[Elecraft] Radio Shack ESD mat doesn't meet spec

[Alan Bloom]

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