that is under 100 deg C and sometimes it shows as the water doesnt truly boil rapidly.

PS:
No Monk movie! Not sure what happened and cant find any more info on this.

 

[Reloadron]

PS:
No Monk movie! Not sure what happened and cant find any more info on this.

LOL, you have mail.

Ron

 

[elfcurry]

Without having to know what the actual temperature is, some sort of circuit (a differentiator circuit, perhaps) to monitor the rate of change in temperature of the water can be used to turn the heat on and off. If the rate of change (either increase or decrease in temperature) is below a certain (small) threshold level, the heat is turned off, otherwise heat on. I think what will happen, however, is that the water will be kept just at the boiling point, which is where small bubbles start to form at the bottom of the kettle and there will no rolling boil. But, does that matter anyway?

Some interesting thoughts but I can see a problem or two. If the heater is controlled proportionally rather than on/off, you need a more complex analogue current control. It can be done with phase delay triac triggering but at 2-3kW it would create a lot of electrical noise. Also it would take longer to reach BP if you keep holding back as you get near the target temperature. And if you don't want to measure temp, what would you differentiate?

From my pov if we measure temp and get a signal to a control circuit, my choice would still be a μC so you can monitor it, drive a display, record data while 'tuning' the process. Some may prefer an analogue alternative or just use a bi-metal strip and timer to cut off when the time's up. I suppose we all lean towards what we're familiar with.

I've thought about the bubbles too. I made some tea earlier in the kettle I currently use. I usually hover and turn it off just as it comes to the boil but with this thread in mind I let it go on a few more seconds and the sound of the bubbles got much more vigorous and loud as it properly boiled. I still believe that's when it reaches BP and the greater vigour is due to the now excess power no longer being needed to heat the water now goes into vapourising some of it.

 

[Mr RB]

I hate to admit it but I'm just happy to know you should start with room temperature water. Will this make them easier to peel?

Eggs don't seem to be as easy to peel as they were 20 years ago, i think the chickens are much less healthy these days and the undershell membrane is weak.

To make hardboiled eggs easier to peel I double boil, ie bring them to boil then turn the pan straight off. Wait 5 minutes, then bring it up to boil again, then likewise turn it straight off. Then allow a slow cool down in the same water. By the time the water is safe to put your fingers in and get the eggs out the shells will practically fall off.

The worst way is to bring the eggs to boil once, then take them straight out and cool them quickly in cold water! You'll lose half your egg by the time you finally break the shell off.

 

[()blivion]

Some interesting thoughts but I can see a problem or two. If the heater is controlled proportionally rather than on/off, you need a more complex analogue current control. It can be done with phase delay triac triggering but at 2-3kW it would create a lot of electrical noise. Also it would take longer to reach BP if you keep holding back as you get near the target temperature. And if you don't want to measure temp, what would you differentiate?

Hummmm... agreed on the phase delay, making use of automatic triac shutdown during zero crossing to do pulse-by-pulse skipping to control power would be the cleanest approach. If it only had a few power levels it could be done with analog easily. In any case, μC implementation would be trivial.

As for taking longer to reach BP because it would cut back the power the closer it got, I'm fairly sure that the rise in temperature would be very constant right up until it actually hit the boiling point. So I don't think there is really a problem.

Finally, maybe I missed something and you really are talking to ccurtis, but I believe it was actually I that was talking about not needing to measure temperature. What I meant was, we don't need to know universal ISO approved standard temperature, because we already know that (1) not all water boils at exactly 100C, do to differences in elevation. And (2) all water DOES stop increasing in temperature when it reaches boiling, no matter what temperature that may be. This makes knowing the exact temperature less than useful, it can't tell us if the water is boiling unless we know other facts that we don't have a way to measure. Now... if we installed a barometric pressure sensor, then we could make use of exact temperature measurements.

From my pov if we measure temp and get a signal to a control circuit, my choice would still be a μC so you can monitor it, drive a display, record data while 'tuning' the process. Some may prefer an analogue alternative or just use a bi-metal strip and timer to cut off when the time's up. I suppose we all lean towards what we're familiar with.

I am probably more familiar with μC than I am analog, and I would still recommend matter-of-fact analog design for this. IMHO It's a straightforward application of occums razor, there is no need to over complicate a simple thing like cooking eggs. Though, if you're just after an excuse to play with μC's, I can relate to that. They are pretty freaking awesome devices.

I've thought about the bubbles too. I made some tea earlier in the kettle I currently use. I usually hover and turn it off just as it comes to the boil but with this thread in mind I let it go on a few more seconds and the sound of the bubbles got much more vigorous and loud as it properly boiled. I still believe that's when it reaches BP and the greater vigour is due to the now excess power no longer being needed to heat the water now goes into vapourising some of it.

Although a novel idea, I think an acoustic cavitation detection scheme would be prone to false positives and not as reliable as alternatives. Cavitation depends a lot on the composition of the water and the exact rate of heat being applied, as well as a hand full of other parameters that are not necessarily constants or under our control. I think that you will find that it would end up having some kind of cryptic failure mode that would be next to impossible to figure out, all do to something simple like a contaminant in the water that messed with surface tension or something silly like that. Also things like placing the pot differently would change how and where the sound was conducted through the medium. May dampen it, may amplify it... can't know or compensate for it at the circuit level though, so it's kind of a deal breaker.


P.S. Half asleep when I wrote this. It looks all correct... but my eyes are not really open either.

 

[MrAl]

Hi,

Yeah analog can be different if you are not used to working it that way, but the beauty lies in the fact that it is continuously variable rather than sampled and stepped and whatnot so the control can be VERY smooth and precise. Not that we need that kind of perfection here though, just more or less mentioning this in passing.

I did a small temperature controlled oven a while back (fairly long while back actually) using analog and for that it is nothing more than a heating element, op amp, driver transistor, and thermistor (plus a few resistors and regulated power supply like LM317 or better). The thermistor measures the temperature via a voltage divider, the op amp compares the thermistor voltage to a set reference (which sets the temperature set point) and the driver provides current to the heating element which may actually be just a large filament bulb. As the interior heats up, the thermistor voltage rises (or falls) and that means it gets closer and closer to the op amp reference set point. The closer it gets, the more the output of the op amp changes and that changes the driver current which lowers the current to the element. With little heat loss from the oven, the current will stay low. With a larger heat loss, the current will increase. So the current follows the heat loss of the oven in a nice analog (continuously variable) way. I suppose PWM to the element would be just as nice however with the pulse width changing as the heat losses changed.

Using a bulb is a little more dramatic because it starts out as bright as it can get, then starts to dim once the oven comes up to the correct temperature. It stays more or less at the same brightness after that unless the oven is opened. Again i suppose that this would still be the case with PWM so that's a good option too.

Nothing wrong with the uC control scheme though, almost the same thing really except for the extra added complexity of programming, etc.

BTW i was trying to figure out what your (Oblivion) sig line means but cant quite read all the symbols or figure it out...some explanation perhaps? Thanks. Is that a line from Eight Legged Freaks or something?

 

[()blivion]

BTW i was trying to figure out what your (Oblivion) sig line means but cant quite read all the symbols or figure it out...some explanation perhaps? Thanks. Is that a line from Eight Legged Freaks or something?

Hummm.... It may not be showing up as the right characters on your system. Here is an image of what it looks like to me...


It is just ASCII art of a funny little guy being mad or scared and throwing a table because of it.



Edit:
BTW, in case you're also wondering, my avatar is the Japanese Kanji "忘却" which in hiragana is "ぼうきゃく" and in romaji "Boukyaku".
All of which of course ultimately equates to..... "Oblivion". (Can also mean "forgetfulness".)

 

[MrAl]

Hummm.... It may not be showing up as the right characters on your system. Here is an image of what it looks like to me...

View attachment 70588
It is just ASCII art of a funny little guy being mad or scared and throwing a table because of it.



Edit:
BTW, in case you're also wondering, my avatar is the Japanese Kanji "忘却" which in hiragana is "ぼうきゃく" and in romaji "Boukyaku".
All of which of course ultimately equates to..... "Oblivion". (Can also mean "forgetfulness".)

Oh ok i see now A table? wow

 

[()blivion]

Yeah, a very short legged table.

And the situation can degrade to "table throwing" when it's Tegenaria duellica that runs across you arm/leg. Fairly common spider in my area. They are not exactly tarantulas, but they are not dust mites either. They also happen to be very very fast when they want to be. Which often times is more startling than their size. Oh, and despite scientific literature, my own anecdotal evidence would suggest that they bite... hard.

Here's the little girl that scared me the other day...



Tegenaria duellica is subtly different from the more common Tegenaria domestica, which has striped pattern on it's legs, and is smaller.

 

[Mr RB]

That looks like a boy, they have much smaller abdomen and very long 4 front legs. Female ground spiders are fatter and more circular looking, that definitely looks very triangular like a male.

And it's tiny. In Australia we don't call a spider big unless it turns around and starts grabbing and biting the broom you are chasing it with, like it thinks it's actually going to "take down" the broom. And if you trap them under a plastic container you hear a "crack crack" as they smash their hard fangs into the plastic wall trying to "kill" it.