DMM Help!

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tmel

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Hello!

I got this DMM today...http://www.amazon.com/gp/product/B000OPDFLM/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1

I have it set Ohms and the lowest setting (200). I put the red lead into the V ohms symbol m a jack and the black lead into the COM jack.

I touch both leads together and the reading just spikes and jumps everywhere, if I hold it steady for long enough, itll read 0.6 which is what I need to subtract I assume..

I have a dual coil build currently, I touch the red lead to the pos post and the black lead to my neg screw and I get 1 ohm.

So, my question is....is this only reading the resistance of a single coil? So, if both show 1 ohm, then I have a 0.5 ohm build, but I need to subtract the 0.6 from that to tell my resistance....is that right?

Thanks
 

proxion

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It will show the combined resistance of both coils since they are in parallel. So if you get 1 ohm with two coils, both are 2 ohms each (if they are equal).

The thing is with a $30 DMM, you won't get much accuracy. It's very possible that it shows 1 ohm for both coils, and when you measure them in parallel, they measure 0.6 ohm. It's pretty hard for a DMM to measure especially low resistances especially at a higher range (the higher the range, the higher the tolerance... think about it, 0.1 error at 200 is 0.05%, but when you're measuring less than an ohm... its another story). I have a $2500 6 1/2 digit Tektronix rack mounted DMM in my lab, and a $120 3 1/2 digit Triplett 9045 DMM for on site work.

Precision is not accuracy, and the $120 Triplett is as accurate as handheld DMMs get. And they still have limitations.
 
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proxion

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I have dealt with a fair amount of pieces of junk that have turned out to be decent equipment. It doesn't mean your meter is crap (okay, maybe the build quality is) but you have to understand its limitations. Your problem may be as simple as a shotty connection between the leads and the jack on the meter, caused by bad lead quality or even bad soldering inside the meter, exacerbated by your attempt to read a low, low resistance. Your meter sends a known current through the measured object and measures the resultant voltage across the leads. So, leads have minimal resistance (but resistance nonetheless), multiply that length by two to account for the return path and you've got yourself some extra series resistance. Just enough to botch your reading. Still, the meter can still be useful to check diodes, etc... so, there's always a reasonable use for something that's not broken. I guess it boils down to the old adage: "you get what you pay for."

PS: Leads are cheap. Even well constructed ones.

EDIT -- If you really want to read up on measuring low resistances, you can read up on Kelvin sensing at https://en.wikipedia.org/wiki/Four-terminal_sensing
 
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crxess

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Not even sure I can trust this piece of junk....I was wrapping up the leads and the black wire pulled out of the plastic part, sigh.

Super glued it back in and now I can't even get a solid reading like I did before.

If you think for one minute Super Glue is the right way to connect wiring - Backup and Punt!!!! You have no business attempting sub ohm builds until you understand how to be truly safe.
Solder the wire back to the lead or(if available) fit back into the compression connection.

ETA: Just flipped my Sperry to 200ohm and touched the leads - Reading = 00.0 :) Yes, it is accurate enough for the type of builds I do.
 
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Rickajho

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If I were me I wouldn't have bought it - for the fact that they don't list the accuracy specs in the product info to begin with. You would be better off with one of the UNI-T meters - at least those will state their accuracy in the product details. Like this one: UNI-T UT61D True RMS Digital Multimeter - Multi Testers - Amazon.com

Costs $14.00 more - but at least you know what you are getting.
:2c:
 
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tmel

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If you think for one minute Super Glue is the right way to connect wiring - Backup and Punt!!!! You have no business attempting sub ohm builds until you understand how to be truly safe.
Solder the wire back to the lead or(if available) fit back into the compression connection.

ETA: Just flipped my Sperry to 200ohm and touched the leads - Reading = 00.0 :) Yes, it is accurate enough for the type of builds I do.

I've been vaping for over 4 years and know plenty about battery safety and ohms law, thank you much.

The wire came out of the plastic lead, so I put super glue around the plastic wire part to stick it back into the inside walls of my probe. I can read my battery voltage fine, however when I touch the 2 leads together, I cannot get a solid reading and it just jumps everywhere.

I also don't believe that my coils are 1 ohm each, they are 7 wraps of 26G around a 5/64 drill bit, no way one of these coils is 1 ohm, probabaly 0.7-0.8ish...

Do you think it is busted and or faulty if I cannot get a correct reading by touching the leads together? Even when I try it on my RBA, it's just jumping all over the place :(
 
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k2zs

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The target resistance for that build would be .3 ohms according to SteamEngine. Before the lead broke you said you read 1 ohm. You also said that the shorted test leads gave you .6 ohm so .3 + .6 = .9; plus or minus accuracy that's what you have.

Leads are cheap, you can go to any Home Depot, Radio Shack, or most auto parts stores and buy replacements. Just bring the ones you have to make sure you get the right plug ends.

At those low ohm style builds you'd better make sure of what you're doing to prevent any accidents...
 
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