You're partly right, Don, but only because a real mat always has some
thickness and isn't always very large compared to the probe spacing.
But even in that case, the difference in measurements isn't very
large, and has nothing to do with any kind of intermediate conducting
layer in the mat.
I know it seems counterintuitive, but it's well known that the
so-called "square resistance" of large, thin flat conductors is
independent of the size of the square. In other words, if you cut two
square pieces of a thin conductor, one a centimeter square, and the
other a meter square, and then you attach electrodes to the opposing
sides of each square and measure their resistances, you'll get the
same value for each. In physics, we say that the resistance of two
dimensional conductors is "scale invariant." Given that fact, it's
very easy to generalize the results to point contact probes, as one
would use with a fluke DMM. BTW, this isn't something I've dreamed up.
It's a well-known result in science and engineering.
73,
Jim w8zr
Sent from my iPhone
> On Apr 12, 2017, at 6:40 AM, "donroden@hiwaay.net" <donroden@hiwaay.net>
> wrote:
>
> Unless you have a perfect conductor under or part of your conductive mat,
> there will be differences as the mat is probed with a Fluke.
> Don W4DNR
>
> Quoting Al Kozakiewicz <akozak@hourglass.com>:
>
>> In Jim's post there were five assertions of fact, one opinion, one typo and
>> a polite closing. Was Don disagreeing with some or all of them?
>>
>> Al
>> AB2ZY
>> ________________________________________
>> From: Amps <amps-bounces@contesting.com> on behalf of Doug Ronald
>> <doug@dougronald.com>
>> Sent: Tuesday, April 11, 2017 4:04 PM
>> To: amps@contesting.com
>> Subject: Re: [Amps] Tubes, transistors, and 'abuse'
>>
>> I just tested the anti-static mat in front of me with an ohmmeter, and was
>> amazed to see the resistance was not linear with distance. The mat was on an
>> insulating surface, and with the probes as close as possible without
>> touching, I got 42 kilo ohm. At the opposite ends of the mat I got 56 kilo
>> ohm. The mat is about 5 mm thick, and seems to be all the same uniform
>> material. The backside behaved the same way. There may be some inner layer
>> that is of much greater conductance - can't tell...
>>
>> -Doug W6DSR
>>
>> -----Original Message-----
>> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of
>> donroden@hiwaay.net
>> Sent: Tuesday, April 11, 2017 12:49 PM
>> To: amps@contesting.com
>> Subject: Re: [Amps] Tubes, transistors, and 'abuse'
>>
>> Disagree.
>> Don W4DNR
>>
>>
>> Quoting MU 4CX250B <4cx250b@miamioh.edu>:
>>
>>> Speaking of high resistance mats, an interesting property is that the
>>> resistance between any two points on the map is the same, no matter
>>> the distance between the points. In other words, it doesn't matter
>>> whether you put your test probes a cm apart or 10cm apart, the
>>> resistance will be the same. That's why the resistance of a flat mat
>>> is always specified in ohms, unlike three-dimensional materials whose
>>> resistivity is specified in ohm-cm. In two dimensions, resistance and
>>> resistivity are the same thing.
>>> 73,
>>> Jim w8zr
>>>
>>> Sent from my iPhone
>>>
>>>> On Apr 11, 2017, at 12:42 PM, MU 4CX250B <4cx250b@miamioh.edu> wrote:
>>>>
>>>> Ah, Wise move on your part, Manfred. I wouldn't wear it either! Your
>>>> former boss needed higher level Technical Support!
>>>> Jim
>>>>
>>>> Sent from my iPhone
>>>>
>>>>> On Apr 11, 2017, at 12:39 PM, Manfred Mornhinweg <manfred@ludens.cl>
>> wrote:
>>>>>
>>>>> Jim,
>>>>>
>>>>>> Manfred, I think you are worrying needlessly. A grounding wrist
>>>>>> strap connects to the mat, not to the device under test. ESD mats
>>>>>> typically have a resistance in the 10E7-10E8 ohm range. The mat on
>>>>>> my workbench has a resistance too high to measure with my Fluke
>>>>>> 87-V. The mats discharge static buildup, but neither they nor the
>>>>>> wrist strap pose any safety hazard.
>>>>>
>>>>> Fine then. But the straps that one boss at the job wanted me to wear
>>>>> were all metal. Indeed they connected to the mat - but to a metal
>>>>> frame surrounding the static dissipative (highly resistive)
>>>>> material, and that frame was grounded. In the end, that wrist strap
>>>>> was grounded with a very low resistance, and I refused to wear that,
>>>>> for safety reasons.
>>>>>
>>>>>> I have lost MOSFETS from not taking adequate ESD measures. Some of
>>>>>> the older devices, especially, are very easily burned out.
>>>>>
>>>>> There are some that don't have the built-in zener protection - those
>>>>> are indeed fragile. Laser diodes (or rather their built-in
>>>>> photodiodes, I think) are also said to be very sensitive to static.
>>>>> I have handled such devices with no more precautions than the basic
>>>>> ones, and never lost any.
>>>>>
>>>>>> There's a reason all semiconductor distributers (Mouser, Digikey,
>>>>>> etc.) pack their components in ESD envelopes!
>>>>>
>>>>> Yes, and that's actually a good thing to do, and I do it too, when I
>>>>> ship something sensitive. My fundamental point instead is that
>>>>> thoughtlessly used grounding straps and the like can CAUSE more risk
>>>>> to the parts than they help prevent! I have seen people who put on
>>>>> such a grounding strap, next to their static-safe workbench, and
>>>>> then think that nothing bad can happen. Then they reach over to a
>>>>> drawer and withdraw a MOSFET by the gate terminal, and !ZAP!, they
>>>>> discharge the entire drawer through that MOSFET!
>>>>> My practice instead is to first get hold of the drawer, to put
>>>>> myself at its potential, then pick up the MOSFET by anything but its
>>>>> gate terminal, then walk over to my desk, touch the desk, then place
>>>>> the MOSFET on it. In doing so, I have already double safety in it:
>>>>> By avoiding to touch the gate first, and by equalizing the potential
>>>>> between myself, the desk, the MOSFET, and anything else, in a safe
>>>>> way.
>>>>>
>>>>> Most of this caution exceeds what's needed, but as you say, it's
>>>>> smart to be careful. And I would add that it's good to be smart!
>>>>> In the sense of thinking where static charges will form, what can be
>>>>> charged relative to what, which items could carry significant
>>>>> leakage current, and so on, and then acting accordingly. That's much
>>>>> safer than using a mat, a strap, and stopping to think about the
>>>>> matter, which is what I have witnessed some people doing!
>>>>>
>>>>> Manfred
>>>>>
>>>>> ========================
>>>>> Visit my hobby homepage!
>>>>> http://ludens.cl
>>>>> ========================
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>>
>>
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>
>
> DonR
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