# Simple Circuit Creating Heat Across Stainless Steel Mesh. Finding Correct Size.

#### caribouuu

Joined Dec 24, 2015
3
I'm not sure how this will be received here, but has anyone ever heard of an MFLB? It's a simple, battery powered herb vaporizer, and I'm trying to make a DIY version of it for myself. A layout of the basic circuit can be found attached to this post. Basically, you're shorting out a battery to generate heat across the "trench" which is comprised of steel mesh. The rest of the circuit is made of bare copper wire.

What I'm trying to figure out is what size steel mesh I should use to get to the desired temperature (170 C) in about 3-5 seconds.

So far, I've used P=(I^2)(R) and deltaQ = cm(deltaT) to get this equation:

T(t) = (V^2/Rcm)(t)

Ideally, this circuit would create little heat in the copper wire and in the battery (obviously) and most would be generated in the steel mesh region. So, my question is, what combination of resistances would create this type of situation? As I understand it, the resistance in the copper wire will depend on the length and diameter. But how do I go about finding the resistance of a square steel mesh?

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#### wayneh

Joined Sep 9, 2010
17,504
Do you have a sample of this mesh, or have any information about where it comes from? You could measure its resistance with a proper (low ohms capable) meter, although the resistance will change with temperature. The increase in resistance with temperature is the only thing keeping this from going poof like an old flash bulb.

Your battery will also have an internal resistance. What kind is it?

Once you know the approximate current in the mesh, you can choose the proper copper wire. Or, you could just rely on overkill. I mean, use some 16 gauge house wiring and rest assured that it is more than big enough to carry the current with minimal loss.

#12

#### #12

Joined Nov 30, 2010
18,224
Measuring a sample piece is easily the best method, compared to trying to math it out.

#### caribouuu

Joined Dec 24, 2015
3
The increase in resistance with temperature is the only thing keeping this from going poof like an old flash bulb.
Could you elaborate on that? Does resistivity decrease with temperature? What would happen in a material where R was nearly constant with temp?

Do you have a sample of this mesh, or have any information about where it comes from? You could measure its resistance with a proper (low ohms capable) meter, although the resistance will change with temperature.
I don't necessarily have a specific mesh that I'm using. I have a rate of heating that I'm trying to hit (Approximately 25 C/s) So I'm trying to relate that to Resistance and therfore to properties of the mesh like diameter and %free space.

#### caribouuu

Joined Dec 24, 2015
3
The point about getting large grade copper wire is well taken though.

#### ronv

Joined Nov 12, 2008
3,770
Could you elaborate on that? Does resistivity decrease with temperature? What would happen in a material where R was nearly constant with temp?

I don't necessarily have a specific mesh that I'm using. I have a rate of heating that I'm trying to hit (Approximately 25 C/s) So I'm trying to relate that to Resistance and therfore to properties of the mesh like diameter and %free space.
Unless you know someplace where you get made to order mesh you should just buy a sample of what other people are using and measure it. We could assume some numbers, but in the long run I don't think that will help you.

#### wayneh

Joined Sep 9, 2010
17,504
Could you elaborate on that? Does resistivity decrease with temperature? What would happen in a material where R was nearly constant with temp?
You want a material with a positive temperature coefficient (PTC) so that resistance increases with temperature. This prevents a runaway situation. If resistance decreased, the rate of heating would accelerate and the mesh would ignite, if the battery was capable of that much current. With a PTC, the current through the mesh and thus the rate of heating decreases as the mesh gets hot. Same deal with a lightbulb. It quickly reaches a steady state.

To calculate what you want to do, on paper, requires a lot of data about the properties of your mesh (the dimensions, temperature coefficient and so on), and some integral calculus. Assuming you're familiar with calculus, the math is usually easier than finding the data.

I think you'll get farther faster with an experimental approach than an analytical one. Just don't forget how much of a factor the battery is. Your experiments may depend more on the battery than on the mesh.