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 _______________________________________________ Elecraft mailing list Post to: [email protected] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
