Nichrome Wire

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  #1  
Old 01-16-14, 06:20 AM
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Nichrome Wire

Hello

I'm making my own vacuum sealer for sous vide cooking.
I ordered a vacuum pump and some flat nichrome wire.
I'm pretty new to electronics so I hope someone can help me out.

The nichrome wire needs to seal plastic bags.
I have the following information about the nichrome wire:
-2mm x 0.25mm (0.078" x 0.0098")
-resistance 2.2 Ω/m (2.2 Ω/39,37") at room temperature rising to 3.5 Ω/m (3.5 Ω/39,37") at 600C.
-Becomes red-hot at about 10A in free air.

I will put a teflon tape tape over the wire to prevent too much heating.

My questions are:
-I need only 34 centimeters (13.38"), is it correct if I say that my Ω is 0.73?
-What is the Amp I need for sealing the bags? I guess 10A (which makes the wire red hot) is too much, I don't want to destroy my plastic bag.
-If I know A and Ω, I can use the Ω's Law calculator which gives me the required Voltage and Watt?

I hope someone can help me, thanks in advance!!

Greetings

Thomas
 
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  #2  
Old 01-16-14, 06:27 AM
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Will you be heating one or both sides of the bag? Do you have silicone barriers for your heat strips? Are you making a continuous hot bar (stays hot all the time) or an impulse sealer (heating element turns on for set period of time)? What type (material) bags will you be sealing?
 
  #3  
Old 01-16-14, 07:00 AM
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OP welcome to the forums.....

Your posts need approval when your new here... You should see your post now....
 
  #4  
Old 01-16-14, 07:14 AM
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Hello

Thank you for reading my questions.
I'm going for a constant heating.
Impulse sealing seems a little bit too difficult for me.
This is my intention for the way of heating:
-The lowest side will have the nichrome wire with a teflon strip above it.
-The upper side will only be for pushing the bag together.

The bags are 7-layered, it consist of:
-Linear Low Density Polyethylene (LLDP)
-Ethylene-Vinyle Alcohol Copolymer (EVOH)
-Nylon
- they are ribbed (necessary for a vacuum sealer)

Thank you!

Thomas
 
  #5  
Old 01-16-14, 11:35 AM
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First off, you have to use silicone, not Teflon. Teflon will melt the first time you use it.

Second, you won't be able to use constant heating. Vacuum sealers must be impulse. If the wire is constantly hot, it will seal as soon as you press down on it, and the vacuum will not be able to do its job.

You will have to either make a manual button to start the seal after the vacuum is complete, or use a vacuum switch to activate it automatically once a certain vacuum is reached (that's how the manufactured units do it).
 
  #6  
Old 01-16-14, 11:40 AM
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Hello

I'm using two different push buttons for controlling the vacuum pump and for controlling the sealer. Thank you for mentioning the silicon!
 
  #7  
Old 01-16-14, 11:43 AM
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Teflon will melt the first time you use it
Odd since it is used in skillets and the plastic bag sealers I used in a warehouse were a stiff cloth like material coated with Teflon.
 
  #8  
Old 01-16-14, 12:01 PM
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It melts at just over 600F (320C). You are not supposed to get Teflon cookware anywhere near that hot - and if you do it's ruined.

That is right around where the wire will need to get to in order to seal that kind of bag quickly. Silicone (melts at over 2500F and is easier to clean) is just a better way to go for a food prep device.
 
  #9  
Old 01-16-14, 12:16 PM
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Heat sealers most commonly use PTFE coated fiberglass to prevent the plastic from sticking to the sealer. It's another reason why you can't leave the heater on continuously as it can eventually get hot enough to melt the Teflon. Silicone is often used as the backing material supporting the heating wire/strip so that there is something firm pressing the plastic together when sealing. The silicone can take the heat and it's available in different durometers (hardness or squishiness) so there is some give when the jaws are brought together. So, you have the metal frame of your sealer, a silicone piece supporting the heating wire, the heating wire and then on top is the PTFE film. Don't forget to make it so the PTFE strip is replaceable. It's a wear item especially if you don't have a timer limiting how long the heating element can be energized.
 
  #10  
Old 01-16-14, 12:27 PM
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I shall order a minimum of 2 meter, so a mistake can be made :-p
I'm thinking of using the body of a broken vacuum sealer and replace the wiring and the vacuum pump. It's pitty that the original sealing element is gone.
So I need to get it up around 300C for a good sealing?

Thank you for the info!
 
  #11  
Old 01-16-14, 01:56 PM
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Can you get wire, vacuum pump and push buttons for less than the cost of a working unit?
 
  #12  
Old 01-16-14, 02:05 PM
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First I tought buying one, but I didn't find a good one unless it was 200 dollars.
I read that the foodsaver vacuum sealer is a good one, but breaks pretty fast.
150 dollars seems a lot of money for the possibility of a fast end.
I have a non functional vacuum sealer which has the buttons and the frame.
The vacuum pump costs around 25 dollars and should be stronger than most basic vacuum sealers.
And when something stops working, I think it's easier to repair.
I hope everything works out fine ;-)
 
  #13  
Old 01-16-14, 05:56 PM
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Note that you cannot control the amperes or watts except by selecting a desired voltage (using a transformer if needed) and selecting the resistance (based on thickness and length of the Nichrome wire).
 
  #14  
Old 01-17-14, 05:38 AM
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I think I've got it.
I read a nichron wire table that said that for bringing it up to 280C I need around 4 Amp.
At the length of 34cm (1 foot) My resistance is about 0.75.
This way I need 3V and 12W.
I believe ther is no way of overheating it this way?
 

Last edited by thodesmet; 01-17-14 at 07:29 AM.
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