Resistance wire manufacturers are happy to post the specs for their wires. Unfortunately, that information is usually incomplete, not tabulated, applies only to their specific alloys, has proprietary terms for each alloy, and is in wildly varing units of measure. I've spent considerable time sorting, converting, and tabulating that data. I hope this post saves someone interested in the subject a little bit of time.
To even figure out what we are talking about, here is a cross reference guide to help decipher the marketing speak.
Before we get to the properties of each alloy or metal element, I'd like to take a whack at some of the common misconceptions on this forum about metal alloys.
Stainless steel is any steel alloy with at least 11% chromium content. Ordinary steel forms an iron oxide film on its surface that is an active oxidizer. In other words, it encourages more oxidation even under its own layer, eventually penetrating and corroding deep into the metal. Stainless steel instead forms a chromium oxide film on its surface that is tough, non-reactive, discourages further surface corrosion, and prevents the corrosive penetration beneath its surface.
Most Nichrome alloys are stainless steel. The composition of 330 stainless steel is Fe 42%, Ni 36%, Cr 20%, Si 2%. The composition of Nichrome 40 is Fe 40%, Ni 37%, Cr 21%, Si 2%. One is called SS, the other NiCr, but they are both solidly in the stainless steel camp. And their properties are not suprisingly almost identical. Nichrome 70 and 80, however, are indeed not stainless 'steel', but their properties make them a very close cousin. In fact, the latest alloys (the FeCrAl series) made for durability, high temperature, high resistivity, and low TCR are, in fact, high iron content stainless steel with a little Aluminum added.
If you are planning to build your own coils, you probably already know what properties you are looking for in selecting a resistance wire. But, in case you're a noob like me, may I suggest the unique properties important to ecig coil design. Foremost is the tiny size required. That makes for short coil windings and in order to prevent the coil from acting like a dead short to the battery, the wire must have a resistance of an ohm or two. For this short of wire, the dia therefore, must be very, very thin. But, of course, now we're talking very fagile, also. So, one of the wire properties to pay close attention to is resistivity. If you choose a wire with very high resistance, the dia can be larger, and the coil would be more robust.
Choosing a wire with a high temperature coeffiency of resistance (TCR), allows you to measure the temperature of the coil, since all you have to do is measure its resistance and decide what to do with that info. Since the resistance rises with the temperature, the voltage across the coil will rise with temperature, and you can measure that easily, even far away from the coil, let's say in a control circuit located back by the battery. Kinabaloo is working on that project now.
Another reason for high resistivity coil wire with a larger diameter is a larger surface area per ohm, for the same length wire. The surface of the wire is where the work gets done. More surface (at the same temp), more vapor.
The HRA entry in the table is one I just found and haven't nailed down any other properties for it, yet. If anyone has some info, please post it. I included it because its resistivity is high enough to make it an interesting alloy, almost regardless of its other properties. HRA is simply my abbreviation for what the literature said was an "High Resistivity Alloy". There are blanks all over this table, but I have grown tired of trying to fill in every detail. Elemental entries usually just have the melting point instead of maximum working temperature that the alloy manufacturers usually publish. The maximum working temp will be from a little below, to a lot below the melting temperature.
The biggest variations of units were the published resistivity data, so I converted all the different units into how many 'ohms per inch of .004" dia (.1mm), 38 gauge dia wire'. The table is sorted by this property as well.
To even figure out what we are talking about, here is a cross reference guide to help decipher the marketing speak.
Before we get to the properties of each alloy or metal element, I'd like to take a whack at some of the common misconceptions on this forum about metal alloys.
Stainless steel is any steel alloy with at least 11% chromium content. Ordinary steel forms an iron oxide film on its surface that is an active oxidizer. In other words, it encourages more oxidation even under its own layer, eventually penetrating and corroding deep into the metal. Stainless steel instead forms a chromium oxide film on its surface that is tough, non-reactive, discourages further surface corrosion, and prevents the corrosive penetration beneath its surface.
Most Nichrome alloys are stainless steel. The composition of 330 stainless steel is Fe 42%, Ni 36%, Cr 20%, Si 2%. The composition of Nichrome 40 is Fe 40%, Ni 37%, Cr 21%, Si 2%. One is called SS, the other NiCr, but they are both solidly in the stainless steel camp. And their properties are not suprisingly almost identical. Nichrome 70 and 80, however, are indeed not stainless 'steel', but their properties make them a very close cousin. In fact, the latest alloys (the FeCrAl series) made for durability, high temperature, high resistivity, and low TCR are, in fact, high iron content stainless steel with a little Aluminum added.
If you are planning to build your own coils, you probably already know what properties you are looking for in selecting a resistance wire. But, in case you're a noob like me, may I suggest the unique properties important to ecig coil design. Foremost is the tiny size required. That makes for short coil windings and in order to prevent the coil from acting like a dead short to the battery, the wire must have a resistance of an ohm or two. For this short of wire, the dia therefore, must be very, very thin. But, of course, now we're talking very fagile, also. So, one of the wire properties to pay close attention to is resistivity. If you choose a wire with very high resistance, the dia can be larger, and the coil would be more robust.
Choosing a wire with a high temperature coeffiency of resistance (TCR), allows you to measure the temperature of the coil, since all you have to do is measure its resistance and decide what to do with that info. Since the resistance rises with the temperature, the voltage across the coil will rise with temperature, and you can measure that easily, even far away from the coil, let's say in a control circuit located back by the battery. Kinabaloo is working on that project now.
Another reason for high resistivity coil wire with a larger diameter is a larger surface area per ohm, for the same length wire. The surface of the wire is where the work gets done. More surface (at the same temp), more vapor.
The HRA entry in the table is one I just found and haven't nailed down any other properties for it, yet. If anyone has some info, please post it. I included it because its resistivity is high enough to make it an interesting alloy, almost regardless of its other properties. HRA is simply my abbreviation for what the literature said was an "High Resistivity Alloy". There are blanks all over this table, but I have grown tired of trying to fill in every detail. Elemental entries usually just have the melting point instead of maximum working temperature that the alloy manufacturers usually publish. The maximum working temp will be from a little below, to a lot below the melting temperature.
The biggest variations of units were the published resistivity data, so I converted all the different units into how many 'ohms per inch of .004" dia (.1mm), 38 gauge dia wire'. The table is sorted by this property as well.
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