Fluke 87V sub ohm accuracy. - Page 1
Forum section: electronics / testgear

author: blewisjr
author_rank: Frequent Contributor
timestamp: October 15, 2014, 03:43:44 pm
content: Hello everyone. I am measuring resistances below 1 ohm. I found a fluke artical on how to setup the 87V to read this in high res mode. The issue I see is acuracy at this level is mentioned no where. I understand the acuracy is 0.2% + 1. Not sure what that means but can anyone explain this for me. Trying to find the best way to measure sub 1 ohm.

author: AG6QR
author_rank: Frequent Contributor
timestamp: October 15, 2014, 04:40:29 pm
content: With any two-probe system, the accuracy of measuring low resistances is limited by the fact that the probe resistance corrupts the measurement. Even if you try to do some sort of relative measurement by shorting the probes together, measuring their resistance, and then subtracting this from the final reading, you still have the issue that the contact resistance where the probes contact the DUT (or where they contact each other when shorted together) may not be constant.As long as the probe resistance is small relative to the resistance being measured, we can generally ignore it. But when measuring very low resistances, the resistance of the probes may not be small relative to what's being measured. If you want accurate measurements of resistances lower than an ohm or so, the four-wire Kelvin approach is the standard technique. Some bench meters support this directly, but it's not a common feature among handheld meters. But you can still do it. The idea is that you use a precision current source to pump a measured current through the DUT, and then use a precision volt meter to measure the voltage developed. The key difference in the Kelvin four-wire technique is that you measure the voltage using a separate set of probes, and those probes have negligible current going through them. Since they have negligible current, their resistance has negligible effect on the final reading.http://www.allaboutcircuits.com/vol_1/chpt_8/9.htmlYou can't do this using just a single multimeter. You need two meters, one for current, and one for voltage. If not necessarily another multimeter, you need some way of measuring the current and holding it constant while you do the voltage measurement.

author: Excavatoree
author_rank: Frequent Contributor
timestamp: October 15, 2014, 05:16:33 pm
content: Dave's got a video showing the "two meter four wire" technique. If I weren't at work, I'd post a link.

author: xwarp
author_rank: Frequent Contributor
timestamp: October 16, 2014, 10:06:51 am
content: Quote from: blewisjr on October 15, 2014, 03:43:44 pm"cloud chaser"?

author: Wytnucls
author_rank: Super Contributor
timestamp: October 16, 2014, 12:47:35 pm
content: Quote from: blewisjr on October 15, 2014, 03:43:44 pmAccuracy is quoted as 0.2%+2 for the normal 6,000 count in the 600 Ohm range and putting the meter in high res mode only improves resolution, not accuracy.Any resistance below about 100 Ohm, should be measured using the 4-wire method for accurate results.As an example, a resistance of 0.5 Ohm would measure between 0.7 or 0.3 Ohm with the 87V, in the best conditions.

author: Fungus
author_rank: Super Contributor
timestamp: October 16, 2014, 01:04:23 pm
content: To measure it accurately you'd have to put, like, 10 amps of current through it to get a decent voltage drop to measure.10 amps of current might alter the resistance of the resistor. Maybe even to the point of it becoming open circuit.

author: Wytnucls
author_rank: Super Contributor
timestamp: October 16, 2014, 02:21:26 pm
content: Not really, the Gossen 30M uses a constant current of 1mA to measure resistance on the lowest 100 Ohm scale with a worst accuracy of 0.005% of reading + 0.001% of range.

author: robrenz
author_rank: Super Contributor
timestamp: October 16, 2014, 05:07:28 pm
content: Not knocking the Gossen but that would be +/-0.1% of reading at 1 Ohm and +/-100% of reading at .001 Ohm

author: Fungus
author_rank: Super Contributor
timestamp: October 16, 2014, 05:58:55 pm
content: Quote from: Wytnucls on October 16, 2014, 02:21:26 pmWell, OK, I exaggerated a bit... :-)

author: Wytnucls
author_rank: Super Contributor
timestamp: October 17, 2014, 02:08:47 am
content: Quote from: robrenz on October 16, 2014, 05:07:28 pmThe manual or the calibration certificate doesn't specify if the accuracy specifications pertain to 2W or 4W measurements.I would assume 4W to achieve those accuracy figures.German DAkkS calibration papers give a meter reading of 100,0068 Ohm for a correct resistance value of 100,00810 Ohm, with an uncertainty of 1.6mOhm.There is no compensation possible for thermoelectric EMFs and no pulsed or reversed current option.Is an accuracy of +/- 1mOhm so shocking for a portable DMM with 2 AA batteries?

author: robrenz
author_rank: Super Contributor
timestamp: October 17, 2014, 03:05:08 am
content: Quote from: Wytnucls on October 17, 2014, 02:08:47 amNot at all. I was simply pointing out the importance of converting specs to % of reading to really appreciate the accuracy of any instrument. The Gossen has .0001 Ohm resolution but the .001% of range means you will always have at least +/-.001 Ohm uncertainty. So the extra resolution is not of much value. Many would read the specs and assume they could make accurate readings of a .001 Ohm resistor yet there is possibly +/-100% error at that level (it could read .002 or 0 and be in spec).Again, I have no problem with the Gossen

author: Wytnucls
author_rank: Super Contributor
timestamp: October 17, 2014, 03:32:27 am
content: It is a multimeter, so the last digit of the 6 1/2 count can be useful on other selections, like the 100mV DC range (0.005% of reading+0.0006% of range) with a worst accuracy of +/- 0.6uV.I suppose Gossen could have suppressed the last digit for resistance measurement, but the extra digit of resolution may be useful in some special cases.

author: DanielS
author_rank: Frequent Contributor
timestamp: October 17, 2014, 03:55:25 am
content: Quote from: Wytnucls on October 17, 2014, 03:32:27 amLike trying to trace a shorted component on a PCB somewhere between a ground and power plane - you do not care about absolute accuracy, you just want to know if you are getting closer to wherever the suspected short might be.