Fluke 28-II 4.5-digit mode tolerance conundrum - Page 1
Forum section: electronics / testgear

author: X
author_rank: Regular Contributor
timestamp: May 10, 2017, 07:04:20 pm
content: When I was looking at the Fluke 28-II specs in the manual*, I came across the following:Quote from: Fluke 27-II/28-II Users ManualThis means the basic DC accuracy goes down from ±(0.05% + 1 count) to ±(0.05% + 10 counts).Standard mode accuracy is ±(0.05x0.01x6000 + 1) = ±(3 + 1) = ±4 counts.4.5-digit mode accuracy is ±(0.05x0.01x20000 + 1) = ±(10 + 10) = ±20 counts.Surely the >10 counts tolerance renders the last digit of the "high-resolution" mode useless. Can this 4.5-digit mode still be used to reliably monitor subtle changes in a long-running process (eg. battery charge/discharge cycles) with any degree of accuracy?What am I missing here?*Also, c'mon Fluke! If you can include a fold-out quick-start guide, a cardboard strip with a life-size well-cropped image of a magnetic hanger (instead of the actual hanger itself), and hard copy of the "safety information" (in ten different languages) in the package, but then have "REFER TO MANUAL FOR ADDITIONAL CAPABILITIES" written on the back of the multimeter along with a warning about the shocking truth of electricity and a logo advising me not to operate the multimeter whilst under the influence of a wheelie bin, the least you lot can do is include a hard copy of the bloody users manual!

author: Gyro
author_rank: Super Contributor
timestamp: May 10, 2017, 07:38:37 pm
content: It looks as if it's giving you what it says in very literal terms - higher resolution, not increased accuracy. Normally, meters in higher resolution mode manage to squeeze a little more displayed accuracy due to reduced display rounding but obviously not in this case.Accuracy is dependent on many factors, reference, range resistors etc. that have not changed. What it is giving you is the added resolution to follow smaller variations / trends, not to measure them to greater accuracy.

author: Robomeds
author_rank: Frequent Contributor
timestamp: May 11, 2017, 02:22:40 am
content: In some cases what is important is looking for a change in those last digits rather than the absolute accuracy. A slow drop in voltage might show in the last digit but not if you have 6k counts vs 20k.

author: X
author_rank: Regular Contributor
timestamp: May 11, 2017, 03:45:27 am
content: Using the last digit to track slow voltage changes makes sense (and this is my intended use case here), but can I be sure that this isn't just some noise or drift within the ADC, reference, or resistor networks?Is this accuracy specification more to do with how the meter comes out of the factory, or does it also mean that the reading can drift during use?

author: Gyro
author_rank: Super Contributor
timestamp: May 11, 2017, 11:54:48 am
content: Quote from: X on May 11, 2017, 03:45:27 amNo, you can't be sure but it's probably unlikely... as long as the battery is in good condition and the meter has in a stable temperature environment for a while. Of course if you are measuring higher currents, or higher voltages (10Minput resistance), you might see the effects of the current shunt or range resistors warming up.QuoteBoth - assuming that you mean 'during use' as the time since new. It will have been calibrated to well within the accuracy spec during manufacture but components will drift over time (particularly on a new unit). The accuracy spec should specify a time period, eg. 90 days or 1 year.You're actually pretty lucky having a 0.05% + 1 digit accuracy spec in a handheld with relatively few digits display. A lot of 'modern' bench meters (apart from the likes of Agilent/Keysight and Fluke) have pretty poor accuracy specs relative the number of digits that they have.

author: X
author_rank: Regular Contributor
timestamp: May 11, 2017, 12:42:09 pm
content: Quote from: Gyro on May 11, 2017, 11:54:48 amI hadn't thought much about the range resistors warming up as this particular case is up to 13.5-14V. Even with 240VAC, that's approx. 34-35uA going through I would imagine. I could imagine the current shunt warming up affecting my DC current reading starting at 8A, but as expected in my setup, it goes down exponentially, and I then switch the meter to 4.5-digit mode at around 30mA to watch the rate of change, but of course the current shunt would also be cooling down well before that.The batteries that are installed were supplied with the meter, which was manufactured in 2015 according to the box. Given that I won't be using the meter in a explosive zone or environment, I intend to swap the alkalines out with Energizer Lithiums (or Eneloops if their self-discharge is comparable) before the alkalines leak, as they often do.Quote from: Gyro on May 11, 2017, 11:54:48 amWhat I meant by "during use" was "during one interval of measuring, from the time the meter is hooked up to the time it is disconnected." For example, could a change in the digit simply be drift while I have the meter on, rather than an actual change in voltage, up until I turn it off again.As for the calibration guarantee period, the manual states that the accuracy "is specified for 2 years after calibration, at operating temperatures of 18 °C to 28 °C, with relative humidity at 0 % to 95 %."Quote from: Gyro on May 11, 2017, 11:54:48 amThis is quite surprising to hear about bench meters. You would think that given they are much bigger, they can fit more sophisticated instrumentation within and can achieve a higher degree of accuracy than a handheld. It is definitely like this for oscilloscopes.

author: Fungus
author_rank: Super Contributor
timestamp: May 11, 2017, 01:29:50 pm
content: Quote from: X on May 11, 2017, 12:42:09 pmYes.

author: X
author_rank: Regular Contributor
timestamp: May 11, 2017, 01:37:24 pm
content: woops, my bad. Edited the post to make that bit clear.

author: Fungus
author_rank: Super Contributor
timestamp: May 11, 2017, 02:03:37 pm
content: Quote from: X on May 11, 2017, 12:42:09 pmThe main advantage Bench 'scopes have is internal temperature control. Their voltage references are actually heated to a specified temperature during use. This makes them a lot more stable over time.Why do some have poor accuracy? I guess the extra case, power supply, larger display, etc., are expensive compared to a handheld. That means you have to spend more to get the same accuracy.