Re: [Elecraft] ESD mat measurements
Alan, Bloom's Law: I like it. Doug out my mat and, no, couldn't measure it with my DVM. It so happens that I do have a nice L/C meter (Almost All Digital Electronics model L/C Meter IIB) and I did discover I can measure capacitance: About 1 pf. Now, while thats not telling me the resistance, at least it's telling me that *something* is happening! If I have time, the sauce pans will come out... Where can I find a nice juicy capacitor... Regards, kurtt Kurt Pawlikowski, AKA WB9FMC The Pinrod Corporation ku...@pinrod.com (773) 284-9500 http://pinrod.com Alan Bloom wrote: A few months ago I reported here on some measurements I made on the Radio Shack portable ESD mat (see below). Basically I found that it doesn't bleed off static charges as it is supposed to. (i.e. It fails the ESD Association resistance specification for ESD mats.) So I recently bought another ESD mat from Jameco. It's their 19.5x23.5-inch anti-static mat, P/N 10584, $16.45. Tonight I measured it using the same technique that I used with the Radio Shack mat. It's even worse! After a half hour, the capacitor was still charged to 75% or so versus 50% with the RS mat. So what's going on here? I can think of three explanations. (1) There's something wrong with my measurement technique. (2) Perhaps you're supposed to treat the mat with some kind of conductive material before use. (3) Cheap anti-static mats are a fraud and are worthless for their intended purpose. I can't figure out how it could be (1). As a sanity check I confirmed that the sauce pans I was using as probes are conductive and the capacitor is indeed 0.1 uF. I doubt it is (2) - I can't believe that the mats are supposed to be untreated as they come from the factory. I suspect (3). Years when when I was a components engineer at Hewlett Packard, I was measuring some of those rubber heat sink insulators and found they didn't even come close to meeting their thermal resistance spec. So I came up with Bloom's Law: The harder a specification is to measure, the more likely it is to be a lie. If you buy a 1k, 10% resistor it will almost certainly be within spec since anyone with a DVM can easily measure it. Measuring surface resistivity by the ESD Association method requires an uncommon, expensive piece of test equipment that people who buy $16 ESD pads are unlikely to have. So it's easy for the manufacturer to cut corners without getting caught. Al N1AL On Wed, 2008-02-27 at 09:51, 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 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
[Elecraft] ESD mat measurements
You may get better - different - results if you use an oscilloscope to measure this voltage peak. The Simpson has ?dampening? qualities and will NOT show the true peak voltage/current. Also, have you taken into account any internal resistance of the Simpson? As I recall that was a 20,000 ohms per volt (a 50 uA movement) meter - Great at the time, but worthless today for anything other than house/auto wiring - which also implies that there is a shunt dropping the meter movement down from 50 to 60 ( which model has a range of 60??? I have seen ones with 50 and 100 uA but not 60) and thus there is resistance added to your test circuit. These small errors are acting like a furculum and greatly skewing your results. Rich, KE0X ___ 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
Re: [Elecraft] ESD mat measurements
On Fri, 2008-12-19 at 08:18, Rich Lentz (KE0X) wrote: You may get better - different - results if you use an oscilloscope to measure this voltage peak. The Simpson has ?dampening? qualities and will NOT show the true peak voltage/current. It shouldn't matter. I was not trying to measure the voltage, rather the RATIO of the voltage before and after the capacitor discharge. As long as the dynamic response of the meter is independent of voltage level, the measurement should be accurate. Also, have you taken into account any internal resistance of the Simpson? As I recall that was a 20,000 ohms per volt (a 50 uA movement) meter - Great at the time, but worthless today for anything other than house/auto wiring - which also implies that there is a shunt dropping the meter movement down from 50 to 60 ( which model has a range of 60??? I have seen ones with 50 and 100 uA but not 60) and thus there is resistance added to your test circuit. These small errors are acting like a furculum and greatly skewing your results. As I said, I don't think it matters. Besides, even if the measurement were off by a factor of two it wouldn't affect the qualitative result. The measured resistance was several orders of magnitude too high. Rich, KE0X 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
RE: [Elecraft] ESD mat measurements
OK, so what's the bottom line? Is the Radio Shack ESD mat any good or should I spring for a 'quality' mat, and if it's the latter, how do I know which mat(s) meet specs? Nelson, KU0A -Original Message- From: elecraft-boun...@mailman.qth.net [mailto:elecraft-boun...@mailman.qth.net] On Behalf Of Alan Bloom Sent: Friday, December 19, 2008 10:49 AM To: elecraft@mailman.qth.net Subject: Re: [Elecraft] ESD mat measurements On Fri, 2008-12-19 at 08:18, Rich Lentz (KE0X) wrote: You may get better - different - results if you use an oscilloscope to measure this voltage peak. The Simpson has ?dampening? qualities and will NOT show the true peak voltage/current. It shouldn't matter. I was not trying to measure the voltage, rather the RATIO of the voltage before and after the capacitor discharge. As long as the dynamic response of the meter is independent of voltage level, the measurement should be accurate. Also, have you taken into account any internal resistance of the Simpson? As I recall that was a 20,000 ohms per volt (a 50 uA movement) meter - Great at the time, but worthless today for anything other than house/auto wiring - which also implies that there is a shunt dropping the meter movement down from 50 to 60 ( which model has a range of 60??? I have seen ones with 50 and 100 uA but not 60) and thus there is resistance added to your test circuit. These small errors are acting like a furculum and greatly skewing your results. As I said, I don't think it matters. Besides, even if the measurement were off by a factor of two it wouldn't affect the qualitative result. The measured resistance was several orders of magnitude too high. Rich, KE0X 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 No virus found in this incoming message. Checked by AVG - http://www.avg.com Version: 8.0.176 / Virus Database: 270.9.19/1857 - Release Date: 12/19/2008 10:09 AM ___ 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
Re: [Elecraft] ESD mat measurements
Here's some more information for the latest swirling controversy :-) If you try to measure the resistance of an anti-static mat, you'll need an ohmmeter good to at least 1,000 megohms. Even my HP/Agilent 34401A 6 digit benchtop DMM can't measure that high! Why is the resistance so high if it is supposed to dissipate static? The answer is here URL:http://www.3m.com/us/office/meetings/rg/pdfs/w4%20choosing%20the%20right%20mat.pdf If you are convinced you need to measure yours, here is a place that sells test equipment for doing just that URL:http://www.botron.com/ESD_product16 73, Lyle kK7PO PS _ I have no financial interest in any of the above companies. This is informational only. Use the information at your own risk. The websites linked may have all sorts of malicious stuff embedded in them. Etc. ___ 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
Re: [Elecraft] ESD mat measurements
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. Willis 'Cookie' Cooke K5EWJ ___ 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
