## Monday, 24 August 2015

### Resistance wires and TC

How it works, and how we can trick the mods?

All materials have a temperature coefficient. In case of metal wires it describes the relative change of a resistance that is associated with a given change in temperature. It means how the coil resistance changes when it is heating up. Just as all materials have a certain specific resistance (at 20* C), they also change resistance according to temperature by certain amounts. For pure metals, this coefficient is a positive number, meaning that resistance increases with increasing temperature. For the carbon, silicon, and germanium, etc. this coefficient is a negative number, meaning that resistance decreases with increasing temperature. For some metal alloys, the temperature coefficient of resistance is very close to zero, meaning that the resistance hardly changes at all with variations in temperature. Generally the resistance values for conductors at any temperature other than the standard temperature (reference temperature, usually specified at 20 Celsius) may be determined through the formula:

The “alpha” (α) constant is known as the temperature coefficient of resistance, and symbolizes the resistance change factor per degree of temperature change.
Within our interest surely are the most popular "temperature" wires:
* Ni200 - temp. coeff. 0.006,
* Titan - temp. coeff. 0.0035,
* Nife30 - temp. coeff. 0.0032,

Let's go to the practice.

For better understanding of above formula I created a chart showing how the resistance changes with increasing temperature of the coil:

And now, how is it used in TC mods?

Let's say we have the 0.1 ohm (measured at 20*C) coil of Ni200 and set the temperature 200*C. How the mod will work?
a) it fires the voltage, and
b) measures the coil resistance (all the time),
c) when the resistance will reach 0.208 ohm mod "knows" that the temperature of coil is 200*C,
d) if resistance exceeds the 0.208ohm the temperature is higher, so the voltage must be reduced to reduce the power (and of course temeperature).

That's it.

How to use other wires with mods dedicated for Ni200?

Much of the TC mods can work only with Ni200 wires. What is happening if we try to use Ti or NiFe wire on it? Let's look at the figure:

As we can see Ti and NiFe resistance steep less than Ni200. So 0.208 ohm means 200*C in case of Ni200 but for Ti it corresponds to 330*C and 360*C for NiFe wire. So the wire other than Ni200 will burn out immediately or at best will burn our mouth.
But we can trick the mod :-) Let's see at Ti in the above figure again... We have to check what resistance corresponds to 200*C for Ti wire? It is about 0.160 ohm. Then we need to find what temperature corresponds to 0.160 ohm in case of Ni200 wire? It is about 120*C. So in order to reach 200*C using Ti wire on mod dedicated for Ni200 wires we have to set 120*C.
Of course such conversion may be a little uncomfortable, so I prepared the ready to use table with temperatures of our interest:

How to use it?
Let's say we have SnowWolf mod (Ni200 only) and want to use Ti wire at 220*C. So we have to find 220 in first row (the red one) and get down to the Titan row - YES: if you want to wape at 220*C with Titan wire on Ni200 dedicated mod you should set 165*C.

Regards :-)