Electronic temperature control.

[pjl83]

Hi All.

 

I'm working on a project but I am struggling with temperature control. Basically, Using an element similar to that of a kettle, I need to keep the water temperature at around 90 degrees C. So what is it in a kettle that tells it to switch off at 100 degrees and how could I make/find one to switch at around 90 degrees.

 

Or can you think of a better way of maintaining that water temperature?

 

Thanks

 

p.s. sorry for the poorly written question, I hope you can understand what I mean.

[Klaynos]

Do you want a control that turns off at 90 or a control that switches on and off to keep the water at about 90?

 

(But the quick answer is they use a resistance based thermometer, so resistance varies with temperature)

[pjl83]

Do you want a control that turns off at 90 or a control that switches on and off to keep the water at about 90?

 

 

Hi. I am wanting to keep the water at around 90 degrees. So switching on and off I think would be the answer.

 

(But the quick answer is they use a resistance based thermometer, so resistance varies with temperature)

 

Could the resistance be varied to achieve the 90 degrees?

 

Thnaks.

[Klaynos]

What you probably want is a PID controller then...

 

http://en.wikipedia.org/wiki/PID_controller

 

I did a course when I was an undergrad that looked into these devices specifically for temperature control, our lecture notes are online:

 

http://newton.ex.ac.uk/teaching/CDHW/Feedback/

[YT2095]

most all domestic kettles use a bi-metal strip as the temp "sensor" switch trigger.

[SH3RL0CK]

How accurate does your temperature controller need to be? How much are you willing to spend on your project? Generally, these two are correlated.

 

If you want something cheap, and don't need a lot of accuracy, the bi-metal switch as suggested by YT2095 is probably a good way to go. If you want a lot of accuracy and are willing to spend the money for it, look into Klaynos' suggestions.

[pjl83]

thanks for the replies guys.

 

The bi-metalic switch idea is a good one, but not right for what I require. The temperature doesn't have to be accurate, basically I need to keep the water as hot as possible without it boiling. There will be a seperate variable timer that will switch it off when required.

 

Think of it like a microwave. (just for explanation purposes) you put your food into the microwave and set the timer. The heat is set by the power setting and it heats at that temperature for the length of time that you specified. Now, what I want to do is have a system that heats with hot water. So......... I put in my "food" in and set the timer. The water will then stay at 90 degrees for that set time and then switch off when the timer runs out.

 

If I used a bi-metalic strip then I think once the switch has activated and cut the power, it may take too long to reset before allowing the power back on again and a lot of the temperature will be lost.

 

It may be a little more complicated than I first thought. I need a good think about this one now. Keep the suggestions coming though. They are recieved with many thanks

[YT2095]

you`ll not get a Really fast response anyway without using an analogue or Fuzzy-Logic method of regulation, there will be hysterisis due to the latent heat capacity of the water itself.

[John Cuthber]

I don't know how accurately the microwave oven models the system you are looking at but I see a potential problem.

When the timer switches the microwave oven off the "heating" stops imediately. With a jug of very hot water the "cooking" will continue for as long as it takes for the water to cool down.

 

Anyway, to answer the question, have you though about a hot water urn. They are designed to keep water hot enough to make tea or coffee without boiling it away. Possibly a bit bigger than whatever you had in mind.

this sor of thing.

http://cgi.ebay.co.uk/BURCO-ELECTIC-CATERING-URN-USED_W0QQitemZ180361049878QQcmdZViewItemQQptZUK_BOI_Restaurant_RL?hash=item29fe5b3716&_trksid=p3286.c0.m14&_trkparms=66%3A2%7C65%3A10%7C39%3A1%7C240%3A1318%7C301%3A1%7C293%3A1%7C294%3A50

[YT2095]

Burcos use a bi-metal strip too, the temp control dial just adds or releases pressure on this to alter its sensitivity.

in a MW, the only temp regs are to prevent overheating, usually disc or stud thermistors.