On Fri, 2008-12-19 at 09:53, wayne burdick wrote: ... > In our tests of the ESD mats we found that if we walk across the room > and then touch the mat, we draw an arc. So, the mat must be conductive, > though very high in resistance.
No doubt Elecraft's industrial-grade ESD mats do meet specifications. That is, between 1 Mohm and 1 Gohm depending on relative humidity. But I was measuring 25-50 Gohms (25,000-50,000 Mohms) on the Radio Shack and Jameco mats. That implies it would take a 25-50 million volt static charge to get a 1 mA arc. ... > Meanwhile, please *do* purchase and use a mat. ... I continue to use mine if for no other reason than for the grounded wrist strap. On Fri, 2008-12-19 at 10:43, WILLIS COOKE wrote: > Al, the explanation could be that the sauce pans are not machine > flat, thus have a much smaller contact area than their measured > diameter would indicate. A machined flat surface with the mat > sitting on a very flat surface would probably yield much different > results. Trying to make laboratory accurate measurements with > jury rigged apparatus sometimes yields poor results. Yes I understand my measurement technique is not very accurate. But it would have to be off by at least 2,500 per cent (Radio Shack) or 5,000 per cent (Jameco) for those mats to be meeting industry-standard specifications. By the way, the Jameco mat in particular is quite squishy. I doubt the flatness of the saucepan bottom makes much difference. On Fri, 2008-12-19 at 10:47, Ron D'Eau Claire wrote: ... > The only uncertainty I can think of is whether your Simpson behaves like you > expect with very brief current surges. It's the old "move a car with a > sledge hammer" issue: inertia plays a huge role as does the action of the > spring in the meter movement. When you whack the meter with the surge, it > may not be behaving in a linear fashion as the surge varies. ... As a sanity check I lowered the power supply voltage to 7.5V and measured the current surge. Sure enough, it jumped just about half the amount as it did with 15V. Admittedly the measurement is not very accurate since I'm looking for a brief meter movement down below 10% of full scale on the meter. So it could easily be off 20-30 per cent. But there's no way it's off 2,500 per cent. > On Fri, 2008-12-19 at 10:59, Kok Chen wrote: > Alan, > > In case the pan is a bad way to measure (as someone pointed out), is > there any way that you can measure the two mats you have (one on top > of the other, facing one another) some way? > > You have two mats, which most of us don't and you may be able to do > some measurements that the rest of us cannot :-) :-). ... Good idea. I turned the Radio Shack mat upside down on the Jameco mat and put my trusty saucepan weighted with 8 pounds as close to the corner with the ground wire connections as possible. I connected the 0.1 uF capacitor between the two ground leads and charged it to 15V. After a half an hour, the meter jumped to 70-80%. So the resistance is comparable, or even higher, using this method than with the two saucepans side-by side (simulating the industry-standard test). Al N1AL _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net 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
