Are these normal with Fluke 289?

[genome2k]

Hi guys. There are a few things with my new Fluke 289 gets under my skin . Please help me take a look and see if these are normal, thanks.



1. DC Volt and DC mVolt cannot be shorted to Zero: I know it is normal with AC volts that when you shorted the probe you still wouldn't get zero. But For DC voltage switches, what i know is you always gets zero when you short your probes, as long as the multimeter is functioning correctly.


But for either "VDC" and "mVDC" switches on my Fluke 289, I can't short the probes to read a zero. I have even tested with different probes:


VDC switch -- the best it can do is 0.0001 volt (@5V range). Very occasionally it gives a whole ZERO reading (0.0000 volts) but this happens like 1 in 500 or even 1 in 1000 times shorting the probes. The VDC switch is the meter's primary DC voltage i guess it's only normal to read zero when shorted, doesn't matter how many zeroes heading the least significant digit.  (I don't know this - nobody has told me so but I just "feel" "zeroing" is the only normal behavior - so i'm asking you guys if my concept is wrong)


"mVDC" switches -- it's worse with the milli volt DC switch. Aside from it never reads zero when shorting the probes, the "shorted reading" is unstable from +/- 0.003 mV ~ +/- 0.03x mV, and most of the time this reading is greater than 0.012 mV (i.e., 2 digits).

I'm clueless why shorting the same probes the same way this reading may change (put the meter on the desk then get a reading, then leave where it is couple minutes and get another reading -- this usually is a different value). 2 things i don't understand about the mVDC switch:

1. why it can't be zeroed (same question as i have with the VDC)
2. For mV DC switch, it shouldn't pick up EMF interference in the air like the AC volt switch does, so why does the "probe shorting voltage" vary from time to time?


2. Fluctuating DC mVolt readings when probes open: The same behavior like you see with an AC voltage switch - floating reading only not that rapidly and radically. But it looks like that although it is a DC measurement, it's affected by electromagnetic fields/energies in the air. How come??


3. The batteries inside multimeter? : This is something i've been think.. Does the voltage of AA batteries the multimeter uses affect the "shorted" reading? I assume more or less there is an effect but does it manifest on the readings??


So guys please help me on this: Is it normal these DC voltage switch cannot be ZEROed when shorting the probes, and why does the mVDC have a floating reading under open circuit?


Thanks.


(PS. I know Fluke 289 is a 50,000 count meter and it's supposed to be very sensitive. But my questions here today is all about DC ranges which is not affected by EMF in the air so there shouldn't be anything it's sensitive about?

[Andy Watson]

It's normal. Most DVMs will fluctuate on the last digit. - It's just noise in the measurement process.

2. For mV DC switch, it shouldn't pick up EMF interference in the air like the AC volt switch does, so why does the "probe shorting voltage" vary from time to time?

0.003-0.03 mV that's microvolts! These voltages could easily be generated from thermal EMFs. Try making a thermocouple from two dissimilar metals, e.g. copper and solder. I'm seeing a 40\$\mu\$V voltage for a temperature difference of 10-15 degrees C.

3. The batteries inside multimeter? : This is something i've been think.. Does the voltage of AA batteries the multimeter uses affect the "shorted" reading? I assume more or less there is an effect but does it manifest on the readings??

The state of the batteries should not significantly affect the reading until the meter complains of "low battery"

[danadak]

Some basic info on noise and other effects in precision measurement.

https://www.dropbox.com/s/2h96beh1fbvz4e2/noise_notes.zip?dl=0

Regards, Dana.

[genome2k]

Thanks a lot Andy!

It's normal. Most DVMs will fluctuate on the last digit. - It's just noise in the measurement process.

The other multimeters i've seen did zero (all digits) when the probes were shorted. The only difference is the other meter had much lower counts (6000- ) but Fluke 289 has 50,000 counts.


So is it correct to put it this way, that the other multimeters i've seen was able to read zero because they had less counts so they display less digits, therefore they were able to round the least significant down to zero, thus masked the effects of the noise. However the F 289 has 5 digit display on the VDC switch and it's not able to round its least significant digit down to zero?


As to the noise you were talking about, it's the intrinsic noise generated by DMM's own circuit but also readable but the DMM itself, as opposed to the noise generated (induced) by the EMF in the surroundings, which is why although the DMM was on a DC range, rather than an AC range, and it was still able to pick up such noise.  <-- is this correct?

0.003-0.03 mV that's microvolts! These voltages could easily be generated from thermal EMFs. Try making a thermocouple from two dissimilar metals, e.g. copper and solder. I'm seeing a 40??V voltage for a temperature difference of 10-15 degrees

Yep it's micro but this not a microvolt reading from a VDC range, but a microvolts reading from the mVDC switch it's different. and its a 2-digit reading out of totally displayed 4 digits. I assume 2 out of 4 digit should be considered a "significant" reading?


I don't have much knowledge on this but I have thought the nature of any EMF induced voltage says it's only readable as an AC voltage? so this is a DC range and its not affected?


Well another reason i felt this was a problem because, every measuring instrument are given a measurement baseline, which is the "zero" that can be referenced to. The reference is certainly not perfect either so it is also given an "acceptable" range of shifting/offset. For voltage on DMM, when the probes are crossed it should be the "baseline" of voltage measurement, so my 0.0xx mV reading - again 2 digit out of 4 and floating around, acceptable for baseline/reference?? And unlike the AC voltage, the DC V doesn't have an "excuse" for this?

[genome2k]

Some basic info on noise and other effects in precision measurement.

https://www.dropbox.com/s/2h96beh1fbvz4e2/noise_notes.zip?dl=0

Regards, Dana.

Thanks a lot for sharing! Dana