Which resistance figure to trust - multimeter or mod?

[reddal]

Hi,

I recently started building my own coils - and I was using the resistance display on a vape-pro regulated mod to get an idea of my ohms.

This mod can't fire anything below about 1.3ohms - but it can display a resistance regardless.

I was worried the resistance shown by the mod might not be accurate - so I bought a multimeter - a Mastech MS8229 (http://www.amazon.co.uk/dp/B000JKMTDM).

This shows a resistance quite a bit higher than the mod for all my builds - (by at least 0.5ohms). However when you measure the resistance of a hard short it sometimes also measures up to 0.5ohms - so now I'm thinking the multimeter might not be measuring resistance accurately.

I just built a dual coil which the multimeter shows at 1.0ohms - but the mod shows as 0.3ohms. If its the lower then its lower than I want to go - but now I've no idea!

Questions :

1. Which figure would you trust more?
2. Any tricks to getting accurate readings from the multimeter - I've just been switching it into Ohms mode and holding the probes onto the screwheads of the build?
3. Did I buy a rubbish multimeter - do you really have a buy one for £100+ to get 0.1ohm accuracy?

thanks - reddal

 

[montara]

I would trust that meter and check the added resistance of the leads. Hold the tips together and see what they read. Should be .3 >.6 make sure you have good contact as dirt/oxidation will also contribute to bad readings. Subtract this from final reading of your build OR some meters allow you to set ZERO, do so if you can.

 

[reddal]

I would trust that meter and check the added resistance of the leads. Hold the tips together and see what they read. Should be .3 >.6 make sure you have good contact as dirt/oxidation will also contribute to bad readings. Subtract this from final reading of your build OR some meters allow you to set ZERO, do so if you can.

Ahhhh - thanks - this is what I was missing.

Holding the probes together measures something like 0.4ohms. I hadn't realized you have to subtract this.

This explains most of the difference I was worrying about - and I guess the meter is more accurate. I wish I'd got one with 0.01ohm accuracy now - its clear this one is struggling at 0.1ohm accuracy.

thanks again for solving that for me.

- reddal

 

[CloudZ]

I don't think that particular meter is going to be any good for measuring low resistance. Usually even cheaper ones at least have some kind of accuracy rating for Ohms (for instance, +/- 0.1). It says it has a buzzer for continuity if resistance is lower than 40 ohms, which means it may not be designed for readings around 1-2 ohms. In this case, your mod is obviously much more accurate because it is designed for reading low resistance in this range. You may get 0.4-0.5 ohms resistance from a dead short, but this is probably not just the internal resistance of the connections and leads, it is just random inaccuracy. It may be off by different quantities as you add some small resistance to the circuit.

I bought the cheapest DMM at the Home Depot or Lowes (don't remember) for $25-ish and it has +/-0.1 ohms. When I dead short it, I get 0.0 or 0.1 depending on how I've wiggled the connectors in their terminals. So I know it must be at least this accurate. To get to +/-0.01 or so, you need a much more expensive DMM or a purpose-built ohm checker (the ones with a 510 connection).

I hope this helps.

 

[Dissonance]

Always thought the ohm checkers with the 510 connections were the way to go... .01 accuracy (+/- a certain %, pretty low), just as cheap if not cheaper, and less room for error since you just screw it on rather than having to do the math + make sure you have the probes in the right spots.