How are you measuring resistance?
Matt W8ESE On Wed, Apr 7, 2010 at 6:53 PM, Alan Bloom <a...@elecraft.com> wrote: > It is important to use an anti-static mat to prevent damage to > electronic devices when working on solid-state equipment. Since > Elecraft is heavily kit-oriented this is a topic of interest for > Elecrafters. > > Quite some time ago I did some testing on the Radio Shack portable > anti-static mat and found that its resistance was way too high to do a > proper job of bleeding off static charges. I then bought another > low-cost mat from Jameco and it also measured way too high. I began to > doubt my testing methodology (described in a previous message, copied > below). > > So now I have bought a third mat. This one is more expensive, but > unlike the cheaper mats it has an actual data sheet that specifies that > it meets ANSI specs for anti-static performance. I figured if it also > measured bad then my testing must be in error. > > But it didn't. Here are the results: > > Radio Shack P/N 276-2370 ($22.99) - 26 G ohms (26,000 M ohms) > Jameco P/N 10584 ($16.45) - 44 G ohms (44,000 M ohms) > Digi-Key P/N 16-1121-ND, (Desco 66164) ($38.18) - 41 M ohms > > The Desco mat's resistance is about 1000x lower than the cheaper ones! > > This mat is quite large, (2 x 3 feet, 61 x 91.4 cm) so if you buy one > you may need to cut it down to fit on your workbench. It comes with a > "common point ground kit" (that you have to install on the mat yourself) > that provides a long wire with solder lug to connect to ground and a > two-socket connector for connecting one or two wrist straps. The wrist > strap and cord have to be purchased separately. It looks like Digi-Key > P/N SCP172-ND for $8.95 should work. > > So the total cost is about $47. But the cheap mats are no bargain if > they don't work. > > I tried cleaning the surface of the mats with some wipes that are > especially intended for cleaning anti-static mats (Digi-Key MTT20-ND) > and they did help. The Radio Shack mat went from 26 to 6 Gohms and the > Jameco went from 44 to about 12 Gohms. But those numbers are still way > too high. The Desco mat's resistance also went down, from 41 to 20 > Mohms. > > The Desco mat came with a small spray bottle of "Rezstore", their own > brand of mat cleaner. > > By the way, the back side of the Desco mat is some kind of > highly-conductive black rubber, apparently to keep the entire mat > surface at equal potential. They warn that you should always use it > blue side up because the resistance of the back side is too low. Sure > enough, it was low enough to measure with my digital multimeter - about > 80 kohms. > > So my recommendation is to buy a mat such as the Desco model that has a > data sheet that specifies that it meets ANSI/ESD S4.1 or ANSI/ESD > S20.20. If you already have the Radio Shack mat, then either replace it > or at least clean it with a cleaner approved for ESD mats. (You're not > supposed to use soap or detergent because it might harm the anti-static > properties.) Gary KI4GGX recommends a product made by Techspray: > http://www.all-spec.com/products/1733-QT.html > > Whatever you use, it is important to keep the mat clean. > > A word about my test procedure. Accuracy is not very good because I am > trying to read the peak amplitude of a brief needle flicker. If I had > an analog meter with a high-impedance input that would work better > because the reading wouldn't change so fast. Also my sauce pans no > doubt do not give the same answer as the ANSI-specified test probes. So > overall, I doubt my measurement accuracy is better than +/- 50% or so. > But for the purposes of this test that's good enough. > > In the test procedure below I had to substitute a 1.0 uF capacitor when > measuring the Tesco mat in order to get a long-enough time constant to > measure accurately. (Also, it's actually a Triplett meter, not a > Simpson.) > > Alan N1AL > > > > On Wed, 2008-02-27 at 09:51 -0800, Alan Bloom wrote: >> 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 > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:Elecraft@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
