Water Heater

Hi just small info , DS18B20 have range from -55 ? ~ +125 ? which is written on link robtillaart give you, but you read only first sentence which describe precision.

Can you just use an ordinary electric kettle element? Perhaps you could even use the body of the kettle as your container, and modify the body to provide your outlet and filling methods.

I don't know if the contents will settle enough to keep hot and cold water separate, but if possible it would be a good idea to design it so that fresh cold water was kept close to the element until it was heater rather than mixing freely with the existing hot water. This would mean using a bottom fill / top outlet and internal baffles to minimise mixing.

Having made a few tea machines over the years here are my suggestions.

1 Standard thermistor you can get them in nice hardened forms as in you can put a probe into the center of the basin. You can find electric tea kettles with them in there already, generally if it's got a LCD screen on it it probably has one.

2 If this is your first go would use a commercial product and upgrade it, the hard bits of making it waterproof and safe has generally been done for you already. I would not worry about uniformity to much convection currents in the water will do that for you. As I find commercial electric tea kettles to be among the most hideous things I used a solid copper stove top type and placed it on a single burner electric "burner".

3 Try something like a solid state switch it can do a very course PWM.

try using peltier junctions to extract heat from the air and heat the water they are able to heat as high as 200 f also if you built a function in that allows reversal of polarity to the junctions you would have the ability to both heat or chill the water ive pulled as low as -25f with a junction and a fan

http://www.mouser.com/_/?Keyword=thermal+electric+cooler

Any hardware store will sell electric heating elements as used with water heaters, although since they thread in it might be a bit of a trick to make it watertight. If you're looking for something more compact you can search eBay for an "injection molding heater".

A standard reed relay will work fine for switching either/any element on and off.

FYI, It takes 3.412 watts to raise 1 pound of water 1 degree Fahrenheit within 1 hour.

Here's my suggestion:
Use a standard drop-in beverage heater like the YL205.

Control it with a relay and an Arduino digital pin. Do not use a reed relay; you need the multi-amp current rating (5 - 10a) of a full size relay.
The most sensitive 5v relay I found still draws > 30 ma ( 70 ) so you'll need a transistor on the output pin to operate the relay.

Mount the temperature sensor on a metal straw.
Mount the metal straw to the beverage heater handle with a hose clamp, or something similar.
Make sure it is thermally distant enough from the heater itself, for example a few inches away, preferably lower.

Now you just drop the heater into any pot, cup, or mug and turn on your Arduino.
Rely on convection, as others have mentioned, to keep the temperature relatively uniform.

I think it would be way cool to mount a small 3 digit digital display also on the handle so you can watch it heat!
(LED displays let you hardwire the decimal point (or comma for Europe?) on.)

FYI, It takes 3.412 watts to raise 1 pound of water 1 degree Fahrenheit within 1 hour.

Pounds, Farenheit, hours ... no wonder the probe crashed on Mars :slight_smile:

The thermal properties of water are;
Boiling temperature - 100 oC
Specific heat water - 4.187 kJ/kgK
Latent heat of evaporation - 2,270 kJ/kg

Degrees Kelvin are the same size as degrees Centigrade or Celcius.
So to raise 1 kg of water by 1 deg C you need to input 4.187kJ.
Since a Watt is a Joule/s you can calculate what wattage you need to heat the water in the time you require.
Once you get the liquid water to 100deg C you need to keep putting in energy to convert it to steam at the same temperature, that is the latent heat of evaporation.

It takes a lot of energy to heat water which is one reason why it is so good at putting out fires.

One thing I would be wary of is mixing water, mains voltages for the heater and LV wiring for your sensor - make sure you know what you are doing from a safety point of view.

...mixing water, mains voltages for the heater and LV wiring for your sensor...

Yes, this is very true, especially with European 220v vs American 120. No experienced electronics person would recommend that a newbie wire a heating element for liquids.

That is why I recommended the $15 drop-in heater. It is an electrically sealed unit. You don't have access to anything electrical with it. You immerse it and plug it in; nothing more. The contacts from your Arduino-controlled relay might even be wired to an extension cord which you cut in half, making that the only HV wiring necessary. It also lets you easily replace the heater if (when) it burns out. I would strongly recommend springing for a DPDT relay, rather than single pole. That way when it is de-energized, say because the Arduino is off, but everything else is still plugged in, both line voltage wires are disconnected, reducing any hazard if the heater develops a leak.

Pounds, Farenheit, hours ... no wonder the probe crashed on Mars smiley

The thermal properties of water are;
Boiling temperature - 100 oC
Specific heat water - 4.187 kJ/kgK

Tch, tch.
sp. "The thermal properties of water are;
Boiling temperature 373.15K"

there was a good reason i advised the use of peltier junctions for this project as the highest voltage one may come in contact with is dependent on the junction used but will nearly always be below 60 volts dc

Thank you for the answers.
I'd say I'm not a completely newbie, I know something about it.

@Techylah
I like your idea.
I will buy a LCD, so I can see the temperature. I pretend in the future make some modifications, but first I need to build a simple water heater to know more about it.

Do you think I can vary the tension applied to that heater with a potentiometer? To get a better tea, e.g. if I want some green tea, I'd would prefer to heat it slower.
Also, do you think I could touch that heater in a metal plate like a cooker, and use the plate to heat the kettle? I think with that arrangement I would make better teas since has some controversy about the ways electric heaters are made.

do you think I could touch that heater in a metal plate like a cooker

No. I think that is ill-advised. It is made to be immersed, which provides a solid, low thermal resistance connection to the heat absorbing object, the water. It would probably overheat and burn-up in flames if not immersed.

Instead, use any regular electric teapot. The electric and mechanical connections to the heating element are done.

Your device can look like a stainless steel spoon that you leave in the pot. Of course it has digits on top and a wire going to your Arduino to bring it to and maintain the temperature you want by cycling power to it via a relay.

I don't know much about tea, but if it is like coffee, heating time doesn't matter.
It is only the final temperature that will affect flavor. I think the better flavors come out at lower temperatures; the bitter components take longer and require higher temperature.
If this is so with tea, making tea at lower temperature but longer steep time, would be better and worth the effort.

You'd still want to reach the lower temp as soon as possible, so the relay is best.
Besides dealing with SCRs and other high voltage, high current "dimming" circuits is tricky.

Your Arduino could also handle the steep time with finesse.
You put your "magic spoon" into any size electric pot, plug the pot into your switched power outlet, and press Start.
You get to watch the water get up to temp as soon as possible, (a settable 85 deg C), give you a "Bong" to add your tea,
and then another "BingBong" x minutes later when it's done! Then it maintains that temperature.

With your pot, potentiometer, that is, you can set the default temperature and steep times.

VirtualDDS:
there was a good reason i advised the use of peltier junctions for this project as the highest voltage one may come in contact with is dependent on the junction used but will nearly always be below 60 volts dc

You know what happens when you drive an "AC" heating element with DC? :wink:

Chagrin:
You know what happens when you drive an "AC" heating element with DC? :wink:

It gets hot?

controversy about the ways electric heaters are made

There is no controversy, just a warning that electricity is dangerous. Normally control circuits are separate and well insulated from power circuits, once you start introducing water there is a risk that people can come in contact with the effects of high voltages or currents.

You say you are not a complete newbie but it seems clear that english is our second language so just take care about safety.

I'm planning to use a digital potentiometer with a relay to turn on my heater (YL205 -
http://www.amazon.com/Lewis-N-Clark-Immersion-Size/dp/B001U0PA7M/ref=sr_1_3?ie=UTF8&qid=1337571828&sr=8-3 ) and change the potency. I found some like AD5171 Digital Potentiometer. Do you think it is a good one or do you recommend me another one?

@TechylahYes, tea and coffee are pretty the same, but teas have more bitter components, so I think it is more difficult to prepare.

@radman Probably you don't know about teas, but tea specialists do not recommend to use any kettle with metal or plastic parts since the flavor gets worse.
I was talking about controversy among tea specialists, maybe I should explain better, but, even if English isn't my first language, it hasn't any relation with the fact that I have or have not any knoledge about electronics.

I looked closer at the reviews of that YL205 water heater.
US people beware - Its "dual voltage" means that it is meant for Europe's 220v and works at half heat in the US 110v!

I don't know what you are going to do with the digital pot chip.
What I think you want is to implement a thermostatic controller.

Side note: I need to do a similar thing to control an otherwise extremely noisy water urn. I don't want to use my Arduino on it.
I found and purchased this cool digital controller for under 10GBP, including thermistor.
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=110880711979&ssPageName=ADME:L:OC:US:3160

Back to your project. The immersion heaters look like they're all 300 watts, so you need a relay with at least 2A contacts.
You can tap into the (non-ground) thermistor wire of the Adafruit thermister/display.
Your Arduino can measure the voltage on that (analogIn) and you can scale the reading to a number that always matches the readout temp.
When that temp is below the threshold you want, you turn on the digital output, turning on the transistor, engaging the relay, and turning on the immersion heater.
Remember to use a back-biased diode across the relay coil.

For me I will use the US 110v socket.

I'm trying to optimize my project, so instead of using the relay to turn on and turn off the heater I will change the potency. So to keep the water warm I just need to set up a potency that equals the heat produced with the heat dissipated. Or if I want to heat just a cup of water maybe it is better the heat it slower, so I have more control and, hence, more precision.

Alright, then. Go for it!
Despite what someone mentioned, I see nothing wrong with putting DC through an AC heater, if you're careful.
I would guess you are going to use PWM to a digital pin, with that digital pin controlling an NPN power Darlington, giving the heater you plug in zero to full duty cycle pulses of 120 volts DC. The 120v DC would come from a full wave bridge rectifier on the AC line, with a large capacitor to smooth the ripple.
If so, I would only say make sure:
1 - Your power on/off switch is double pole to switch both 110v wires. Off should not leave one side connected, since a reversed plug could leave things hot with respect to ground.
2 - You include a fuse or circuit breaker. If you don't have one and there is a short circuit, much current will flow, producing one heck of a vaporizing flash.
3 - You include a bleeder resistor across the capacitor 120vdc to ground. Otherwise, like in some very old tv sets, you unplug it to work on it and come to the shocking realization that the 120v remains for quite some time (minutes, even)!
4 - Use low enough gauge wire, not going through any breadboard pins.