Temperature Coefficient of Resistance is how all but one Temperature Control mod works. TCR is the amount of resistance change a material has when it is heated. A higher TCR means the material changes more resistance over a given temperature change. Say the mod is setting in the living room at 72F and I vape at 360F. As that coil heats up to 360F, it's resistance changes by a predictable amount based on the coil materials TCR.
Typical single point TCR numbers:
|Material||TCR||Normalized resistivity*||TCR Resistivity Product|
|1C High carbon steel (Music wire)||0.00448||4.26||0.019085|
|Iron Wire (Binding wire)||0.00537||2.07||0.011116|
Nifethal 70 has twice the resistivity of Ni200. So if I make a coil with NI200 and it comes out to 0.05ohms then the same coil dimensions will be 0.1ohms with NiFe 70. Which can be very important as some mods even TC mods in TC mode, will not fire a coil that is less than 0.1ohms. Also Nifethal70's TCR is 87.5% of NI200. So 200% resistivity times 87.5% TCR makes the total resistance change over the same temperature range 175% of Ni200. It's difficult to measure small resistances. So in other words, Nifethal70 is easier to measure the temperature difference over the same temperature range than Ni200. This is why I have the table oriented by the [TCR * Resistivity product], higher is better. This is also why Stainless Steel which has a very low TCR works for temperature control, because it has a much higher resistivity.
*But watch out, I have measured a typical ceramic heating element that supposedly uses Tungsten at a TCR of .0035. And with no way to calculate it's Resistivity, I would bet that is off too. I would bet it's some form of Tungsten but not pure Tungsten. In practice, these ceramic heating elements can wreak havoc combined with a TC mod so I always start with a low temperature setting and work my way up gradually until I get the vape taste I know is right. And there is more data here and here.
*1/29/2019 - In revisiting my pages after three years, I forget how I calculated the resistivity numbers. The units are not ohm.meter nor ten to the minus 8. I do know that I compensated for diameter and length but I can't remember how. So do not take these as classical physics/material properties Resistivity numbers per the rho formula and units. I did normalize these somehow so they could all be related and relative to each other. They say memory is the first thing to go, well it's not the first, but it sure is one of the first LOL.
The other thing to notice is this: SS ~.001, TI ~.003, Tungsten ~.004, NI ~.006. So a mod that supports Stainless Steel, Titanium, Ceramic and Nickel modes covers the TCR range of vaping wire materials from lowest to highest. So if the mod doesn't have a mode where I can set or adjust the TCR for the specific wire material, I pick one that is close and adjust temperature setting slowly upward from it's lowest setting on the mod until I get the taste I am searching for. For example. Let's say the mod doesn't have a Nifethal70 mode, nor a TCR mode where I can adjust TCR. Pick Ni200 mode and start at 200F and gradually increase temperature until I get the taste I am used to. If I want a 360F vape then a setting of 360 X 87.5% = 315F. And guess what, that is what I find in practice. If I set a mod to NI mode and I am using NiFe 70 wire, then a temperature setting of 320F gives me the taste I am used to getting at 360F with Ni200 wire. That's why the more modes a mod has is important if it does not have a TCR mode where I can enter the TCR by hand. I can get close using other modes and a temperature offset. That is also why it is more important for a mod and atomizer to be consistent. That is every time I pick it up, it is giving me the same vape experience. In other words, really good connections and resistance is NOT jumping around because of lousy 510 connections or flaky coil leg posts.
A consistent mod is better than an inaccurate one. I can fix the inaccurate one by a temperature setting offset. But inconsistent ones are a real pain. A perfect example of this is weak springs in a spring loaded 510 connector on the mod like the Hohmtech Wrecker G2 and Slice. After I replaced the 510 connectors with better more stiff spring loaded center pins from Fatdaddyvapes and Varitube they suddenly became a lot more consistent. Another example is Velocity style or other grub screw posts where instead of capturing and securing the coil legs they cut the wire legs leaving the leg to make and break contact as it heats up. These intermittent contact issues make a TC mod extremely inconsistent as well as inaccurate. Another example is an atomizer that uses a spring for current flow. This is different than spring loaded pins. On spring loaded pins the spring does not carry current it just adds mechanical force to make a solid connection between contacts that DO carry the current. But on the Kayfun V4, there is a spring that carries current from the 510 center post to the positive post on the build deck. Well guess what? First, this spring doesn't have consistent electrical contact between it's two ends and that spring acts as part of the heating element chain even though it is no where near the build deck. Yes that spring has a TCR too. For those reasons I do not use the Kayfun V4. Out of the box it is a very inaccurate TC atomizer. There is a fix though. I have to remove the spring, add a washer under the bottom 510 center pin that is filed or ground to the square shape needed for it's insulator. And another thing about the V4, it is NOT 100VG friendly. Though I can drill out the juice holes to make it friendlier. Given excellent atomizers out there that do not suffer these problems, the V4 is just not good. And they repeated the same problems with the V5. When it comes to Kayfuns, I like the V3.1 or earlier.
With a thermocouple or other temperature measuring device and a DNA mod using ESCRIBE which is a PC based application from EVOLV, I can determine what the real TCR is for the exact wire or heating element. I will show an example of this below:
The method of test is very simple. I used a KF deck and make a coil with the wire to be tested so that I could insert a temperature probe in the coil. Since my temperature probe came with a digital multimeter and is metal, I wrapped a single layer of paper towel around it then inserted into the coil to keep from shorting out the coil or causing hot spots. Although mine came from Lowes they have since discontinued it but inexpensive multimeters and immersion type k thermocouple probes can be found on Amazon and ebay. The KF is attached to a Hotcig DX200 which is a DNA200 device. Then the DX200 is attached to a computer via USB cable and ESCRIBE connects to the DX200 to read real time values.
The below example is with a DNA200. If I have a mod that enters one number for a TCR value vice a file with many points describing a curve, then I can do this test at the temperature I normally vape assuming the mod shows live resistance and not base resistance. In that case I need two data points to calculate TCR. Lets say I want to vape at around 400F. Take one measurement at around 400F and one at around 450F then calculate TCR using this equation:
Where R is in ohms and T is in degrees Celsius.
It won't be as accurate as a curve of points but I have found a single TCR setting even in a DNA200 to be close enough as long as the TCR was calculated from data measured in the vaping range.
Set the TCR in the TC mod, in this case a DNA200 close to the wire type being used. Connect the DNA200 to a PC via USB cable and open ESCRIBE. Go to device monitor and set the temperature starting low at 200F. I set power to max but I start at 50W as most coils will take 24W or less to reach sustained temperature in a reasonable amount of time. Ensure only live ohms is checked. Then click record and set an output file. Then fire the atty for 10 seconds by clicking on Puff. Repeat until the meter stabilizes. This may take a minute or two in order to overcome the thermal mass of the temperature probe.
When the meter has stabilized record the temperature and stop recording in escribe. Open the file, I use MS Excel and select the last 10s worth of data and use the Average function to find the average resistance over the puff. Record the resistance. I now have the temperature and resistance at that temperature.
Increase the temperature setting by 50F and repeat. Stop when the thermocouple reads around 500F. I now have several data points and can generate a TCR profile. I use Excel:
I set the multimeter to Celsius and calculate Fahrenheit in excel.
And then I generate a .csv file to upload into the DNA200 for a custom wire material, I use Notepad:
Most .csv files for the DNA200 are "normalized" like the one below from steamengine:
But in practice I have found "normalization" isn't necessary. After I create the file, I then load into the DNA200, set the Temp to 360 and recheck the thermocouple reading. Using wires with TCR of .004 or above, my method results in thermocouple measured temperature being within one or two degrees of set temperature.
There is another method where it requires submersing the coil in a bath of mineral oil and heat it measuring with a resistance checker or ohm meter. But I find my method far easier, less messy and accurate enough. This also has the added benefit of figuring in the mod and atomizers internal resistances so I don't have to mess around with the DNAs internal resistance setting which few mods come with that setting from the factory. And even when they do or I test it myself with a 510 copper shorting plug, this method doesn't need it. I have also shorted the posts on the atomizer with heavy gauge copper wire. This will accommodate for BOTH the mod's internal resistance AND the internal resistance of the atomizer. Then enter that setting in ESCRIBE for the Mod resistance. As long as I use that same atomizer in theory, it should be accurate. However 510 connectors and post for the coil legs get flaky. Now if I want to test atomizer internal resistances, then I use a copper shorting plug in the mod's 510 and set the mod resistance to that and then test atomizers by shorting the atomizer posts with copper wire and measure away. Though it's anecdotal because atomizer selection is based on other more important factors than it's internal resistance which can be accommodated for by my method and just leaving it at that.
All the information contained in these pages are only the opinions of the author and the author is not an expert at anything.