Fluke 289 mV(~)  erratic open circuit behavior  - Page 1
Forum section: electronics / testgear

author: genome2k
author_rank: Newbie
timestamp: February 29, 2016, 11:29:26 am
content: Hi guys,I've just got a brand new Fluke 289 with 6 FRESH AA batteries and I have some issues with the mV(~) (AC) switch. I am not sure if this is normal since I don't own another meter that has a separate mV(~) switch.Well my problem is still with floating readings on the mV switch when the probes are open or not plugged into the meter at all. I know this is an old question asked and answered many times, but my case is a little different.Just for the record, I know that (1) EMF/RF etc. electromagnetic energy in the air (2) high impedance of the meter are the reasons you get floating readings on AC Voltage reading, especially when it comes to the sensitive mV range.I did the inspection with my new Fluke 289at home, and have these observations described in below. My environment:1. Typical household electromagnetic profile - residence power supply, wifi, laptops, bluetooth, 2.4GHz wireless headphone. No known significant EMF/RF source in the vicinity.2. I have tried to power off all the interference i could think of - wifi, wireless headphones, etc. But it didn't make much of a difference.Now the two MAJOR problems I have with my Fluke 289 on the mV switch:1. Way too much for a floating readingSometimes you put the meter on desk, with the probes touches nothing and close to each other but not shorted, the mV-AC gives erratic readings ranging 7x mV - OL. And many times i switch to mV-AC, the reading fluctuates from 300mV - OL. Even after the probes were shorted there was still 0.2xx mV left.Those of you who have the Fluke 289 -- is this the same with your meters??I mean I expect there will be some floating reading with the probes open -- but readings take 60% of the range, and even overloaded it sometimes -- I really don't know if this is normal with Fluke 289?2. Another behavior I can't explain:The Fluke 289 was switched on and put to the mV-AC stop, then i put it (probe included) on the desk. After that there was nothing in the environment that changed, and the meter was exactly where it was when i put it on the desk. As you would expect the readings would fluctuate but not too much.Now, if there was a slightest disturbance on the meter -- like iputmy hand on the rubber part of the meter (just put my hand on it really gently,without causing any movement of the meter), even a slightest disturbance like this could make the the readings drastically change -- from a fluctuation of 100-200mV to overload.Is this the same with your 289's?Thanks guysBy the way, I also did side by side parallel test - the 289 against Fluke 28 II, as followed:===Please note:even the mV-DC switch cannot be zeroed(after the probes were shorted ---- is this normal with 289??)

author: mzacharias
author_rank: Frequent Contributor
timestamp: February 29, 2016, 03:49:52 pm
content: My 289 in a fluorescent lighted environment: 3.3 mV with no leads. Wave hand over it - anywhere from 16 to 35 mV to OL.With leads attached: about 15mV residual, easily goes OL waving my hand over the unit near the input jacks.

author: Performa01
author_rank: Super Contributor
timestamp: February 29, 2016, 05:16:53 pm
content: Why do you think your home would be a particularly clean environment? You do have mains power at home, don't you?What you describe sounds perfectly plausible for an instrument with 1MOhm input impedance, mV sensitivity and a bandwidth of supposedly some 100kHz. Try the same with an oscilloscope (you'd need an adapter in order to connect the DMM test leads to the scope input) and see what you get ... granted, the scope has a much wider bandwidth, but you will see predominately mains hum nevertheless.Only reason to worry would be if the meter shows fluctuating values with shorted inputs - shorted with a shorting bar, that is, since the shorted test leads would form a big magnetic loop antenna, capturing all sorts of electromagnetic garbage.A low static value with shorted input is ok, since this is just an undesired feature of the RMS converter, which limits the lowest measurable value, but has no impact on accuracy once the signal level exceeds that basic offset level.

author: genome2k
author_rank: Newbie
timestamp: February 29, 2016, 07:32:54 pm
content: Quote from: mzacharias onTodayat 02:49:52 AM>My 289 in a fluorescent lighted environment: 3.3 mV with no leads. Wave hand over it - anywhere from 16 to 35 mV to OL.With leads attached: about 15mV residual, easily goes OL waving my hand over the unit near the input jacks.

author: genome2k
author_rank: Newbie
timestamp: February 29, 2016, 07:49:55 pm
content: Hi Performa01,Thanks for the comments but if you look at my OP, I said:QuoteAnd Yes I agree the high impedance of the 289 is a reason we see these readings. But unfortunately i dont have an oscilloscope (I'm just a beginner) . I enjoy your explanation in the third paragraph, thanks !

author: TheSteve
author_rank: Unknown
timestamp: February 29, 2016, 08:00:27 pm
content: Sounds completely normal.

author: mos6502
author_rank: Frequent Contributor
timestamp: February 29, 2016, 08:03:03 pm
content: Quote from: genome2k on February 29, 2016, 07:32:54 pmIt's called capacitive coupling. The same way that your phone detects your finger through a piece of glass.

author: genome2k
author_rank: Newbie
timestamp: February 29, 2016, 08:33:26 pm
content: Quote from: mos6502 onTodayat 07:03:03 AM

author: genome2k
author_rank: Newbie
timestamp: February 29, 2016, 08:59:10 pm
content: Thanks to you all who have replied to my questions. But like I have appended to my OP: when i was testing the 289 I did side by side parallel measurements with Fluke 28 II:And pls note that with Fluke 289, even the mVDC cannot be zeroed (probes shorted) . Normal with 289??=======>So guys if you have Fluke 289 please read my OP and my comparison table with Fluke 28 II, and post the data with your 289's. Much appreciated !I'm fully aware different environment may cause the mV readings vary a great deal but the behavior i described -- I want to know if they have occurred to your 289 as well.Pls don't forget the mVDC shorted reading above. Thanks a lot!

author: electr_peter
author_rank: Unknown
timestamp: February 29, 2016, 10:03:59 pm
content: I do not see anything extraordinary - DMM behaves normally as it should. You seea)high values on sensitive ranges when nothing is connected andb)you see few uV (micro-volt) offset with shorted probes. Both a) and b) is normal behaviour for such class DMM.a) is explained by noise/high impedance/op-amp related offsets. b) is explained by non-perfect DMM input circuitry, thermal noise, non-perfect nulling.You have very high end handheld DMM, so it shows some sensitivity to "the real world" in terms of weird effects. Use it properly, study datasheet with specifications, learn and don't be afraid of strange readings. Good luck.

author: Scottjd
author_rank: Frequent Contributor
timestamp: February 29, 2016, 10:57:16 pm
content: Quote from: genome2k on February 29, 2016, 08:33:26 pmThese meter you mention are in a different class on how sensitive they are and their accuracy tolerances.Mine does the same as you mentioned in my house.I don't have a mV AC to measure. But I have something for DC and it is not calibrated, it does better with 1.00mV DC and higher.If I put it on a 0.01mV DC output and probe it with my 289 I get a steady reading between 0.029 and 0.030mV DC.When it sensing the voltage the reading is stable and waving my hands around the probes won't change it.But disconnected and floating open with min/max enabled I see -1.003 to 0.170 and its sitting in the -0.042 range instead of zeroing out.If I move 3 feet away from the probes it goes down to -0.006 BUT I have to be still, even moving my feet an inch will cause this to raise.wavingmmymhand or moving the probes will get a reading of min -33.609 to max 44.08 and OL. The min/max doesn't record over limits.Again, when I give it a source it stays fairly steady between 0.029 mV DC and 0.030 mV DC and that's all that counts when your taking a reading.The Brymen BM235 also jumps around the same way. When given the same source it will steady and read 00.02mV DC. But this is a 6000 count vs a 50,000 count. So the Brymen probably rounds down the least significant number in the reading. I would say for this level the 289 is probably the correct reading. So out of curiosity I took the same source and out it in my calibrated HP 34401A and it reads 000.0294mV DC

author: genome2k
author_rank: Newbie
timestamp: March 01, 2016, 03:54:54 am
content: Thanks Electr_peter and Scottjd,It’s good to know my meter is normal as it supposed to be.But Scottjd:You meter reads 0.029-0.030 mV with a 0.01mV DC output — is this ok?? That’s 200% off. I’m not familiar with bench meters (i suppose the HP34401A).So the HP should be the one of greatest confidence — therefore your experiment suggests the 0.029 mV by 289 was actually correct, and that error from 0.01 actually comes from the DC power supply. Am I right?