OK, now you have sent me back to the bench.
I recently made a 0.01 ohm shunt with 18 AWG wire, now I can see that I need to make a 0.001 ohm model.
Very useful tutorial, I will use your technique sometime when I need to measure higher than 10A currents.
Thanks for sharing!
Thanks for posting this. I’ve been turning over the idea of a better shunt in my head since earlier today when I saw your discussion with RMM in the other thread. Looks like you’ve got the problem under control! 
For calibration I do not think another DMM is really necessary. A consistent, known load (such as 7135’s) should allow someone to tune their shunt.
WOW, great post. Now I know!
Also, are those really cheap HFT DMM's? Or Free DMM's? look what I found this month in my Autoweek magazine! Plan to go tomorrow and pick them up!
HF DMM, "the choice of flashaholics." TM 
^^ I hope no one tries to print that!
HF might 
Their biggest product endorsement is “Chosen by Truckin’ Magazine,” which they use for several items.
Just a warning about these free harbor freight meters. I have three of them and they all read quite differently of one another. I changed out the batteries with fresh new tested energizers and they still read incorrectly. Since I have a few calibrated fluke DMM’s, I didnt bother to look any further and threw the HF meters in the parts box. There might be a calibration pot inside to dial these in. Just dont rely on your readings until you have checked them against a known calibrated DMM and adjusted if necessary. Then recheck for accuracy every month or so. I read that a lot of cheap meters are overly sensitive to the state of the battery voltage that powers them and can also make them read incorrectly. So as the cell ages, your calibration gets thrown out the window, even after a recent calibration. :Sp Maybe thats why these are free and have been for several years.
I might dig mine out and practice my throwing sling techniques. Just hold it by the leads and give it a good throw into a concrete wall 
FlashPilot,at risk of being called a shill for HF, I must say that it seems HF is upping their game lately. It used to be that I could just not touch their stuff. Totally unusable, but lately their stuff has improved. Take for example this meter. It used to be the shunt resistor was longer and not mounted directly to the jacks. When passing current it had to travel through the circuit board, Now they are right next to the jacks. Here is an older #90839 and the newer #90899
Also, I was shocked when I recently was forced to buy some more alligator clips. In the past they were terrible, totally unusable as the wire was hair thin. Compare the ones I just picked up last week with the ones from a few years ago. They package claims 18 gauge.
The copper step flashing I used was cut with the free HF scissors (shears). Their regular price is $1.99, on sale almost always for .99. Or you could go to Dollar Tree and get the same exact pair, in white, for $1
As far as these meters go, unless there is something grossly wrong with one, they pretty mcuh agree with each other.
Woops, not that one, this one.
and this
Anyway, I am not pitching HF meters in this post but an idea. That idea is central to BLF, If you don’t own it, or can’t buy it, you can make it. And have fun doing it!
Really all a meter has to do is provide you with a basis, that is, a reference.
If it is not spot on the exact value, it does not matter, what matters is that you use the same meter and record the difference before and after mods.
Most cheap meters will work quite well once you have upgraded the leads, preferably soldered them as dchomak has outlined.
As they say, “it ain’t rocket science”
Some of you guys get sooo bent on a battery charger charging .01 volt over or under like it’s the end of the world.
Then HKJ will chime in and say, BFD, it’s within acceptable spec.
This just in, the battery isn’t going to explode because your I4 charged it to 4.22 volts.
Likewise, your flashlight is going to look the same at 4.185 volts as it does at 4.20 volts, and really that battery will be down under 4.15 volts before you can type this post if your light has any kind of decent draw, so big deal.
I like specs, I just will not be lured into being, or acting like a Spec Head.
JMHO, your amperage may vary:)
Keith
Great post dchomak… you silly HF DMM mega-shill! :D. Holly smokes! Thats a lot of meters you’ve got there. :bigsmile: That pic says it all and Im impressed with the close voltage spread. Mine were off by more than .5V, as I recall. I’ll have to locate my 3 copies and see if they resemble yours. I think I got them last summer so they are less than a year old. Is there an adjustment pot anywhere on the board? Maybe hidden on the front side underneath? Hopefully one for current and another for voltage.
I got a free pair of black scissors like yours from HF instead of the free DMM last time… and I like it! Maybe I’ll keep going back with my free scissors coupon and become the HF Scissors Mega-Shill. How’d that be? :bigsmile:
Careful folks, dchomak is just trying to lull us into a false state of complacency as he silently builds his secret army of DMM's that will be used to attain complete and total World DOMINATION!
Sorry, I keep surprising myself as I continue to reach new levels of corniness.
Nice thread dcho. Very interesting and informative.
That’s right, if I can get ahold of enough meters………, well resistance will futile.
Actually there’s nothing to worry about, I’ve moved on to the free tape measures. Look for a post in the near future about my “collection” (Tongue in cheek, of course)
FlashPilot, it’s appropriate that you mentioned the Sling. A sling illustrates just what I was putting forth.
Ever hear the story of David and Goliath. Goliath was a warrior, a mercenary that fought for the enemy. He was equipped with the most modern weapons of war. He was so greatly feared that none of David’s countrymen would meet him in battle. The only one that was willing to take him on, one on one, was David. He was just a little sheep herder, not even experienced in the art of war. Nor did he have any sophisticated weapons, just a homemade sling and 5 smooth stones. And you know how that went, he got the job done, he slew Goliath.
I have a saying, “The true measure of a man is what it takes to stop him”
If it doesn’t take much to stop someone, then he’s not much of a man.
So, don’t have the tool? Don’t let that stop you!
The OP was my way of showing, members here that sometimes you may not need fancy equipment to get the job done. After all, some of us don’t have all the “toys”. Maybe they are young, or just starting out, or don’t have the money. 2 individuals that come to mind that do very well without the fancy tools is Old Lumens and ComfyChair.
And I wasn’t down playing having good tools either, that “hack” works with a Fluke also.
Lotsa luck doing that… at 0.001 ohms (even 0.01 ohms) even a tiny amount of change in wiring/component leads/soldering can be more than the resistance value you want to make. I have meters that can read well below 0.000001 ohms and getting anywhere near 0.001 ohms takes a lot of careful work with them.
That’s exactly right TP. I was able to do it by cutting a little long, making my connections, and allowing small bits of solder to wick into the wire. It wasn’t easy, but I got ’er done.
dchomak, I didnt mean to detract from your thread. I think it was an interesting post that demonstrated some good problem solving skills… and free is always better. Especially if its useful. :bigsmile:
The only reason I saved my HF DMM’s was because I might have a use for them as parts in future projects if they couldn’t easily be calibrated correctly. I picked them up thinking that a 0.1-0.2V variation would be good enough to give to others to check their cells from time to time. 0.5V or more just didnt cut it. But you know, there’s always some humor to be found while criticizing a cheap product thats completely useless. BUT, Ive been more than happy with my free HF scissors! I think there was a similar thread on BLF posted a few years back where someone used a pair of DMM’s in tandem to read higher amps but I cant find it now. Im lucky that I have access to a calibrated clamp meter if I need it.
Ive always enjoyed the saying, “dont bring a knife to a gunfight.”
Interesting thread. I’ve always looked at meters as “black boxes” not understanding how the results were obtained. Learned a bit today, thanks. Mostly I just use mine to verify that all the 7135’s are functional or that the driver is in the ballpark but even for that I’ve had to replace the stock leads with shorter, thicker ones. This mod should get a link in one of the DIY sticky’s.
I can't believe I still having followed the advice given in the OP and made one of these external shunts yet. I generally use very short leads made each time using the emitter lead itself (use extra length for testing and then shorten for final assembly). Probably not too bad a set up, but this is much better.
I hope to build one this weekend. Thank you again for this thread. Good stuff.
I really want to make myself one of these external shunts, but I'm having trouble picturing how calibrate it. In the OP, you wrote:
From the resistance chart, it works out that 4.75 inches of 14 gauge is about .001 ohm. So I cut off a 5 inch length of wire and soldered leads a little greater that 4.75 inches apart. I calibrated it with another HF meter on the 10A scale. At first it read a little high as it was a little too long, but I reduced the resistance slightly by allowing solder to wick into the wire until is was exact.
Could you elaborate on how you did the Bold texted part above?