Kanthal - Increased resistance

[KeyLo]

Hi. I appreciate any insight and help anyone can provide

I am trying to wrap my RSST. Its not my first time around the block. I use a .4 ribbon kanthal.

3/4 wrap use to get me 0.7 to 0.9 ohms. I bought the kanthal in July from temco and it was great.

Now when I use the same technique and material, I get 1.8 to 2.0 ohms. I used 2 ohm meters and a multimeter. I also know its about 2ohms by the way it lights up. 3 or 4 pulses at 0.7ohms and it was bright. it takes about 8 or 9 pulses to get the same effect.

I did a voltage drop test on the mod and Im getting 0.23v drop at the posts.

I tried torching the wire. Not torching the wire. Lightly torching the wire. Its a new rebuild. So theres no gunk. No hot spots. I did a ohm test just on the wire and I get the same reading. Before torching... after torching.


I tried rebuilding using new wire. all the same results.

 

[Vaslovik]

Well I'll take a stab at it. In my experience ribbon wire can be a real pain in the patootie to use, and I've run into some pretty strange things like this too. You can get ohms creep with round kanthal too, but usually not that bad. What I've seen with the ribbon wire is that it's pretty springy stuff, even when you anneal it with a torch, and unless it's locked down real good on both ends it can unwrap a little from your coil, increasing it's length over the coil, and increasing the ohms as well.

On an RSST you have a spring-loaded capture on the center post, which is a nice feature, but might it be that the spring has weakened under heat over time? I know that with my AGA-T2 the nut on the center post sometimes comes loose from heat, and the ohms jump upwards. I just tighten it back down, pulling the kanthal tight and that fixes it.

I've goofed around with ribbon wire a bit, but I always seem to end up just going back to 28g. kanthal, which seems to work with fewer problems, although....

Yesterday I was re-coiling, and somehow, I have no idea how, it was way sub-ohm, like .34 with a 4/3 wrap! I threw on another loop for a 5/4 wrap, and got .51 ohms! I have NO IDEA how that happened! A 4/3 wrap has always gotten me between .7 and .82 ohms with 28 g. kanthal. Very weird.

All I can say is that sometimes bizarre things happen, and we may never know why....

[cue twilight zone theme...]

 

[Oomee]

Look up the specifications for the wire .
It will give you a resistance per length . EG 1Ohm per 1 cm (just an example) .

Measure a known length of wire and compare to the specifications .
If the wire is good , then the problem is elsewhere , probably as Vaslovik said , in the terminals of the atty or the 510 connectors .

Word of advise , batteries in multimeters do wear out , and sometimes the cheap meters can misbehave before the it signals battery low .

 

[KeyLo]

Thanks Vaslovik. Super frustrating. I tried tightening it too. I measure the ohms just on the kanthal before installing. It read 2.2ohms. After torching and cutting, it lowered to 1.8ohms. I tried a tight wrap. Than a loose wrap. Maybe my wire has gone bad. i dont see how that could happened.

@Oomee. I thought about that. I used a voltage drop test to see if there were any weak points. 510 connectors are good. 0.2v drop at the posts.

I tried it in my provari. I tried it in a dedicated 510 ohm meter. I used my multimeter. all gave the same reading +/- 0.1 ohm.

Last thing. Specs of a 0.4mm ribbon Kanthal are on par with a 28guage round. 4/3 wrap on most genny tanks are around 0.7-0.9ohms with round kanthal.

I have 28 round. Its 0.8ohms right now in the RSST. I have 20ft of 0.4 ribbon. I dont want to throw it away.

 

[Coastal Cowboy]

You might be the victim of normal variations in manufacturing processes, and you're dealing with a length of wire that's somehow out of spec, even though it's on the same spool as the stuff you were using before and reaching the target resistance.

Maybe you don't have to throw away the whole spool (I hate wasting anything).

What you might want to try is unspooling a few feet of wire and weight it down on the table/desk/countertop at each end. Use your multimeter leads to test the resistance of a few inches of wire at a time, starting at one end and working to the other. If your hands are steady enough, you can get about the same distance between the leads as you move down the wire. If you have a yardstick or tape measure, you can get dead on.

As you know, the resistance measured will be for the length of wire between the two leads, and you can see how the resistance changes by moving the leads closer together or farther apart.

I actually had the reverse happen with a spool of 30ga wire. My resistance went down when using the same length of wire on a Protank coil rebuild. I knew I had the right length of wire and the right wrap count and tightness. But I went from ~2.0 ohms to 1.4! I stopped rebuilding a while ago, but I recall going through about six feet of wire before I found a section of it that had close to the spec resistance.

 

[ScottP]

First, resistance has nothing to do with number of wraps. The resistance only has to do with the length of the wire from post to post. It will not matter if the wire is straight, mildly curly or a micro coil. So if you pre-measure a length of wire and check resistance, after wrapping and then putting it on the post, the resistance should drop slightly to account for the part of the wire length that was "lost" after wrapping around the posts, since the resistance will only count from the first point of contact with the post and the rest around the post is in essence "lost".

Second, I would ask what you are using to wrap the wire around. If you are not pre-measuring the wire length but are wrapping directly around a "soft" wick such as cotton or silica, counting 4 wraps, then cutting off the wire, then there will be a variance due to how tight/constricting the wire is around the wick. I have found it all but impossible to be consistent wrapping that way. If you are wrapping around a fix diameter item such as a drill bit, it should be more consistent.

Finally, your wire may be out of spec, or they sent you wire different from what you ordered. You can easily test the spec though.
According to TemCo that wire should be 11.4 ohms per foot (0.95 omhs per inch) and according to cigtechs the same wire should be 35.4 ohms per Meter (10.79 ohms per foot and 0.9 ohms per inch). I have provided the source links below. What you can do is cut off 1 ft of wire and test that with a meter and if you do not get something between 10.79 and 11.4ohms per foot the wire is either bad or they sent you the wrong stuff. You can test by just cutting 1 inch but the shorter the length the less accurate the test.

Flat Ribbon Kanthal Wire RW0320 - 0.4 x 0.1 mm (0.157 x 0.004 in) 8 oz 2809 FT Series A-1 Resistance
Kanthal Ribbon wire 0.4x0.1mm @ 35.40 ohm/m

 

[KeyLo]

Thanks everyone!

I use a cable wire wick. It's pretty stiff.

I'll have to test the rest of the spool. Sucks that you go into thinking you have 50ft of wire and then half of it is out of spec.

I love this forum. I was hoping I wasn't the only one with this problem. And I wasn't.

 

[Izan]

I use a cable wire wick.

I just spit coffee all over my desk...

Thanks!
I

 

[Thrasher]

sounds like the coil as grounding out through the wick. how do you have the coil insulated from the rope?