IET LOM-501A Micro Ohm meter

[billclay]

Nice video, and great job on the front panel!
Can you post the data from the spreadsheet/graph?  Thanks.

[robrenz]

Here is the excell spreadsheet of the low ohms accuracy comparison.
Vertical axis is logaritmic maximum +/-% of reading error.

EDIT:  See chart as picture with additional meters in later posts

[robrenz]

I just got my LOM-510A calibrated by IET labs.  The meter is 25 years old and had no signs of previous calibrations. Cal data shown below.  The stability of this meter at 1 micro Ohm resolution is truly amazing. I have done enough testing with it to know that anything other than a rock steady reading or a perfectly consistent LSD bobble between two readings means something thermal is going on.  This seems like the "Power Designs" of micro-Ohm meters, no frills, no programming, just exceptional accuracy even by today's standards.

 I also made a stupid low $200.00 offer on another unit on EBay and got it . Perfect condition and I checked/calibrated it against the IET calibrated unit as shown below.  All ranges were in spec as recieved.  Even though I adjusted the second unit to agree exactly with the calibrated unit on all ranges, some could argue there could be transferal errors. Even if I add 1LSD to the calibrated units error on each range it is still well within the .02% of reading + 2LSD + 2microOhm specification. So I feel confident the second unit is well within spec.

Calibration data


Both units warmed up and on the same range and are measuring in switched mode. Cables are clamped so that nothing moves when I switch the plug from one unit to the other.


Piece of tinned copper wire used for the .010 Ohm test.  I just adjusted the spacing of my ultimate Kelvin probes to get a nominal 10 mOhm reading.


The Thermal coefficient of resistance of the copper wire requires the use of a insulated box to prevent variations from drafts and my body heat.  just made one out of a piece semi rigid foam.


Chunk of iron holding the foam box tight to the table top.


The calibrated meter with the reading stabilized at 10.022 mOhm.  The probe plugs are then interchanged between the two meters.


The second meter's 20 mOhm range trim pot is adjusted until the reading is stable at 10.022 mOhm. The probe plugs are interchanged a couple of times making sure the readings are identical between the meters.  The meters cover is kept on all the time except to make a trim pot adjustment to keep the meter internal temperature consistent.


Comparison and adjustment on the other ranges was done using my decade resistance box.  These are 25 ppm resistors and are heat sinked so they were stable without any additional thermal control.

[HLA-27b]

Amazing!

[Daniyal]

great post!!

[SeanB]

Lovely technique, like the isothermal block you made, and the nice simple ways used. Very nice, makes my measurements look crude. All I can get is a 10mOhm resolution on a 10R resistor, but it is not 4 wire, just a bridge.

[robrenz]

I finally figured out how to convert excel to pdf to jpg so I could post the low Ohms comparison chart as a picture.  Also added the Keithley, Megger, and AEMC to round out the comparison.  Chart is % of reading error values.  8846A and 34411 are identical above 13 Ohm.  It is interesting to see they all have thier strengths and weaknesses. I would put the 34420 as the winner and the LOM-510A as second for the 1 Ohm to 1 mOhm span.

Max. drive current and resolution.
MEGGER DLRO 10  10A drive  0.1u Ohm resolution
AEMC 6250            10A drive  0.1u Ohm resolution
KIETHLY 580     100mA drive   10u Ohm resolution
LOM-510A              1A drive   1.0u Ohm resolution
AGILENT 34411    1mA drive  100u Ohm resolution
FLUKE 8846A       5mA drive    10u Ohm resolution
AGILENT 34420  10mA drive   0.1u Ohm resolution
AGILENT 3458A  10mA drive    10u Ohm resolution

[andyb]

Wow! What is that resistor decade box? Did you make it? I googled a bit for Versa-Ohm, and came up empty.

[robrenz]

I was wondering how long it would take for someone to notice that.  It is my own design and name.  I did the  same thing you did, I thought up a name and then googled it to make sure no one else had used it. It is TWO 8 decade units in one box. The resistors 10 ohm and up are 0.1% .25 watt 25ppm and are heat sinked. The heat sink fins are on the back and that is why it is sitting in that orientation for optimum convection cooling.  It also has a forced fan cooling attachment for higher accuracy or dissipation. All the individual decades are available separately so you can get many resistance values out of it simultaneously with supplied calibrated jumpers.  Special jumpers allow paralleling up to 20 resistors to get down to .01 ohm at 5W.  I will be posting a thread/video about all its features soon and there will be a whole series of detailed videos on its design and construction. And yes they will be for sale, I made a first run of 25.  I am just finishing up some of the jumper attachments now.  Stay tuned!

[andyb]

Very cool! I fall into the camp of folks who've never really seen a use for substitution boxes, but this may change my mind! I'll be watching!

[sorin]

Has anyone got a schematic, or the service manual?

[robrenz]

The Manual talks about how the circuitry works including chip numbers and referring to a nonexistent schematic.  I have not found a schematic so far but I don't really need one the circuit is a very simple 2 sided board.

[sorin]

I want to build from scratch my "Micro Ohm meter" based on this schematic.
Thanks for your reply.

[ass20]

hi
where i can see shematic of 510micro-ohm meter?