[Elecraft] easy source of static dissipating work mat?

[Alan Bloom]

Beware that the Radio Shack 276-2370 portable ESD mat does not properly
bleed off static charges.  Appended below is a message I sent on this
subject back in February.

Al N1AL


On Thu, 2008-12-11 at 21:04, Dick Dievendorff wrote:
> Radio Shack.
> 
> Dick, K6KR
> 
> 
> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Gary Smith
> Sent: Thursday, December 11, 2008 9:01 PM
> To: elecraft at mailman.qth.net
> Subject: [Elecraft] easy source of static dissipating work mat?
> 
> My new K3 is on it's way. I have anti-static material that my 
> motherboard came in but that's not enough to work on & be static 
> free.
> 
> Where can I easily & quickly get an anti-static mat and grounded 
> wrist strap?
> 
> Thanks1
> 
> Gary
> KA1J

=================================================================

-----Forwarded Message----- 
> From: Alan Bloom <n1al at cds1.net>
> To: elecraft at mailman.qth.net
> Subject: [Elecraft] Radio Shack ESD mat doesn't meet spec
> Date: Wed, 27 Feb 2008 09:51:25 -0800
> 
> Well, I've convinced myself that the Radio Shack portable ESD mat, P/N
> 276-2370 doesn't work properly.
> 
> The "ESD Association" http://www.esda.org has promulgated an
> industry-standard test for ESD mats, ESD S4.1.  It is the standard
> specified by most commercial mats.  I decided not to spring for the $70
> to buy a copy of the standard, but other information I found on the web
> describes the test in general terms.  It uses two circular electrodes,
> each weighted with 5 pounds, spaced 10 inches apart on the mat.  The
> "Point to Point Resistance" is specified to be:
> 
> At 40-60% RH: 10^6 - 10^7 ohms
> At 20-40% RH: 10^7 - 10^8 ohms
> At 10-20% RH: 10^8 - 10^9 ohms
> 
> I don't know what the RH here in Santa Rosa was yesterday when I did the
> test, but I don't think it was very low since it has been raining
> recently and the ground is still damp.  For sure the resistance
> shouldn't be below 10^9 ohms (1 gigohm) and probably more like 10^8 or
> 10^7 (100 or 10 megohms).
> 
> I measured 2.5 x 10^10 ohms (25 gigohms), which puts the Radio Shack mat
> way out of spec.
> 
> Test procedure:
> 
> I didn't find a specification on the electrode size, but in the photo of
> a popular tester they look to be maybe 3 or 4 inches in diameter.  For
> my test, the electrodes were two saucepans, each about 7 inches in
> diameter and weighted with 5 pounds.  They were spaced 10 inches apart
> on the mat (3 inches edge-to-edge).  I connected a 0.1 uF film capacitor
> between the two pans and charged it to 15V with a power supply.  
> 
> I set my ancient Simpson analog volt-ohm meter to 60 uA full scale.  If
> I touch the leads across the capacitor immediately after charging, the
> needle momentarily jumps to about 6 uA (1/10 full scale) as the
> capacitor discharges through the meter.  If I wait half an hour (1800
> seconds) for the capacitor to partially discharge through the mat
> resistance, the needle jumps to about 3 uA.
> 
> An R-C network discharges to 3/6 of original voltage in about 0.7 time
> constant.  So the time constant must be 1800/0.7 = 2571 seconds.  That
> implies the mat resistance is 2571 sec / 0.1 uF ~= 2.5 x 10^10 ohms.  
> 
> Al N1AL
>