Thanks, it's worth looking at alternatives. I know there isn't just one method to boil eggs so if I build it my way, I could still try your method - boil from cold and let it cook in the water's heat for a known period. That period to cook as desired must depend on the amount of water present though, and the ambient temperature. I still prefer my cooking method as it's repeatable and you can play with the parameters to see what's best, probably moreso than doing it 'manually' unless you're very methodical.I've always been taught to put the egg in cold water, bring the water to the boil, turn off and wait 7 minutes for a soft to medium boiled egg or 10 mins for a hard boiled egg.
Obviously depends on the volume of water and quantity of eggs being cooked but as a rule of thumb seems to work.
A thermistor to detect 100 degrees C - or a car temperature sender would also do - some of the radiator fan switches have a 95-100 degree cut off point. Interface to a relay, one shot switch and time and job done.
Thanks Ron - same cooking method as picbits. I also start with the egg in cold water but time it from boiling and keep it just boiling. That's what I'm trying to automate.Given a choice I would just use an SSR for handling power to the heating element. A SSR with a 3 - 32 volt trigger. The heating element is a resistive load and there is no reason to get fancy. That said, I would use a thermistor embedded in epoxy as a sensor, get creative and make your own sensor housing, a drinking straw might be a good start. I would not use any form of microphone to detect boiling. Why? The eggs are in their shells and there are plenty of non-toxic temperature sensor solutions. While I would not bother with a uC and likely use a simple comparator circuit On/Off with hysteresis if you want to use a uC then have at it. Configure the thermistor in a voltage divider configuration and run the output into an analog input of your uC.
Water boils at5 sea level at 100C so write your code so 100C is the SP (Set Point) and actual temperature is your PV (Process Variable). When the PV is below SP the output is 100% or code it however you wish. If you want to pulse the element write code so when the PV is say 90C put it in your code. The bottom line is the water at sea level will never get hotter than 100C at which point you are making steam. Using a uC you have the benefit of programming your control however you wish. Just a matter of your creative imagination. Use another DO from the uC to raise/lower your basket.
<EDIT> PC Bits beat me.</EDIT>
Yeah, I start with the eggs in cold water, bring to a boil, wait and then place the eggs in cold water.
Just My Take...
Ron
My concern is that measuring temperature won't be so much use to keep it boiling properly as it'll be at the same temperature all the time until it goes off the boil. The idea of going by the sound it makes is because I suspect the vigour of the boiling will change with power while temp won't nearly so much.
I'm thinking that measuring the vapor pressure developed would be a better indicator, and a fast responding process variable as well. Pressure sensor could be mounted in the cover of the kettle.
Seems more complex to me. As water is heated it expands before turning to a gassious sate at 100C. No problem when open to atmosphere. But also no pressure to detect when vented to atmosphere. There can be a big problem when it is not vented and pressure builds up. This is why a pressure cooker has a relief valve and even those things scare me.
Ron
Nice thought and in a sealed container this could work but my kettle is plastic with a far-from-airtight lid. I don't want to make it airtight and if I did it would affect the boiling point and complicate things.I'm thinking that measuring the vapor pressure developed would be a better indicator, and a fast responding process variable as well. Pressure sensor could be mounted in the cover of the kettle.
Well, thinking back to school science lessons, I think there's a bigger problem than altitude or atmospheric pressure: latent heat of vaporisation.The potential problem that I see with using just a temperature sensor to control something like this is that if you aim for a set point of say 100c, and you are actually at a higher altitude, or the barometric pressure changes a large amount, you run a modest risk of *NOT* properly detecting boil, and thus not shutting down the heat. This happens because water boils at whatever temperature it boils at... and no higher... for a given pressure.
This same physics is why you can boil water in a paper cup.
For example, when a pot of water is kept boiling, the temperature remains at 100 °C (212 °F) until the last drop evaporates, because all the heat being added to the liquid is absorbed as latent heat of vaporization and carried away by the escaping vapour molecules.
Hi Ron, I've had a look at SSRs on ebay and found one which can control 25A at 240V with a logic input. It didn't say whether it's resistive load but as mine is, it's fine. I do want to learn about triggering a triac to control mains but that can wait. SSR will do for this.Hi Ya elfcurry,
OK, let me clear up a little of what I mentioned. My suggestion to use a SSR to control your power to the heating element was simply to control the power. You could build your own triac circuit but I see an off the shelf SSR as a simple solution that can use a nice 5 volt control voltage to turn your 220 VAC on and off.
A thermistor is something I know nothing about so it's another thing to learn if I go that way. But see my post above for why temperature may not be the best thing to measure. Toxicity wasn't a major concern, just something I thought I should not ignore.As to the sensor? A thermistor is but one of several ways to go about sensing the water temperature. For example a thermistor like this one has a cost of about $1.95 USD and is pretty inexpensive. Looking at the data sheet it has an upper range of -40 to 125 degrees C which is adequate. When it comes to the sensor I figure it this way, both my micro-wave oven and my kitchen gas stove oven have probes that can be inserted into meats (or other foods) that are totally non toxic and leave no residue in the foods. If I can determine when the internal temperature of a roast beef is 140 degrees F (I like medium rare beef) it can surely determine when water reaches 100 degrees C. Anyhow, the sensor is your call, I merely suggested a thermistor but there is no shortage of other sensor options out there that would do just fine. A Google of "thermistor arduino" will bring up a dozen hits or more about interfacing a thermistor to for example an Arduino uC.
I wasn't aware that DS18B20 could go past boiling but I am now.As to using any form of for example PID (Proportional Integral Derivative) control I would just run with On/Off control. Assuming around sea level water will boil at 100C. At 300 meters above sea level that only will drop by a few degrees. Matters not how much heat our elements are supplying the hottest the water will get id the boiling point. If we turn the elements off the water will stop a rolling boil pretty quick. So given a choice using a uC I would start my eggs in cold water and apply the heat. Once the water temperature reaches 100C start a timer function where the digital out of your uC remains on for about another 15 or 20 min. My attempts at hard boiled eggs always suck. Truth is if I want to hard boil 5 eggs I start with 10 or more just to get 5 good eggs. My wife can make perfect easy to peel eggs every time but mine are a disaster! So after a prescribed time turn off the heaters. This is typically called a ramp and soak cycle in heating applications, we ramp the water temperature up to 100C and then start a timed soak cycle. When the cycle times out the digital out to the SSR goes low and that is about it.
Actually the DS18B20 has an upper range of 125C I believe and it could be used in conjunction with a uC. Back to choosing a sensor.
Ron
The potential problem that I see with using just a temperature sensor to control something like this is that if you aim for a set point of say 100c, and you are actually at a higher altitude, or the barometric pressure changes a large amount, you run a modest risk of *NOT* properly detecting boil, and thus not shutting down the heat. This happens because water boils at whatever temperature it boils at... and no higher... for a given pressure.
This same physics is why you can boil water in a paper cup.
Re: no limits - yes, I agree that there are all sorts of projects possible around the home or garden or some workplaces (though safety regulation may complicate things ....)Funny, as when you start really thinking about this, there is no limit as to what can be done. Reminds me of some of the projects over the years at work. Someone wants to do something and you say OK, we can make that happen. So you start kicking around a few ways to make it happen. Then someone comes along and says it would be nice if it also did this or that, so the basic design has a few new features added. Finally you look at the project and remember what it was originally and how it evolved, which is pretty cool.
You start with a plan for a basic egg cooker and begin adding features. Each feature is another "how to" like you just mentioned for example, what if I lose my water? What if all my water boils off? How do I sense if this happens? What sensing method to use? Ah, a fail safe smart egg cooker.
I should share the story of how once, when I was much, much younger I set 6 eggs on the stove to boil and promptly fell asleep. This was after a night that involved several pubs. I never knew 6 eggs could make so much horrible smoke and stink so bad! It was ugly!
I hope some other forum members chime in with some ideas as to making a good, reliable, egg cooker. I would like to see some other thoughts on how to go about it beyond my thinking.
Ron
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.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.
By the way, the process of cooking is basically "Denaturation", which is an endothermic process.
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