TTI BS407 Precision Milli/Micro Ohmmeter-Just bought one used.

[djsb]

Just bought a used (without leads-shout out if you have a spare pair I can buy off you) TTI BS407 Precision Milli/Micro Ohmmeter for £455.00 (over £1K new).

https://resources.aimtti.com/datasheets/AIM-BS407_precision_micro-ohmeter_data_sheet-Iss1A.pdf

It has 1u Ohm resolution, so should be good enough for basic fault-finding on connectors etc. Does anyone else have one of these? I did have a quick look at a UNI-T UT620B, but it doesn't seem to have the same level of resolution. I wonder how much a set of new leads will cost me? Thanks.




P.S Can anyone recommend a basic precision resistance standard I can use to do a sanity/functionality test? Not too expensive, please. Thanks.

[djsb]

Got a set of these 4 lead Kelvin probes as well from Warwick test supplies.

https://warwickts.com/8312/PJP-KITKELVIN-4934HA-Kelvin-Probe-Lead-Set-2m

Cost £130 including postage and VAT.

I hope they are OK.
The instrument is 7 years old and was calibrated about 5 years ago, and the internal batteries are OK according to the seller. Should be arriving this week, so I'll have everything by the end of the week. Might do a quick review on my YouTube channel at some stage.

[Randy222]

How much current on DUT is needed by that meter to read u-ohm ?


There are so many.
Not sure which ones are good or not.

https://duckduckgo.com/?q=milliohm+meter&iar=images&iax=images&ia=images

[djsb]

250mA according to the datasheet I provided a link to in my first post. The resolution is 1 micro Ohm on the 1999 micro Ohm range.

[Randy222]

250mA according to the datasheet I provided a link to in my first post. The resolution is 1 micro Ohm on the 1999 micro Ohm range.

I asked because some of models, like the x-branded Lutron MO-2014 (aka MRC Lab MO-2014) spec sheet calls for 10A in the 0-60k u-ohm range. Where does a small handheld get accurate 10A from? It's internal fuse is a 10A fuse.

[djsb]

So with the 4 lead set mentioned above I can only get down to around 10 milli Ohm (3.5 milliohm with pressure on the leads). That's fine for most practical uses. Are there any tips for getting down to the micro-Ohm range? I might buy a set of the OEM leads a bit later on. Thanks.

[J-R]

As hinted at by the diagram on the front of the BS407, the test leads you have are not actually a Kelvin setup.  The force connections need to be separate from and outside of the sense connections.

[Gyro]

Sorry to say that you paid over the odds for the test probes. As J-R indicated, they are not true kelvin probes, just force and sense wires soldered into the back of standard probes (so you are measuring the resistance of the length of the probe tips).

For satisfactory very low resistance measurements you need proper kelvin clips, with force and sense connections on opposite jaws of each clip. You can find adequate ones cheaply on ebay...  https://www.ebay.co.uk/sch/i.html?_from=R40&_nkw=kelvin+clips&_sacat=0&rt=nc&LH_PrefLoc=1


P.S. If it's any consolation, I bought a set of the kelvin clips shown in the BS407 datasheet photo from CPC years ago. They are truly horrible, it's hard to make them grip anything satisfactorily, unstable readings etc. The ebay ones are far better.

[djsb]

Thanks for the link. Which ones from the link would you recommend? I've just emailed Warwick Test supplies and asked them if I can return them for a refund. Hopefully I should get all of my money back less the postage cost. I'll report back here later.

[Gyro]

The first 3-4 items, the £8 - £11 ones.

[J-R]

I actually have had a set of the Kelvin leads shown in the BS407 photo for a few years.  Ran into an eBay listing with an impossibly low price ($35) so snapped them up without doing too much research.  They are made by Pomona and sold as the "Wide Jaw Kelvin Lead Set".  The "Wide Jaw" part is why they are not ideal for typical uses.  Their "Kelvin Clip Test Lead Set" is a bit more normal.  Either way, the list prices on these are pretty ridiculous.

You can buy quality Kelvin clips and make your own set, but also the cheap ones do work well enough.

I haven't spent too much time below 10uOhms, but I think you'll be surprised at how easy it really is to get good results with even the most basic Kelvin setup.  Of course typically the best option is a dedicated fixture, but down from there is two sets of alligator clips, or you can use alligator clips for the force and then use DMM test probes for the sense, and finally you can actually get your tongue at just the right angle and use two sets of DMM test probes and just hold them with your fingers like chopsticks.