Cheap Microohm Meters

[HighVoltage]

To be honest, I had no idea that e-cigarettes is such a market. I feel like I have been living under a carpet, if it comes to this subject.

[dr.diesel]

To be honest, I had no idea that e-cigarettes is such a market.

I had to Google the word vaper, didn't have a clue.

[HighVoltage]

To be honest, I had no idea that e-cigarettes is such a market.

I had to Google the word vaper, didn't have a clue.

Makes me feel better, that I am not alone on this.
My bad excuse is that I never smoked in my life.

[TimFox]

Another feature found on very-low resistance meters is low-frequency AC drive, rather than DC, with four-wire connections to avoid the effect of thermal EMFs between the different metals in the circuit.

Oh, that is quite interesting. Can you explain how that avoids the EMF from the bimetal junction? Is it because it can then sense a different absolute voltage between the two polarities, and using that estimate the voltage from the thermal EMF and subtract that out?

See this link for the manual of an older Keithley unit that uses the AC technique.
The minimum full-scale value is 1.0 milliohm.
The four-terminal connection is done with two AC cables on XLR connectors.
AC excitation also allows phase-sensitive detection to reduce the measurement bandwidth.
http://electronicsandbooks.com/eab1/manual/Hardware/K/Keithley%20www.keithley.com/Product/503%20Milliohmmeter/Models%20503,%20503C%20Milliohmmeters%20Instruction%20Manual%20Rev.%20A%2020040929%20%5B36%5D.pdf

[ywara]

This post of mine shows that accuracy below 1 milli Ohm is not trivial.

You're certainly correct that getting a high-precision (let's say 3-4+ digits) measurement at low resistances requires some doing. However, the 2-3 digits needed in most common applications (contact resistance, heating elements, wire & cable, etc.)  can be easily obtained with a stable current source, range-appropriate drive current, and a normal voltmeter. Probably my "trivial" statement was not nearly specific enough.

I am familiar with contact resistance measurements (0.5-10 mOhm, required accuracy of ~0.1 mOhm) and we have no trouble getting repeatable, accurate measurements with an old power supply and a cheap 3.5 digit voltmeter by using a 1A drive current. There is effectively no heating to worry about - solid metal parts of even small size are not too concerned with 10 mW of heat when there are two big 16 ga. copper heatsinks hanging off them. If we're really desperate we reverse the polarity of the current source and average out the TE voltages.

They also claim that Kanthal resistance doesn't change in respect to temperature. I am not sure if that is 100% true, but so far I haven't seen any changes yet.

It's more accurate to say that Kanthal does not change much. See http://kanthal.com/en/products/material-datasheets/wire/resistance-heating-wire-and-resistance-wire/kanthal-a-1/. 5% across the range. Kanthal A is slightly worse (6%). I assume the rest of the family is similar.

I am not sure what is the temperature used in a vaporizer, but your cold resistance is probably within 2.5% of the resistance under power.

[TheBloke]

It's more accurate to say that Kanthal does not change much. See http://kanthal.com/en/products/material-datasheets/wire/resistance-heating-wire-and-resistance-wire/kanthal-a-1/. 5% across the range. Kanthal A is slightly worse (6%). I assume the rest of the family is similar.

I am not sure what is the temperature used in a vaporizer, but your cold resistance is probably within 2.5% of the resistance under power.

For vaping purposes, Kanthal barely changes resistance at all - a few mOhm at most.

When we do temperature controlled vaping (resistance-based temperature measurement using high-TCR wires such as Ni200 and Titanium), the maximum allowed temperature will be 300°C and the usual desired temperature range is 200 - 250°C.

With Kanthal in non-temp-controlled situations the temperature may occasionally go higher than that, but generally speaking no vaper ever wants to heat his/her coil above 300°C as a maximum.

The Kanthal A1 datasheet doesn't shown any resistance change until 500°C.  A standard resistance for a Kanthal coil is between 0.4 and 1.5 ohms.  Taking 1 ohm as an average, at 500°C Kanthal A1 will increase by 10 mOhm (1.00 -> 1.01).  The TCR seems to be increasing as the temperature does, so I'm guessing the rate of increase from 20-300°C is less than the rate of increase from 300-500°C. 

So I'd think definitely less than 5mOhm by 300°C, probably no more than 2 or 3 mOhm for a 1.0 ohm cold coil.

All that said, accurate resistance reading is pretty unimportant for non-Temp-Controlled/Kanthal vaping.  Most non-TC vaporisers read resistance with an accuracy no better than 100 mOhm, whereas the best TC devices read accurately to the mOhm level (or at least, claim to! - though my testing suggests they generally are pretty good at this.  Only two-wire measurements, but using multiple amps of current.  They certainly are accurate in terms of the achieved coil temperature, something I've tested quite carefully using thermocouples.)

[nctnico]

The DE-5000 LCR meter will do a great job of measuring to mili Ohm resolution as shown in my short video below. It is 4 wire Kelvin and uses AC excitation which eliminates thermal emf errors which is a must at this level and below.  The standard alligator clip attachment needs the clips replaced with true Kelvin clips to get this level of performance.

That is really cool. However, making those nice Kelvin clips like you did there is impractical for 99.99999% of the people here

Thanks, but any stock Kelvin clip can be used to replace the alligator clips of the standard DE-5000 attachment to achieve the results shown in the video.

Can you point to a Kelvin clip in particular? I'm having some trouble to use a HP4274A in the milli-ohm range.

[TheBloke]

The DE-5000 LCR meter will do a great job of measuring to mili Ohm resolution as shown in my short video below. It is 4 wire Kelvin and uses AC excitation which eliminates thermal emf errors which is a must at this level and below.  The standard alligator clip attachment needs the clips replaced with true Kelvin clips to get this level of performance.

Damn, and they're only £60 on eBay.  Now I feel a compelling urge to get that as well, not least just to see exactly how accurate these Chinese YZXStudio devices are   Actually I could have bought one DE-5000 for the price I paid for two of these 'micro' ohm YZXStudio devices..

Like nctnico asked, would be great if you could point to a 'standard Kelvin clip'.  How much does the quality of such a clip matter?

These are the clips that the YZXStudio device came with - four lengths of 20AWG silicone wire soldered to (seemingly) gold-plated plates on a clip.   I'm guessing these cost approximately 1/500th the price of a 'real' Kelvin clip. 



But how much does that matter for milli-ohm accuracy?

[TheBloke]

However,  that said, I think I just got re-excited about these YZXStudio devices.  Now I think I understand the oscilloscope trace I earlier got off my YZXStudio 'micro' ohm reader which I could not understand at the time.

Now that @robrenz mentions:  AC excitation which eliminates thermal EMF errors..

Would this be an example of that?  This is what I get when I put the YZX device on my new Rigol scope



And, because I have owned a scope for no more than 48 hours and am still a complete newbie at all this, could you please confirm that this would be a valid way to probe it?  I clipped the ground lead to the battery negative terminal on the board, and then clipped the Kelvin clips together so that it's measuring its own resistance, and then attached the probe to that:




Assuming this does confirm the YZXStudio device is using the AC method Rob described, then I would think (hope) that makes it a rather good device for £35 / $50?   At least as a milli-ohm reader, if not more?

Is there any chance it might be accurate to 4 digits?  It certainly appears to be so when I check my reference resistors, but I can't be completely sure.  As I mentioned in my earlier (too long) post, it sends somewhere between 85mA and 101mA of current, if that helps to judge its accuracy.

[TheBloke]

One more scope output on the YZXStudio reader. 


The first screenshot was when I had the two Kelvin clips connected together.  This time I used two probes, one per Kelvin clip.


This shows the same 50hz AC signal on each probe, but they are offset and varying. 






Both show a Vpp varying between 1.5 to 2.5V.  Probe 1 has a VMax in the region of -500mv and a VMin in the region of -2.5V, and probe 2 has a VMax in the region of 2.5-3V and a VMin between 0V and 0.5V.  The Vpp/VMax/VMins are varying a fair bit as the AC signals jump around the scale, but the the frequency and amplitude and Vrms are consistent.





I have no idea if this is meaningful information in understanding what the ohm reader is doing, so apologies if it's not   I'm still very much at the stage where I'm looking for any excuse to measure/test things with my scope

[motocoder]

The DE-5000 LCR meter will do a great job of measuring to mili Ohm resolution as shown in my short video below. It is 4 wire Kelvin and uses AC excitation which eliminates thermal emf errors which is a must at this level and below.  The standard alligator clip attachment needs the clips replaced with true Kelvin clips to get this level of performance.

That is really cool. However, making those nice Kelvin clips like you did there is impractical for 99.99999% of the people here

Thanks, but any stock Kelvin clip can be used to replace the alligator clips of the standard DE-5000 attachment to achieve the results shown in the video.

Yes, after I posted that I finished watching your videos. I saw towards the tail end of video #1 where you had the attachment apart. Looks like it would be straightforward to attach different clips to the TL-21 attachment, which can be found on eBay for $15.

[motocoder]

To be honest, I had no idea that e-cigarettes is such a market.

I had to Google the word vaper, didn't have a clue.

Makes me feel better, that I am not alone on this.
My bad excuse is that I never smoked in my life.

Same here. I was thrown off by someone saying that vaping was their "hobby". I used to like to drink beer, but I would never call that a hobby

[TheBloke]

Quote from: motocoder on Today at 05:31:10

Same here. I was thrown off by someone saying that vaping was their "hobby". I used to like to drink beer, but I would never call that a hobby

But you've heard of people who brew beer and have that as a hobby, right?   Similar concept to vaping as a hobby

I never would have considered smoking a hobby - it was just something I did.  But with vaping it's very different.  There's a huge amount of hardware to experiment with - and/or try to design/build oneself - different techniques to use, liquids to try, and so on.  The experimentation with all of that, and discussion of it with like minded individuals, is what constitutes the hobby.


In my case, it's vaping that has been directly responsible for getting me into learning about EE.  Experimenting with and testing the vaping devices, in particular with regards to temperature controlled vaping - hence the interest in accurate ohmmeters, and also now an oscilloscope - and discussing that with other vapers.

[BillW50]

@TheBloke: Ah another vaper here. That is great news. Now someone will know what I am talking about.    Say with your YZXstudio meter, I don't think that 50Hz is coming from your meter, but from your AC house wiring that your scope is picking up. As I am not seeing anything like that here. What I see is 3.32VDC with the leads open and when you add resistance to be measured, the DC voltage drops.

How it looks like it works to me is two leads supplies a constant current of 100ma. And the other two leads reads the voltage. And figuring out the resistance is easy, you just multiply the voltage times ten to give you the ohms. Hell I got a variable power supply right here that I can do that with. Just tried it on a 2.15 ohm coil and yes it works. It looks like my constant current could use some calibration. As I had to dial it down to 94ma before it came up with the correct resistance.

Ah why does it show 0.110 amp vs. 83.73ma? That is pretty normal. As my UT33B inserts a 0.20 ohm resister in amp mode and a 2.0 ohm in ma mode. Thus why the difference in current. Say I just noticed my UT81B is dead in amp mode, ma works. I bought it slightly used and the guy won it and has no need for a meter. Maybe the fuse is loose or missing or something.

Yes it would be a good idea to add a 510 connector to our YZXstudio. Yes just wire it up just like the clips are wired. And yes, once that lithium battery dies, I'll be replacing it with a 18650 battery too. Say did you know that the Tesla car is powered by 18650 batteries?

http://www.wakeandvape.com/blog/18650-vape-faqs/

Yes those Eleaf meters are totally crap for sure. Today it is reading 0.07 ohm higher. The other day it was only 0.04 higher. And some days it is 0.27 ohms higher. Well it can read volts pretty well. Just wire some test leads or battery holder and just use it for a battery checker.

Wow your main thread on ECF looks excellent. Great job!

[motocoder]

Quote from: motocoder on Today at 05:31:10

Same here. I was thrown off by someone saying that vaping was their "hobby". I used to like to drink beer, but I would never call that a hobby

But you've heard of people who brew beer and have that as a hobby, right?   Similar concept to vaping as a hobby

I never would have considered smoking a hobby - it was just something I did.  But with vaping it's very different.  There's a huge amount of hardware to experiment with - and/or try to design/build oneself - different techniques to use, liquids to try, and so on.  The experimentation with all of that, and discussion of it with like minded individuals, is what constitutes the hobby.


In my case, it's vaping that has been directly responsible for getting me into learning about EE.  Experimenting with and testing the vaping devices, in particular with regards to temperature controlled vaping - hence the interest in accurate ohmmeters, and also now an oscilloscope - and discussing that with other vapers.

Yes, in fact I used to brew beer myself until one day I dropped a large glass carbuoy on my foot, shoving a shard of glass through it and turning my wife's kitchen floor into a mixture of Wort and blood. That was the end of that hobby, mostly because of a desire to keep my wife around

I have to admit I am totally ignorant of all things vaping. I had no idea it was something like cooking or brewing beer or making wine. I will have to look into this further. I think it's great that you're learning about EE, and I support that 100%.

[BillW50]

Say I just noticed my UT81B is dead in amp mode, ma works. I bought it slightly used and the guy won it and has no need for a meter. Maybe the fuse is loose or missing or something.

Pop the back off and yes the 10 amp fuse is blown. Well one mystery solved.   

[TheBloke]

Yes, in fact I used to brew beer myself until one day I dropped a large glass carbuoy on my foot, shoving a shard of glass through it and turning my wife's kitchen floor into a mixture of Wort and blood. That was the end of that hobby, mostly because of a desire to keep my wife around

Oh dear

I have to admit I am totally ignorant of all things vaping. I had no idea it was something like cooking or brewing beer or making wine. I will have to look into this further. I think it's great that you're learning about EE, and I support that 100%.

Well, isn't that vaping is just like brewing beer.  I just meant, that is a good comparison - you said "I drink beer but it's not a hobby", so I used an example as to how beer could be a hobby.

Vaping for most people is a replacement for smoking.  A much healthier way to ingest nicotine and handle an addiction to inhaling smoke/vapour    I am sure for most vapers, the vast majority, that is all it is.  But for some of us it becomes a hobby - the ECF forum, by far the biggest vaping forum, has over 15 billion posts in 8 years from 250 thousand members.

There are loads of aspects that can turn into a hobby.  Actually there is one that is a lot like brewing beer/making wine: mixing one's own e-liquids.  You can buy different concentrated flavours, and then mix them together in different proportions along with nicotine and a couple of other ingredients, to try to get the nicest flavours when you vape.

For me that's not a big thing, mostly because I suck at cooking/working out flavours!   For me, the main hobby aspect is the hardware/technology.  Experimenting with and testing different vaping devices.  Different vaping wires, especially for temp-controlled vaping.  Trying different coils that vape differently.  All that sort of stuff.

[TheBloke]

@TheBloke: Ah another vaper here. That is great news. Now someone will know what I am talking about.    Say with your YZXstudio meter, I don't think that 50Hz is coming from your meter, but from your AC house wiring that your scope is picking up. As I am not seeing anything like that here. What I see is 3.32VDC with the leads open and when you add resistance to be measured, the DC voltage drops.

How it looks like it works to me is two leads supplies a constant current of 100ma. And the other two leads reads the voltage. And figuring out the resistance is easy, you just multiply the voltage times ten to give you the ohms. Hell I got a variable power supply right here that I can do that with. Just tried it on a 2.15 ohm coil and yes it works. It looks like my constant current could use some calibration. As I had to dial it down to 94ma before it came up with the correct resistance.

Ah why does it show 0.110 amp vs. 83.73ma? That is pretty normal. As my UT33B inserts a 0.20 ohm resister in amp mode and a 2.0 ohm in ma mode. Thus why the difference in current. Say I just noticed my UT81B is dead in amp mode, ma works. I bought it slightly used and the guy won it and has no need for a meter. Maybe the fuse is loose or missing or something.

Yes it would be a good idea to add a 510 connector to our YZXstudio. Yes just wire it up just like the clips are wired. And yes, once that lithium battery dies, I'll be replacing it with a 18650 battery too. Say did you know that the Tesla car is powered by 18650 batteries?

http://www.wakeandvape.com/blog/18650-vape-faqs/

Yes those Eleaf meters are totally crap for sure. Today it is reading 0.07 ohm higher. The other day it was only 0.04 higher. And some days it is 0.27 ohms higher. Well it can read volts pretty well. Just wire some test leads or battery holder and just use it for a battery checker.

Wow your main thread on ECF looks excellent. Great job!

Oh OK so you already have the YZX?  I thought you were asking whether you should get one.  That's great then.

Re the scope trace, yeah you're right   I think I must have got the grounding wrong, I wasn't properly attached to the battery negative wire.  I've just done it again and I see what you see, 3.32V without a resistance to measure, then it drops.

This begs a new question:  how is my house AC leaking into an improperly grounded scope trace??  Is that normal?  I did have the negative lead on the probe connected to the YZX, just I guess it wasn't making good contact with the negative battery terminal.  So the AC is coming through the floor, up into the desk, and through the casing of the device?  Do I have some bad wiring in my room/house or is this to be expected?

Anyway that's a shame that the YZX is not using the AC excitation method.  Maybe I will get that 5000 LCR meter as well!  If only to compare accuracy.

Thanks for the re-assurance on the DMM different on A/mA!   

[BillW50]

@TheBloke: Nothing to worry about your AC house wiring leaking throughout the house. You can touch the scope probe too and probably get a better and stronger signal. I often use this as a test signal. I just fired up my LeCroy scope and I just got 1.6v of AC. That is pretty normal. But here in the states most of our outlets are wired for 120VAC. Yours are wired for 220VAC or is it 240VAC, right? Some homes in the states has a few outlets 220/240, but equipment that requires 220/240 are usually hardwired in like central air or something.

Yes I have one of those YZXstudio meters. I have got it down to >3 micro-ohms by shorting the leads. But those clips are the worst cheap clips I have ever used. And it looks like to me when you turn it on, you should short the clips out and press the button quickly. This zeros the meter out. When you press and hold to power down, it also does a zero out (not good when you are measuring resistance). I am still puzzled if it saves the calibration or not when you power off. Sometimes it seems to and sometimes it doesn't.

I don't think checking coil resistance will benefit from using the AC excitation method. Although if I am wrong I am sure someone will correct me.