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Thermocouple

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by stubborn66, Mar 10, 2012.

  1. stubborn66

    stubborn66 New Member

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    Hi
    I am new to this site and have limited knowledge, just enough to get me in trouble! I also apoligize if this question does not fit this forum or if it is mundane to this group.

    I am building a bio diesel processor to make fuel from vegetable oil. Part of what I need to do is warm the vegetable oil to about 140F to get a good seperation of the fuel and the glycerin. I got an idea to use the heating element from a hot tub (220volt AC) to warm the oil as I circulate in the mixing process. My question is this, do they make a thermocouple and controller that would sense a low temperature and trigger the heating element to come on, then when the temperature reaches 140F cut the power. This is opposite of what I know Thermocuoples normally do. If that is not possible, does anyone know of another way to control the heating element that I can use without getting too complicate?

    Thanks in advance
     
  2. Steaphany

    Steaphany New Member

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    Right off, your description reminds me of the temperature control of a water heater. Most will switch two phase 220 VAC and provide a variable set point from about 100F and up to near boiling. With a water heater thermostat, it will sense the fluid temperature and apply power to the heating element until the temperature exceeds the set point after which is maintains the temperature by reactivating the element as the fluid temperature drops below the set point.

    As long as the heating element current draw is within the operating range for a water heater thermostat, it should drop in and do just what you need.
     
  3. stubborn66

    stubborn66 New Member

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    Thanks for the help. I didnt think of a water heater, but I need to see how it senses temperature.
     
  4. dave

    Dave New Member

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  5. kubeek

    kubeek Well-Known Member

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    They use bimetal thermostatic switch.
     
  6. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    Recently there have been temperature controllers similar to this unit pouring in from China. Whiile I can't say I have tried any of them they are unbelievably inexpensive. A unit like this used with a SSR (Solid State Relay) to power the heating element might be a good solution. The next time the wife does one of her Amazon orders I plan to get one just to see if they are any good. I have seen these things on Amazon for quite sometime.

    Looking at your control temp of about 140 Degrees F there are a number of sensors and methods you could use including a thermocouple. What I am suggesting based on your post is a simple canned off the shelf solution. Building your own from scratch may get a little complicated. Also needless to say wiring something like this up to power 220 or 240 VAC heating elements requires some caution.

    Ron
     
  7. Diver300

    Diver300 Well-Known Member

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    One of these:- http://www.screwfix.com/p/resettable-immersion-heater-11/31397 contains a thermostat, like this http://www.screwfix.com/p/dual-safety-thermostat-7/21275

    That can be set from 30 - 70 °C, or about 86 - 158 °F.

    That sort of immersion heater seems to me to be perfect for heating the oil for you, to a controlled temperature.

    You can get a flange to screw the immersion heater to here:- http://www.toolstation.com/shop/Heating/Central+Heating/Mechanical+Flange+2+14/d230/sd2708/p38614
     
    Last edited: Mar 11, 2012
  8. user_88

    user_88 Member

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    What volume of fluid are you trying to heat?
    What means are you using to circulate the fluid about the heating element?
     
  9. stubborn66

    stubborn66 New Member

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    I am going to heat 50 gallons at a time and I am using a teflon impeller pump. I just saw on version of these processors where they used a hot water heater in the process. I also saw an immersion heater used.
    I have an old hot tub that had a new heater installed in it before we quit using it and had a brain storm to use it. I am using a clear plastic cone tank so I can see when the fuel seperates.
     
  10. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    This reminds me a lot of one of the lab experiments the professors have the students do at the college I work at. It's used to control the temperature of a soldering iron--to keep it within ±10° of the solder melting point. The students use a type J thermocouple attached to the tip of a soldering iron. They connect the + and - leads to a digital multimeter, which is connected to a computer via a GPIB cable. The students put together a LabVIEW VI that reads the voltage on the multimeter (through the bus). When it reads a voltage below the one that translates to a temperature that is 10° below the desired one, it tells a DC power supply (also connected by GPIB) to switch on. The power supply is connected to a solid state relay, which connects the soldering iron to mains. Then, when the temperature (voltage on the thermocouple read by the DMM) becomes higher than the one for 10° more than the desired temperature, it shuts off the power supply, which causes the SSR to disconnect the iron from mains.

    This is a fairly drawn-out explanation, but it sounds quite similar to what you're trying to do. The whole process described above requires a lot of benchtop equipment and expensive software, but the whole thing could be done with a single microcontroller. It can use the ADC to read a voltage and turn on/off a relay depending on the value it reads. There could even be simpler ways to do it, but my point is that switching the heater off when it reaches a high enough temperature, and turning it on when it cools is actually quite simple. It is a setup that is used in many different applications.

    Regards,
    Der Strom

    P.S. I just re-read that and I see I have a lot of confusing sentences in there. I hope you can figure out what I mean :p:rolleyes:
     
    Last edited: Mar 11, 2012
  11. aljamri

    aljamri Member

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    Heating element is a good solution.

    for such low temperature no need to go for expensive thermocouple, you may use as low price as your budget and accurecy you require. You may use RTD, NTC or PTC, or even a diode as a temperature senser.

    Your heating element may have Normally Open, Normally Closed and a Common contact, so you just need to alter the connection to get work the other way around. If It has only one connection, connect it to a suitable relay with three contacts where you can get the required connection.
     

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