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l4m2



Joined: 15 Jan 2015
Posts: 648
l4m2
it says 20Mohm if I directly link the two pins


Last edited by l4m2 on 08 May 2015, 12:07; edited 1 time in total
Post 25 Feb 2015, 14:02
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
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revolution
An 8 digit display? Which international standard is it calibrated to? Oh, wait, 20MΩ for shorted terminals ... hmm, I see. Neutral
Post 25 Feb 2015, 15:08
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AsmGuru62



Joined: 28 Jan 2004
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AsmGuru62
It should say zero ohms.
Smile
Post 25 Feb 2015, 15:57
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revolution
When all else fails, read the source


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revolution
No, it should say 0.0000000Ω Razz

Although you'd need some very good probe wires made of superconductors.
Post 25 Feb 2015, 16:13
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gens



Joined: 18 Feb 2013
Posts: 161
gens
my body has some 5MOhm Smile

the only time i got a 0.0Ohm was when a friend brought some expensive space plastic case
putting the probes on it gave 0
the instrument never showed 0 until that day, it was surprising

@revolution
i think they compensate for the probes when calibrating the instrument, no ?
Post 25 Feb 2015, 16:48
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revolution
When all else fails, read the source


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revolution
gens wrote:
i think they compensate for the probes when calibrating the instrument, no ?
You are probably correct. I've never owned an 8 digit meter so I wouldn't know what happens. You might also need temperature compensation or a temperature controlled oven to get that many digits. Question
Post 25 Feb 2015, 16:51
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gens



Joined: 18 Feb 2013
Posts: 161
gens
wait, 8 digits ?
never seen one like that, and i sold them

PS all newer multimeters are fairly precise, even the small cheap ones
Post 25 Feb 2015, 17:01
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l4m2



Joined: 15 Jan 2015
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l4m2
number mode can't put my easily so a long one appears
Post 25 Feb 2015, 18:34
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Matrix



Joined: 04 Sep 2004
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Matrix
revolution wrote:
No, it should say 0.0000000Ω Razz

Although you'd need some very good probe wires made of superconductors.


in real life, technician just hits the zero button that trims offset of measurement value
Post 25 Feb 2015, 20:23
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
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revolution
Matrix wrote:
revolution wrote:
No, it should say 0.0000000Ω Razz

Although you'd need some very good probe wires made of superconductors.


in real life, technician just hits the zero button that trims offset of measurement value
So you've never used a high precision instrument then. If only life was always so easy as that.
Post 26 Feb 2015, 01:13
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JohnFound



Joined: 16 Jun 2003
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JohnFound
In order to measure resistance with 8 digits of accuracy (not resolution) one will need 4-wire probes (simple copper, not super conductive).
Post 26 Feb 2015, 05:45
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Matrix



Joined: 04 Sep 2004
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Matrix
revolution wrote:
Matrix wrote:
revolution wrote:
No, it should say 0.0000000Ω Razz

Although you'd need some very good probe wires made of superconductors.


in real life, technician just hits the zero button that trims offset of measurement value
So you've never used a high precision instrument then. If only life was always so easy as that.


you're right, i just design tthem and make them, i never use them
Post 26 Feb 2015, 08:00
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revolution
When all else fails, read the source


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revolution
Cool. So do 8 digit ohmmeters need temperature compensation?
Post 26 Feb 2015, 08:05
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Matrix



Joined: 04 Sep 2004
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Matrix
revolution wrote:
Cool. So do 8 digit ohmmeters need temperature compensation?

no

but copper is about as good temperature sensor as platinum
Post 26 Feb 2015, 12:29
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revolution
When all else fails, read the source


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revolution
So when the meter applies current to the DUT how does it account for the resistance change due to temperature rise (or fall for NTC devices) caused by the current? Wouldn't that make the last digit or two kind of meaningless?
Post 26 Feb 2015, 12:37
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Matrix



Joined: 04 Sep 2004
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Matrix
revolution wrote:
So when the meter applies current to the DUT how does it account for the resistance change due to temperature rise (or fall for NTC devices) caused by the current? Wouldn't that make the last digit or two kind of meaningless?


you just round off the 0.5% that will be caused by self heating (use low current, 10-47k NTC ?)
also, you do calibration at your desired temperature range to get your 1% precision with your NTC temperature sensor.
Post 02 Mar 2015, 10:14
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revolution
When all else fails, read the source


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revolution
8 digits is more than 0.5% accuracy.
Post 02 Mar 2015, 10:34
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Matrix



Joined: 04 Sep 2004
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Matrix
revolution wrote:
8 digits is more than 0.5% accuracy.

ok well i assumed reality, i never had a need for an 8 digit precision temperature sensor, 0.5C precision is good (and maybe 0.1C resolution), 0.05C precision is excellent in real world
( note: precision != resolution )
Post 02 Mar 2015, 10:42
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revolution
When all else fails, read the source


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revolution
Oh, well not specifically a TS, but anything that measures to 8 digits I would assume needed to be mindful of temperature variations. So do you make 8 digits ohmmeters?
Post 02 Mar 2015, 10:47
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Matrix



Joined: 04 Sep 2004
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Matrix
This is how it works:
if you use a microcomputer, and you process data digitally, you first need to convert analog voltage into digital information, using an ADC, this will limit the precision.
(Resolution can be much greater than precision)
read some technical information and you can do the math you wish
state of the art 24 bit ADC with 4uV RMS noise

In software, one can display many useless digits, any amount.
Post 02 Mar 2015, 16:27
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