First time on AAC, don't know much about electronics. Been googling to find the answer to my question but no can do. Last search got me to AAC, so here goes. Given amps required and resistance/ft, can someone tell me how to calculate the power required to heat a resistance wire to a specific temperature? The wire is René 41, will be 1' long, and will be used to cut foam.
Thanks.

 

Power consumption is v2/R . That is, voltage squared divided by resistance.

Temperature is difficult to calculate (a question of how fast energy is pumped into the wire vs extracted. I would put 25 watts into it and control with a Pwm controller. Then slowly turn up the power until I was happy.

You could also work the other way, how much energy will it take to melt the foam to achieve a satisfying cut rate. For the width of your wire.

 

If you read the details here, it says you can use the nichrome calculator here and just make a small adjustment to get your answers.

That René 41 sounds like great stuff!

I get about 5W (1A at 5V) for 350°F, using 28 gauge nichrome to simulate 27 gauge René41.

 

Power consumption is v2/R . That is, voltage squared divided by resistance.

Temperature is difficult to calculate (a question of how fast energy is pumped into the wire vs extracted. I would put 25 watts into it and control with a Pwm controller. Then slowly turn up the power until I was happy.

You could also work the other way, how much energy will it take to melt the foam to achieve a satisfying cut rate. For the width of your wire.

GT,
Thanks for your response. I've decided I'll have to answer my question empirically.
Western PA really runs from Erie south past Uniontown, but it usually refers to the Pittsburgh area. I live in Nashville now but I'm from there as well. Small town called Verona, NE of The city. Whereabouts are you from, if you don't mind my asking.

 

If you read the details here, it says you can use the nichrome calculator here and just make a small adjustment to get your answers.

That René 41 sounds like great stuff!

I get about 5W (1A at 5V) for 350°F, using 28 gauge nichrome to simulate 27 gauge René41.

Thanks, Wayne. I bought my wire from Jacobs so I was familiar with the data in your link. Bottom line, the Jacobs site gives enough info to do what I want to do, but I wanted to know how the wire info was calculated. I'm not sure it was, except, perhaps, the first time the table was created - most sites refer to virtually the same table of data!

 

When I built one I just did it empirically. A lenth of wire that you like and a variable power supply. Adjust for the "feel" you like.

 

... I wanted to know how the wire info was calculated.

Heat INTO the wire is easy, volts times amps. Heat OUT OF the wire is more complicated. Heat leaves by conduction, radiation and convection. One can estimate the magnitude of each of these 3 modes using basic engineering approaches. The rate of each mode will depend on the temperature of the wire. You then solve for the wire temperature that gives a steady state; equal heat in and out.

My hunch is, if the wire is quite hot, that radiation dominates. So I'd start by looking up how to estimate black body radiation from a long wire and add maybe 20% to that for the other heat losses. See what value you get and recheck the assumptions.

Or, collect a buttload of data and fit it with an equation.

 

GT,
Thanks for your response. I've decided I'll have to answer my question empirically.
Western PA really runs from Erie south past Uniontown, but it usually refers to the Pittsburgh area. I live in Nashville now but I'm from there as well. Small town called Verona, NE of The city. Whereabouts are you from, if you don't mind my asking.

I'd also say empirically is the best approach if you're going to use the wire to cut material ... although I find wayneh's black body observation very interesting

 

That René 41 sounds like great stuff!

wow... you're right, that wire looks like the best there is... I might start doing some tests with it for a dynamic wire cutter that I've been working on...
EDIT: and it's expen$ive too!