Resistance Readings on an Agilent U1272A DMM - Page 1
Forum section: electronics / beginners

author: J.P. Samson
author_rank: Newbie
timestamp: January 28, 2012, 10:53:56 pm
content: With the tempting discounts and rebates available on Agilent DMM's in the past few months, I decided to finally invest in a 1272A. This replaces a very old, basic analog Radio Shack multimeter (one with a deflection needle)!I've seen some chat about erratic behaviour of the 1272A, particularly when trying to measure resistances. I've noticed that as you wiggle the probe cables on my particular unit, the resistance numbers jump around; once the cables stop moving, the readout stabilizes and the resistance value seems to be mostly consistent. For example, with the probes not touching anything (i.e. open circuit), the readout jumps around in the 100 Mohm range when the cables are wobbling around rather than displaying the expected OL. It is the same effect when measuring a resistor, albeit the numbers jump around at a lower range. Again, once the cables settle (stop moving), the display shows a reasonable (repeatable) resistance value.As this is my first autoranging DMM, I'm not sure if this "annoyance" is to be expected from the Agilent or DMM's in general. I'd appreciate if someone could let me know if this is an expected behaviour and why it occurs. Thanks.-- JP

author: saturation
author_rank: Super Contributor
timestamp: January 29, 2012, 12:21:36 am
content: Its normal, when you shake the cables you are creating tiny voltages from many sources that are normally not seen except in sensitive high impedance devices, like your DMM or modern oscilloscopes. Electrostatic fields, piezoelectric effects from movement, cable capacitances and inductances etc., are some of the few phenomena. ESD protection is common because body movement alone can cause large enough charge creation and build up that can discharge into other devices, even in environments where static doesn't seem to visibly build up [ such as high humidity hot climates like Pacific islands.]

author: Lesolee
author_rank: Regular Contributor
timestamp: January 29, 2012, 06:13:16 pm
content: Quote from: J.P. Samson on January 28, 2012, 10:53:56 pmI just checked out the spec. Wow! That is a seriously upmarket bit of kit. When a DMM is measuring high ohms it is measuring at high voltage, 1V or more on some DMMs. It should not be affected by wobbly leads. Ifyouare part of the circuit then it will read badly. For example if you hold the thing you are measuring and effectively shunt yourself across the reading. If you have a mobile phone near the measurement then turn it off or move it far away. If you are near any RF transmitter (wifi etc) then turn it off or move it away. Even a crappy £20 hand-held will give a stable reading. Use the built in filter. Any DMM will have the last digits "running around" especially with that many digits at that speed.If it is moving more than the last two digits, and it is not an RF field problem, you may want to consider complaining about it.

author: saturation
author_rank: Super Contributor
timestamp: January 29, 2012, 08:24:16 pm
content: Alas, its 'normal' for these class of Agilent DMMs, the same happens to the 1252a. On megaohms or nS range, it picks up body movements easily.On the Fluke 87V its suppressed, but you'll see the same effect once the nS mode is engaged, but not as severe as the Agilent.I work around it using the minmaxave mode to filter the noise, you can use the statistical ave as the final reading as it reduces the effects of transient values; just let it linger a few seconds so the average overcomes the outlier values. Compared against an HP 3456a on 4 wire ohms, it comes near par in accuracy.

author: J.P. Samson
author_rank: Newbie
timestamp: January 30, 2012, 12:55:40 am
content: Quote from: Lesolee on January 29, 2012, 06:13:16 pmAs I mentioned, there were some significant discounts available (in this case, more than 40% off). Otherwise, I would have settled for a much lesser model!There are certainly more than two digits of instability when the cables are moving around. For example, measuring a 68 kohm resistor, the reading fluctuates around +-5% if the cable leads are swinging/moving around. Again, the reading is acceptably stable and consistent so long as the cable is quite stationary. I'm not shorting across the probe tips in any way as the leads of the resistor are being held in place in a breadboard, and I'm taking care to ensure the probe tips are held steady on the resistor when wiggling the cable.-- JP

author: Unknown
author_rank: Unknown
timestamp: January 30, 2012, 01:18:29 am
content: Did you try different test leads? Sounds like a broken wire to me. I don't see any reason for a 68 kOhm reading to fluctuate either, you're not dealing with very low voltages (at the bottom end of the range), nor with extremely low currents (tens to hundreds of Mohms).

author: J.P. Samson
author_rank: Newbie
timestamp: January 30, 2012, 02:30:00 am
content: Quote from: alm on January 30, 2012, 01:18:29 amYeah, it's a similar outcome using the leads from my old analog multimeter.-- JP

author: Lesolee
author_rank: Regular Contributor
timestamp: January 30, 2012, 08:21:07 pm
content: Quote from: saturation on January 29, 2012, 08:24:16 pmThat was an excellent video. Thanks very much for posting it.That Agilent meter is utter crap!I would want a refund. Reason:Not fit for purpose. Not EMC compliant.It must maintain spec despite static (or they can quote a degraded spec if they wish).

author: Rufus
author_rank: Super Contributor
timestamp: January 31, 2012, 01:53:10 am
content: Quote from: Lesolee on January 30, 2012, 08:21:07 pmBit harsh. 10M source impedance, unscreened cables, what do you expect? The U1252B specs a 14Hz update rate for resistance measurement (which you might appreciate in some circumstances) the Fluke only 4Hz, maybe the fluke does some software filtering or just sticks some capacitance across the inputs on ohms ranges.The Agilent does seem to have a bit of an issue with static generation on the case and differential strays. I would say could do better, not utter crap.

author: saturation
author_rank: Super Contributor
timestamp: January 31, 2012, 11:40:39 am
content: I would agree. These DMM have flaws which was brought up to Agilent during the review process here at eevblog, but its not unusable. It does have more functionality than its competing Fluke 87V, for nearly the same money, after the incentives are in. Also, as I mentioned earlier you get a similar effect with the 87V by shaking the probes in conductance mode.Quote from: Rufus on January 31, 2012, 01:53:10 am