Cheap Microohm Meters

[BillW50]

I was tired of my many multimeters only measuring down to only a tenth of an ohm. As I build my own heater coils for my electronic cigarettes (aka vaping). And my Eleaf ohm meter made for this purpose that can read down to 0.01 ohm is off by like 0.17 ohm (it was like 8 bucks, so what do you expect and you can't calibrate it). So I needed something better.

I found some really accurate low ohm meters, but they were hundreds to thousands of dollars. Why so much? If you know the voltage and the current applied, it is super easy to calculate the resistance. And millivolt and milliamp multimeters are dirt cheap. Why nothing cheap for milliohm (or microohm)?

Well I might have found something. It is suppose to read down to 1 microohm (yes, that is 0.000001 ohm). It wasn't thousands, nor hundreds of dollars, but about 50 bucks. Nor was it big and bulky, but tiny. I thought it was worth a shot.

http://www.ebay.com/itm/111378757065

I am not sure of the logic of these low ohm meters, but they have two leads per connection. The idea is that they null out any resistance the leads has (I thought that is what the ohms adjust was for on analog meters?). Anyway I am not sure how that is suppose to work like constant current or voltage or something. This one too has the same thing. Every time you press the button, it zeros out the meter. So that part is good.

I quickly discovered if you are charging the 3.7v 1400mah lithium battery at the same time through the USB port. That you can kiss off any accurate resistance reading, at least on microohms. It would jump around +/- 800 microohms easily. Without USB, I could get at best jumping around 10 microohms. But you have to wedge it pretty tight to get that, otherwise it would jump around +/- 200 microohms easily. The spring clips are pretty weak and while they claim the clips are gold plated, either they are not or so thin it doesn't help much.

I dunno, I was looking for something cheap that can read at least one milliohm accurately. Well I guess this can do this very well. But has anybody found anything better?

[motocoder]

They use a 4-wire, aka "Kelvin" connection. When measuring low ohm values, the voltage drop from the resistance of the test leads can be significant. With a 4 wire ohms measurement, this problem is reduced greatly by  using two of the leads to send a constant current into the device under test, and the other two leads to sense the voltage across the device under test. Because the leads sensing the voltage have very little current flowing through them, the voltage drop due to the resistance of the test leads is negligible. The other two leads, the ones sending the test current, may have significant voltage drop, but this does not impact the measurement at all.

Many desktop meters can do 4-wire measurement. Micro-ohm meters have some additional features to go below what is even possible to measure with a normal meter with a 4-wire ohms capability. I believe mostly this is that they send a much larger current into the device.

Another feature that is sometimes useful is the ability to "null" the measurement. This just takes the current value the meter is reading and stores it away, and then for future readings it subtracts this value. This somewhat compensates for the lead resistance. You can manually do this, even with your existing meter by just subtracting that 0.17  ohm value. See if  you can get access to a 4-wire capable meter, though, to see just how accurate this is and whether that will meet your needs.

[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.

[motocoder]

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?

[NoItAint]

Check this out for some idea's on how you can use a Mooshimeter to do some in circuit measurements,  but this is a pretty high ampere circuit.

http://hackaday.com/2014/01/27/measuring-185-%C2%B5%CF%89-in-circuit/

Mooshimeter does two measurements at once.
I don't know if other meters, or two meters can do this unless you plot several measurements.

[motocoder]

Check this out for some idea's on how you can use a Mooshimeter to do some in circuit measurements,  but this is a pretty high ampere circuit.

http://hackaday.com/2014/01/27/measuring-185-%C2%B5%CF%89-in-circuit/

Mooshimeter does two measurements at once.
I don't know if other meters, or two meters can do this unless you plot several measurements.

That  is a pretty cool device. Too bad it doesn't support Windows Phone

[ywara]

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.

What HP does in their DMMs - much simpler - is to disable the current source and measure the residual voltage. It's called "offset compensation". The residual voltage should be the sum of any TE potentials and whatever else is in the circuit. It is easy to do manually if you don't have a good four-wire ohmmeter.

Measuring low resistances is pretty trivial provided you have a stable current source and a decent voltmeter. You don't really need a high-quality micro- or milli- ohmmeter.

[robrenz]

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.

[robrenz]

Measuring low resistances is pretty trivial provided you have a stable current source and a decent voltmeter. You don't really need a high-quality micro- or milli- ohmmeter.

It is not that simple if you need any level of precision.

[TheSteve]

We whipped up a bit of a low resistance measuring box the other day at work for fun. It is nothing more then 4 AA batteries feeding an LM317 in constant current mode set to 100 mAh. The current can be set with a single resistor of the proper value. It doesn't get much simpler or cheaper really. Use clip leads to connect to your wire and then measure the voltage across it to determine the required resistance needed. We were using it to measure the resistance of the joints in a clip together slot car track and of copper tape. It is nothing more then a very budget 4 wire setup that should work great for your application.

[edavid]

What is the point of measuring the cold resistance of a heating element?
Wouldn't you be better off measuring it at working temperature?

[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

[Vgkid]

Another option would be to grab a hp3456a and a set of ebay kelvin clips. It gives you 100microOhm resolution, and statisticsa

[Someone]

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?

Agilent has one of their videos going into some details here https://youtu.be/vydZQRqUm9M

[Vgkid]

For low resistance measurements, the 3456A features an offset compensated Ohms function.

[motocoder]

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?

Agilent has one of their videos going into some details here https://youtu.be/vydZQRqUm9M

Thanks, it was what I theorized in my post; you measure the value with opposite polarity currents, and can remove the EMF contribution by comparing the two values. Probably a better way to say it is that you do a differential measurement.

[Gyro]

Maxim App note AN106

www.wentztech.com/radio/resources/Projects/AN106.pdf

Use your own choice of reference and op-amp. Subtract any meter reading you get before / after to eliminate any thermal emfs. Test current up to 1A, battery powered.

[kripton2035]

here on my web repository you can get schematics of simple devices that can measure the milliohm
http://kripton2035.free.fr/continuity-repos.html

I made the cypress one and it works quite nicely. I'm trying to upgrade it by adding a 100ohm range, and a speaker to ear the low ohms.
http://kripton2035.free.fr/Continuity%20Meters/continuity-an220.html

[robrenz]

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.

[robrenz]

This post of mine shows that accuracy below 1 milli Ohm is not trivial. That chart is plotted in % of Reading error on the left side. My point is that getting micro Ohm resolution with a setup has no bearing on the accuracy you are achieving. Notice how quickly that very expensive meters exceed 100% of reading errors at 10 micro Ohms.  The LOM-510 I used in the video has a maximum error of 13% of reading at 10 micro Ohms as seen on the chart.  Another very interesting thing about that chart is seeing that which meter is more accurate depends greatly on what resistance you are measuring.

[HighVoltage]

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

Great chart, robrenz.
It seems the Agilent 34420A is getting such great results with only 10mA of drive current.
The LOM-510A is using 1A drive current, interestingly, I never measured that.

[robrenz]

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

Great chart, robrenz.
It seems the Agilent 34420A is getting such great results with only 10mA of drive current.
The LOM-510A is using 1A drive current, interestingly, I never measured that.

That is because it is also a nanoVolt meter and has at least 100x voltage resolution to make up for the 100x reduction in drive current.

[BillW50]

Wow! I had no idea how interesting this topic would become. Thanks everybody!

@motocoder: Ah I should have elaborated more about the 0.17 ohm error on the Eleaf meter. One it changes from day to day. I have seen it be off by 0.27 ohm before. Now today I checked it with three different coils (2.1425, 0.1510, and 0.1093 ohm) and it is only off by 0.04 to 0.05 ohm higher. Later on today it will probably be some other error. Maybe the position of the moon and the planets has something to do with it, who knows?

@edavid: Why measure cold resistance of a heating element? Well the most common wire used for this purpose is called Kanthal, which consist of mainly iron, chromium (20–30%) and aluminium (4–7.5%). And what many people do is to build the coils where each loop touches each other. Sounds strange I know, since you would think the coils wraps would short to each other. It does in fact at first. But they have a method they call checking for hot spots. You apply current and the coil should turn red in the center and expand outward. If it does, you are good to go.

If it doesn't, that means one or more wraps are shorting. Easy to fix, you just stroke the coil with a screwdriver or something and recheck for hot spots. What happens is when Kanthal is heated, it forms a protective layer of aluminum oxide (alumina). Aluminium oxide is an electrical insulator but has a relatively high thermal conductivity. Thus even when the wire touches each other, they are electrically insulated. 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.

Another popular wire is nickel wire (Ni200). Now nickel wire does change resistance with heat. In fact, that is why they use it for devices that does temperature control. And how these generally work is by reading the resistance at room temperature, this is your baseline and gets stored. Now as it heats up the resistance changes (I believe it increases in resistance). I never saw a huge change personally. Say a cold nickel coil reads 0.10 ohm. When heated to 400°F it might be at 0.11 ohms. Apparently these temperature control devices can read far more accurately than their resistance display shows. As mine for example can be set between 200°F to 600°F in 10°F steps. And during my experiments it does a pretty good job of it. As it is generally within 20°F of the setting.

Now they say you are not ever suppose to use nickel wire without temperature control. As over 600°F it gets really soft and it could cause shorts, etc. Nor does nickel oxide either, so the coils shouldn't touch each other (unlike Kanthal). Then there are some health concerns about inhaling hot nickel wire too. Although it is generally believed that 600°F and cooler should be fine.

[edavid]

Thanks, that clears up a lot.  I was also wondering why people make their own heater coils... do you burn through (ha ha) a lot of them?

[TheBloke]

Ooh I somehow missed this thread, and it's very much up my street.  And I see now it was started by a vaper, awesome

Vaping is my current hobby and passion - @BillW50, look for me on ECF if you want to discuss a lot more about accurate Temp Control and the technical stuff  that goes with it, such as accurate ohms reading, adjusting for Static Resistance in the atomizer/510s, and adjusting TCRs to vape with different wires.  My main thread on the subject at ECF is here: TC beyond Ni200: Nickel Purity, Dicodes; Ti, SS, Resistherm NiFe30; Coefficient of Resistance

Anyway, back to the topic at hand - to answer your original question Bill, I have already bought two of the very device you are asking about (and for the same reason - vaping!)  :



(Click on any of the images in this post to see them larger)

I also bought two high quality current sense resistors so I could validate their accuracy, and the accuracy of other ohm readers I have or might get.  As you can see, on the left I have a 0.01 ohm resistor with +/- 0.25% tolerance, and on the right it's 0.1 ohm with +/- 0.1%.  (I bought these resistors from eBay - they're used, having been taken out of a military-spec current meter.)

And according to these readings, these micro ohm readers are accurate at least for these resistance ranges - both are reading within the tolerances of the resistors.  I have some 0.001 ohm 1% resistors on order from eBay so I can do further verification at the milli-ohm level; would like to get some 0.001 ohm 0.5% or 0.1% as well if I can find some that are affordable.

I don't believe that they're actually accurate to micro ohm or that close to it.  For certain they're not accurate to six decimals, because the last digit always fluctuates.   The fifth digit was fairly stable for these readings, but does sometimes move about a bit.

I'm generally happy to regard them as definitely accurate to milli-ohm, and at most accurate to 0.1 milli-ohm; 4 digits, 1-in-10,000.  That's certainly good enough for me.

I checked out the current they put out, as my (beginner's) reading on micro-ohm/milli-ohm readers (in particular this document from Megger about low-ohm reading) suggested that the current used by the reader was very important for the accuracy?

In that sense, it's good news/bad news for these micro ohm readers:

 

0.101 A or 84mA - but I'm not sure why I get different readings depending on whether I'm in A or mA mode on the DMM?  Surely 0.101A should show 101 mA or 85mA should show 0.085A?  Not quite sure what's going on there?

Anyway, I believe this is pretty good news because it's way more than the average DMM ohm reader sends - which is usually about 1 or 2mA I believe?  But it's bad news because it's nowhere near the multiple-amps that a true micro ohm reader sends.  (Though in this case, it made me realise that probably, for my vaping purposes, I would never want a true micro-ohm reader - I can't send 10A of current through the wires I would be testing!)

Finally, here's a look inside:

They use a 3.7V Lipo battery, hard wired to the board.   I have been thinking about replacing this with a battery socket.  It's great that it uses 3.7V as I could use any standard 18650 battery, of which I have loads (again, for vaping.)   For some reason, I got very different battery capacities in my two models!



The PCB is quite simple.  I'd be very grateful for anyone's comments on how this is designed and how it does the calculations?  I would really like to make my own reader, and that's one reason I bought the second of these - so I could have one to play about with and maybe change components.



The chip in the middle is the ADS1232 24-bit ADC.  To the left of that, the smaller chip is the STM32F030F4P6 - a 32bit 16kb microcontroller, which presumably does the calculations.   

I'd really like to build one of my own, doing the calcs on an Arduino.  It'd be a fun project. 

Though I just saw in this thread that someone has linked to a plan for a micro-ohm reader, so I'll be studying that first!

Anyway, in conclusion I think these devices are pretty great considering I paid £35 per device ($50) on eBay.   I am very confident that they are definitely accurate at least to three digit / milli-ohm, and quite possibly even to 4 digits.  This is more than good enough for my purposes, and it's very awesome not having to worry about the contact resistance / resistance in the cabling.

My next plans are to:
1. On one, put on better quality cables with stronger clips, so it's easier to grab onto wires/resistors to test.  This is easy to do as they come with a 4-wire screw terminal block.
2. On the other, mod the case to put in a vaping 510 connection, so I can directly screw down an atomizer onto the reader and get a super accurate reading for it; in other words, a home-made 4-wire micro-ohm version of the ELeaf type reader!   I'm hoping this will work out OK as a 4-wire reader if I simply solder two wires onto the positive and two on the negative?  I might need to get them in appropriate positions and/or alignments - that Megger guide I linked earlier has some discussion of that.  Worst case, I'll simply re-create the clips, with permanent clips onto the positive/negative rather than soldering the wires directly.


PS. Bill, throw that ELeaf reader in the bin.  It is known to be not worth the plastic it's made out of.  I made the mistake of buying one too.  Utterly useless if you want a figure close to accurate.  Inside is a hot glue mess with crappy wiring, and it doesn't even have a proper 510 connector.

I started out using my DMM as a reader, because it can do two decimal places.  So before I got the YZKStudio devices, I was measuring atomizer resistance like this:

 

That's a Fat Daddy Vapes v3 Shorty 510, connected to two 4mm banana plugs to plug into the DMM.  Worked well!  That's the sort of socket I plan to put on one of the YZXStudio readers, which will then become my standard vaping ohms reader once I've done it.