DLRO Digital Low Resistance Ohmmeter Wanted

[Reloadron]

While I am familiar with DLROs what I am looking for I can't seem to find. What I want to do, using a kelvin configuration is measure in a range of 20 (19.999) Ohms with a current of 1 amp. I can find ranges like that and down to mΩ but a range like 19.999 Ohms with 1 Amp is something I can't find. Even a 1.999 Ohm range would be fine.

The goal here is to have receiving inspection remove about 10 feet (OK 10" +/- 1") from new spool of AWG 16.5 wire and measure it. We wind motors, very unusual hand built motors. The problem is we use a coil winding machine to make the coils (think 700 feet plus) of continuous copper enameled wire. This leaves a hell of a lot to go wrong. If the tension isn't monitored the wire can get stretched. But lately we have a new problem. We are supplied by Essex and another wire manufacturer. Lately we get wire that is not in specification. The ***** is they are winding using it and we don't know till the finished coils are checked.

Screw source inspection and all that. I want them (receiving inspection) to draw 10 feet off the beginning of a spool and measure it before we scrap 700 feet of wire plus the setup time and time to wind the coils.

Currently when the coils fail the engineer in coil wind drags the junk to me so I can hand it to a lab technician and have them run 1.0 amps from a DC Lab Standard through the stuff and use a lab bench meter to to measure the IR drop. This is getting old real quick. Additionally the wire temperature is allowed to get stable, the temperature is recorded and correction factors are applied.

I have looked at Biddle (Megger) and other manufacturers but can't find that damn 1 amp. Not for the range I want anyway. I am guessing a price range in the 2K to 3K USD. If anyone has any ideas other than build one I would love to hear them as to a manufacturer.

Thanks
Ron

 

[KeepItSimpleStupid]

Ron: I.m too lazy to do the math, but check out keitley's line of SMU's, e.g. **broken link removed**

Don't use the resistance range, but rather have the unit source 1 Amp and measure the voltage and it's at the top of your price range. I used this kind of instruments a lot.

 

[Reloadron]

Thanks Kiss. Sort of what if possible I want to try to avoid. Been hoping for a one simple box solution. Right now they are using a Fluke 1725 DC Current standard when necessary.

Ron

 

[crutschow]

I don't see why you have to measure the resistance at 1 amp. A lower current will give just as accurate a resistance measurement as measuring at 1 amp will. Current doesn't affect resistance other than temperature variation due to self heating. If you measure at a lower current, IR heating won't be a concern and you can just use room ambient as the wire temperature.

There are, of course, many kelvin type ohmmeters that can measure with milliohm resolution.

 

[Reloadron]

Hi Carl

That is a sticking point and not quite with me. For reasons I don't understand I am being pushed with this one amp by one of the coil wind engineers who seems to feel for reasons I'll never understand that one amp is some form of holy grail as it will afford a higher voltage drop. (Wow !).

This is what happens in a manufacturing environment when other areas tell test engineering what they seem to feel they want for test purposes rather than allow test engineering dictate what they need for testing.

They are going to end up with between a 1 mA or 10 mA and told this is as good as it gets and will work just fine in receiving inspection. When that happens I will be able to look at them and with an absolutely honest face tell them there is no such animal.

Thanks Carl
Ron

 

[crutschow]

...That is a sticking point and not quite with me. For reasons I don't understand I am being pushed with this one amp by one of the coil wind engineers who seems to feel for reasons I'll never understand that one amp is some form of holy grail as it will afford a higher voltage drop. (Wow !).

This is what happens in a manufacturing environment when other areas tell test engineering what they seem to feel they want for test purposes rather than allow test engineering dictate what they need for testing.

They are going to end up with between a 1 mA or 10 mA and told this is as good as it gets and will work just fine in receiving inspection. When that happens I will be able to look at them and with an absolutely honest face tell them there is no such animal.

So it's like the old joke ---They don't tell you how to measure resistance and you don't tell them how to wind coils.

 

[carbonzit]

So it's like the old joke ---They don't tell you how to measure resistance and you don't tell them how to wind coils.

Heh; good one.

But seriously, Ron, what's the industry-standard method for checking resistance of wire? Or is there none--just whatever ad hoc method the engineer in charge happens to come up with (no matter how crazy it is)?

How hard could it be to accurately measure the resistance of ten feet of wire?

 

[Reloadron]

Very good Carl and very on target. The coil winding group is a strange group.

Carbonzit, what they are proposing is something I advocated for years just to preclude this sort of problem every now and then. While there is no standard I am aware of it just involves a simple Kelvin measurement using a DLRO. We get the wire in 1,000 foot spools and a typical set of coils (depending on the motor they are winding for) will use about 700 to 750 feet as I recall. Accurately measuring a 10' section is not at all difficult down to mΩ. There is no shortage of very good and very accurate DLROs out there. They do check the coils (3 sets for 3 phase motors in a wye) after winding.

Following the last fiasco I again suggested doing samples from each spool (yet again) by lot # prior to doing any winding. A simple quick check. Enter the newer engineer from coil wind who seems to feel I need 1 amp to do this right. Currently he is away at some trade show for coil winding machines (Thank God) and I should see him again around Friday or next Monday so I can yet again make my case. This as I cling to my sanity.

I want to thank you guys for your inputs. Much appreciated.

Ron

 

[The Electrician]

Have you asked Essex how they do it?

You haven't said what kind of accuracy you need.

These guys have an instrument that can probably do the job:
**broken link removed**

The 2 ohm range apparently only uses 1 milliamp, but further down the page there is a heading that says "High or Low Current Configurations"; in that paragraph they say:

"Low or high current configurations for the Model 5060-06XR are available for the various testing applications that are required for specialized procedures. High current is often required for the accurate resistance measurement of some materials. This is often true in the measurement of products having low resistance values. The option to increase maximum current to 1 amp is standard on the Model 5060-06XR."

It's probably not cheap.

What I would do is to use a cheap variable bench supply, with about a 12 volt output capability at 1 amp. Put a quality (meaning stable) 10 ohm (with appropriate wattage rating; overrate it) resistor in series with the output and also in series with a DVM on amps range. You can then adjust the supply to output 1.000 amps to a short (or your length of magnet wire).

Cut off 11 feet of wire, and using a good steel tape, mark two spots exactly 10 feet apart, with each spot being about 6 inches in from the ends. Strip the enamel off the ends of the length of wire and pass the 1 amp through the wire at the ends. Adjust the supply to get exactly 1.000 amp. Using a couple of probes with needle points and another DVM, pierce the enamel with the needle points at the marked spots and measure the voltage drop across the 10 feet of wire. A little arithmetic and you've got the ohms per foot.

You could dedicate a couple of reasonable quality DVMs and a bench supply to this if you're going to be doing it a lot, which it sounds like may be the case from now on. You shouldn't have any trouble getting .1% accuracy with this setup.

 

[Reloadron]

Hi Electrician

I will look at that Harris unit. I already estimated the cost between $2,000 and $3,000 USD so I am not overly worried about cost. I like this feature:

"Low or high current configurations for the Model 5060-06XR are available for the various testing applications that are required for specialized procedures. High current is often required for the accurate resistance measurement of some materials. This is often true in the measurement of products having low resistance values. The option to increase maximum current to 1 amp is standard on the Model 5060-06XR."

I have been doing exactly what you suggested using a Fluke 5720 precision current source and a 6 1/2 digit bench lab meter to measure their wire when this problem arises. The problem is I am tired of using my departments lab people to measure wire. My solution is to have receiving inspection draw samples from each spool by lot number and include the data on an inspection sheet. For this to happen it becomes a matter of buying receiving inspection a DLRO. That went fine till a newer engineer in coil wind decided he wanted the wire tested at 1 amp. I don't want my assembly and test department of my lab involved. As to how Essex or the other manufacturer qualifies or test the wire prior to ship? That should be between purchasing and the manufacturer. Can you tell I am trying like hell to distance myself from this problem?

Holy crap, I just noticed Harris is in Delaware, Ohio less than 100 miles from me. Talk about convenient! I'll give them a call and maybe take time to run down there and see what they got. I could drag a few samples with me.

As to the specifications I'll sit down with the other engineer either Friday or Monday (he is out of town) and go over all this in more detail. Remember I am not dealing with electrical / electronic types on this.

On another note, yes, we use peculiar wire with gauges like 16 1/2 or 17.

Ron

 

[KeepItSimpleStupid]

How about a single edged razer blade to pierce the insulation for the voltage probes.

How accurate depends on R=pL/A. You know p (a constant), you know L and you know the cross sectional area A.

Using a higher current is equlivelent to using a smaller length or better meter.

Just figure it out: I+-?, V+-? ---> R +-? based on the resolutions of the instruments.
If A & R are constants, then L is measured to what +- length.
Convert that to percentage.

Done.

 

[The Electrician]

I have been doing exactly what you suggested using a Fluke 5720 precision current source and a 6 1/2 digit bench lab meter to measure their wire when this problem arises. The problem is I am tired of using my departments lab people to measure wire.

The difference in what I suggested is the use of a low cost variable supply rather than a Fluke precision source, and instead of your lab DVM, a couple of these:
https://www.tequipment.net/ExtechEX530.asp

only $219 each. Have that other new engineer who wants 1 amp used for the test buy the stuff, set it all up and teach one of his guys to do it.

Can you tell I am trying like hell to distance myself from this problem?

Yes, I certainly understand your desire to just buy a single piece of equipment and wash your hands of the whole thing! I hope the Harris people can satisfy your needs.

 

[MrAl]

Hi,

There's more to wire testing than just knowing the ohms in a given sample length.

When we run a 1ua current through a wire we find out the resistance, and thus the resistance per unit length and resistivity, but the key point is we only measure the resistance which is one single quantity.

When we run a much higher current that actually heats the wire and compare it to a lab standard we are testing more than one single variable. We are actually testing not only the resistance but also the temperature coefficient. Add to that we are also testing the ability of the wire to dissipate heat which may be important. With that one higher current measurement we can tell more about the wire than we can at a lower current.

It's always better to test something at or near the rated specification rather than a lower amount. What has to be assessed is the risk one would incur with a change in policy...how likely is it that a wire tested at a lower current is going to measure good when it's not. You could perform a few test to get a better idea.

 

[Reloadron]

Thank you all for the inputs. I spoke with these guys this morning. What I will propose is if they only want to check the resistance of a 10' section off each new spool they run with a benchtop version of the unit I linked to. It has a few options including automatic temperature compensation and the 1 amp available if they want it. Additionally with the options I will suggest it will come in at about $1,500 which is well below my forcasted amount. Then too, I always high ball things.

Electrician, many thanks for pointing to the Harris unit.

Overall I try real hard to avoid getting sucked into things like this. I like to focus on my own departments and workscope. Sometimes when you find yourself up to your ass in alligators it becomes apparent you forgot your primary mission was to drain the swamp. I have been here over 20 years and we have never been as busy as we currently are. While this is a "good thing" it is also a time when I try like hell to manage every man hour my areas expend.

Ron

 

[Mr RB]

I often use the method described by The Electrician above, I have a bench supply and set it to 1 amp constant current then clip it on the wire (usually I'm measuring an inductor etc) so it has exactly 1 amp through the wire, then touch the multimeter voltage probes on the wire just inside the PSU clips (this eliminates the voltage drop between the clip connection to the wire).

Since the multimeter reads in 1mV steps it reads neatly in milliOhms.

 

[Reloadron]

I often use the method described by The Electrician above, I have a bench supply and set it to 1 amp constant current then clip it on the wire (usually I'm measuring an inductor etc) so it has exactly 1 amp through the wire, then touch the multimeter voltage probes on the wire just inside the PSU clips (this eliminates the voltage drop between the clip connection to the wire).

Since the multimeter reads in 1mV steps it reads neatly in milliOhms.

Hi Ya Roman

I totally agree with the method. This is fine for you and I and anyone (just about) active in this forum. Unfortunately, whatever I propose for things like this needs to be simple, real simple. Our receiving inspection department is quite good with physical dimensional measurements. Unfortunately they suck with anything with wires, even with a good procedure. They need a simple one box solution. Go & No Go lights are a great tool.

Ron

 

[Mr RB]

Hmm, then you might need to build one to get something as simple as "go/no go" as it will need specific ranges where each range is set to your standard wire sizes.

Back to your original goal of getting one off the shelf, couldn't you use a commercial low-ohms meter, and let them measure ohms per foot? Even if you print a small table and stick it on the back of the meter or something, so they can check compared to "good" results.

 

[Reloadron]

Hi Roman

I am going to propose the Harris Unit that Electrician linked to. I spoke with them yesterday and they can provide what this other guy wants. This also gets me out of what I really wanted nothing to do with. I planned to bounce it off them on Friday but I have bronchitis so may be out a few days. Either way, the Harris unit will make them happy and has settings for high & low limits. I am not going to argue with them on this. They want an amp? They can have it as well as other choices.

Ron

 

[KeepItSimpleStupid]

Wow, take the easy way out. I don't blame you.

BUT everything does depend on resolution. You have it made because of the HI/LO limits. As long as they know to take 10'6" off the roll and use clips on the end and precisely use probes (I'm suggesting single edged razor blades in your own holder).

Hey, you could make a simple 10'-6" hinged gizmo, a couple of clamps. Have the wires fit into a cutout. You then clamp the wire into place and lower the razor blade gizmo onto the wire and bingo.

This wire is likely Formvar insulated, so you do have to deal with the ends separately.

I take it, you do a lot of this and therefore a jig is practical. When you start returning the wire to the manufacturer, maybe they will get the message.

 

[Reloadron]

Hi Ya Kiss

Yeah, sometimes in a large business structure the easy way out is a "good thing".

The additional spools this time around will be returned for contract violations, that is an issue between purchasing and the vendor. I will say in all fairness that Essex has always been a good supplier but every now and then as we say stuff happens. Additionally we use I believe REA Wire and they are another good supplier.

For some humor attached are a few images of Essex. One shows a label for AWG 17 (told you we use strange gauges) and the other the container it comes in. Note the bump on the top where the wire exits. Pretty cool huh? If you note the label you will see it was a 79 pound spool.

The wife feeds the birds and buys bird seed in bulk, like 25 or 30 pound bags. The wire pails we throw away make perfect bird seed containers. That plus other things like mixing paint or whatever.

Ron