EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: do2 on December 10, 2018, 09:35:16 pm
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I've got myself a Fluke 87V and I am a happy camper ^-^
But there is one thing that's puzzling me: According to the specs, a switch into HiRes-mode also reduces accuracy, namely the number of least significant digits increases by 10; see e.g. https://www.alliedelec.com/m/d/8cf21be95af06ae474858dd790972ba3.pdf (https://www.alliedelec.com/m/d/8cf21be95af06ae474858dd790972ba3.pdf)
So, for instance if I measure DC Volts, instead of 1 least significant digit in standard mode I will have 10 in HiRes-mode.
Where is the benefit, if the decimal point is shifted one count to the left side when you loose one digit of accuracy on the right at the same time?
Do I oversee something here?
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That's a nice DMM that will give you many years of flawless service!
In HiRes mode the accuracy (https://en.wikipedia.org/wiki/Accuracy_and_precision) of the instrument remain the same, so your measurements will have more or less the same error with respect to the true value.
What one gains is increased precision, in the sense that having one more digit will make it easier to see trends and small variations, as the instrument has a finer resolution, and being a quality one, gives very repeatable measurements.
E.g., my power supply, when set to 1.5V nominal, measures 1.499V in normal mode and 1.4991 in HiRes mode.
If I slightly heat the DAC I can see a positive temperature coefficient in HiRes, reading 1.4992, 1.4993 etc., that I would otherwise miss.
BTW, these values exactly match the ones on my 50000 counts 121GW DMM (same 0.05% accuracy, + 5 counts).
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It looks like a similar scenario as Brymen's BM857 and its high res mode. Check the related discussion at:
https://www.eevblog.com/forum/blog/eevblog-432-brymen-multimeters/msg197499/#msg197499 (https://www.eevblog.com/forum/blog/eevblog-432-brymen-multimeters/msg197499/#msg197499)
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FWIW, here's mine compared with my HP 3455A.
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Where is the benefit, if the decimal point is shifted one count to the left side when you loose one digit of accuracy on the right at the same time?
When I was troubleshooting my car battery, it was helpful to put the 87 into 20,000 count mode so I could see if my battery was slowly losing or gaining charge.
I wasn't worried about accuracy, just which way it was going without having to stand in -30C weather for several minutes in low resolution mode.
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The accuracy you're reading is the accuracy guaranteed by Fluke. If the meter's outside that range you need to get it fixed.
Those meters usually will do better than that though, the extra digit isn't wasted when you switch to hi-res.
Plus you can watch slow changes.
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Thank you to newbrain and the others for your informative replies!
When cold starting the Fluke, I can see the last digit sometimes changing slightly. But this is just during the first few minutes. After that, each measurement shows the same value; that is, if the source is constant, of course ;)