Measuring low resistance

[cobalt327]

Hi all, total noob here so be forewarned!

I need to accurately measure resistances two places right of decimal. As an example, 0.05 ohm. I have a Fluke 77 Series 2 that has served me well for many years but need better resolution(?) that it provides. Someone had mentioned a UNI-T UT61E but I haven't had time to research it.

[rx8pilot]

Are you measuring a wire or resistor?
You may be better off measuring the voltage drop across the device and back calculating the resistance instead of measuring resistance directly. Look at current sense resistor measurements and calculations

[Simon]

You will need to look carefully at the ranges available and resolution (counts ?), if you can get a 0.1 or 0.01 range then all well and good or you will need something with so many counts that at the lower range it measures that sort of value reliably.

[cobalt327]

I'm measuring resistance of ~3" lengths of 28 to 30 gauge Ni200 (nickel) wire.

[Zero999]

You can probably calculate it.

One way is to use a constant current source and measure the voltage. If you use a 1A constant current source, then the 200mV range on your meter should enable you to measure low resistances fairly easily.

[cobalt327]

Thanks all for the suggestions, I will look into which might be best for me. In the meantime, is there no "affordable" meters that fit the bill?

[Lightages]

DER EE DE-5000

It is an LCR meter that also does four wire measurement down to what you want. Watch this:


They can be found on ebay for under $100 or a bit more with all the accessories.

[Dago]

I wrote a blog post about this topic: http://www.dgkelectronics.com/tip-measure-small-resistances/

[Grapsus]

The Fluke 77 you have has 1 mV resolution. Just make a 1A current source with a linear chip like LM317. The same DMM can be used to check the current source. Then pass 1A through the piece of wire you want to test and measure the voltage drop: 1 mV equals 1 mOhm. The precision is not super good, but it may be good enough and the price of this setup is unbeatable.

[fpliuzzi]

I too use my DER DE-5000 when doing low resistance measurements, but I always thought that the simple low Ohms DMM adapter shown in the following PDF was pretty clever. It uses an LM317 while incorporating the four wire measurement technique to maximize accuracy.

www.aeroelectric.com/articles/LowOhmsAdapter_3.pdf

The DMM's REL button would take care of what appears to be the device's 2 milliohms of parasitic resistance.

[Lightages]

I wrote a blog post about this topic: http://www.dgkelectronics.com/tip-measure-small-resistances/

Short pieces of nickel wire are very likely to self heat very quickly and throw out the measurement.

[Lightages]

I too use my DER DE-5000 when doing low resistance measurements, but I always thought that the simple low Ohms DMM adapter shown in the following PDF was pretty clever. It uses an LM317 while incorporating the four wire measurement technique to maximize accuracy.

www.aeroelectric.com/articles/LowOhmsAdapter_3.pdf

The DMM's REL button would take care of what appears to be the device's 2 milliohms of parasitic resistance.

That device is pretty cheap and probably has very good repeatability, but the accuracy will probably be pretty bad, like on the order of 10%. I could be wrong but it isn't going to be better than 1% for sure I think.

[Grapsus]

+1 Don't measure small chucks of wire with big currents for more than a second or two because they will get red hot and the measured resistance will skew.

[cobalt327]

More good ideas here- thank you for taking the time to post! 

The deal here is a temperature "protected" e cigarette device used to 'vape' e cig liquids. FWIW, the resistance change when current is passed through the Ni wire and the wire is heated, is the very reason it's used. By using Ni wire and circuitry that reads the resistance change as the wire is heated, and then using this resistance change to modulate the wattage going to the device's Ni atomizer coil (the part that heats up to vaporize the liquid), the temperature can be selected for the best result. Also, this can prevent overheating the liquid, which is thought to eliminate or greatly reduce the chance of harmful compounds being formed.

[Grapsus]

In this case, wouldn't it be simpler to measure the temperature of your wire at thermal equilibrium ? You power it with whatever source your cigarettes provide and stick a thermocouple to it ? Getting 1% accuracy for a temperature in the range of 100° C is a lot easier than getting the same accuracy for a milliohm resistance.

[hli]

Look at the Halfohm project from Jaanus Kalde. I think it also appeared in one of Daves Mailbags.

[cobalt327]

@ Grapsus: The thermocouple scheme would work, but ideally if there's a problem you want to know about it prior to installation. This is due to the time and work involved w/making and installing the coils. 

@ hli: I will read this over lunch.

Again, thank you to all who took the time to respond. It is sincerely appreciated!