"Anti static" mats

I got a "static dissipative" mat and measured its resistance between two points. On the underside, which is black, it's just kilohms. But on the top surface it's too high to measure - I checked the DVM with a 50Mohm resistor and it measured 51M, so I trust the DVM. I asked the manufacturer how this insulating surface is meant to prevent static building up and damaging components... tellingly, no answer was forthcoming, "we'll, uh, get back to you".

I reckon it is possible that these mats work on the principle that static flows over surfaces, so it leaks round the edges to the conductive layer below. I can quite imagine this being worked out by some genius in the 1920s and no one remembers why it works any more: "we've always made them that way"; there seems to be an increasing number of technologies where the fundamental knowledge is lost. But here, the top surface does really seem to be basically a plastic insulator. I don't have an electrometer or other handy device to see if it holds charge, can anyone reassure me these mats really do work? Or a test to prove they do not, like levitating paper after rubbing the mat to build up a charge?

TIA

Reply to
Nemo
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They are quite well understood, if not by the person in charge of fielding phone calls.

The details are easy to find. As best I recall from when I bought mine and actually looked this stuff up (which I'm not going to repeat - you can do it yourself) there are problems with using the highly conductive side up due to too-quickly discharging things. The more-resistive side conducts through to the more conductive side at a moderate but adequate rate - the more conductive side keeps everything equipotential.

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Reply to
Ecnerwal

Or, the upper surface does indeed have a high resistance per square (more than you can measure conveniently) but is still low enough to allow static to dissipate over a period of a few seconds.

For a static-dissipative safety mat, you don't want a low resistance on the upper (contact) side, for a couple of reasons:

- Bringing a charged object into contact with a low-resistance surface would likely cause a rapid discharge to ground (i.e. a spark) which could damage sensitive devices. You want the static drained away to ground slowly.

- Human-safety hazard... if you're working on equipment which might possibly have AC line voltage or similar voltages present in its circuitry, it's a bad idea to have an efficiently-grounded "plate" where human contact is possible. Touch the circuit accidentally when it's powered up, touch the mat with your other hand... you really don't want a lethal amount of current flowing right through your torso. Static-grounding wrist straps have a high-value resistor in series with their ground cord, for this very reason, and the high resistance of a static-dissipative mat would serve the same purpose.

I doubt that one needs high conductivity to discharge incident static effectively enough.

Consider laundry "fabric softener". One of its attributes is to reduce static cling on clothing, and it can perform a similar function for plastic dials and panels if you dilute it in water and simply wipe some across the surface. Many "plastic cleaner" sprays have a similar anti-static property. If you were to treat a plastic insulator with one of these and then try to measure the resistance with a DMM, I expect you'd see a similar "off-scale" reading.

A low-tech approach to running such a test would be to build a simple electroscope out of household materials. There are plenty of sets of plans on-line - one guy's approach is at

formatting link
A more sensitive electroscope would use a lighter type of foil - gold leaf is the standard, and I've heard of people using foil-and-paper chewing gum wrappers.

Run a wire from the top of the elecroscope out sideways and suspend it from a piece of nylon fishing line. Charge up the electroscope and see how long it holds a charge (longer on low-humidity days). Charge it again, lower the end of the wire until its bare end contacts the static-dissipative mat (with its bottom side connected to a good ground) and see how rapidly the electroscope discharges this time. Try soldering a penny or brass washer onto the free end of the wire to increase the contact surface area, and see if that makes a difference.

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Dave Platt                                    AE6EO 
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Reply to
Dave Platt

Thanks. I'll have a go at the electrometer but the responses are already reassuring.

Nemo

Reply to
Nemo

Flip it over.

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Reply to
John Larkin

If you could measure surface resistance with a DVM it would be too conductive to use with a powered circuit. There is a special meter to test ASM. At one time I had the 'pleasure' of calibrating over one hundred soldering irons, and testing 80 ASM.

Reply to
Michael A. Terrell

I would use an old VTVM. any deflection can easily be seen. I don't remember trying this test. I just believed.

Greg

Reply to
gregz

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              AE6EO
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There's a 1 meg ohm resistor in the grounding strap that I put one when working on sensitive (expensive) stuff.

(go measure your's)

George H.

Reply to
George Herold

Just for the sake of comparison, I looked up the characteristics of a couple of brands of static-dissipative floor tiles, and floor mats.

The tiles were listed as having a "point-to-point and point-to-ground" resistance of anywhere from 10^6 to 19^9 ohms. The mats floor ranged from around 500*10^6 up to 8*10^9 ohms.

The floor tiles were rated as being able to discharge a human body (wearing ESD-conductive shoes) from 5000 volts, to ground potential, in

0.5 seconds or less.

The document didn't say, but I'd offer a guess that this "human body" test may involve something like the MIL-STD-883G "charged human body" ESD model (i.e. 100 pF of capacitance, in series with 1500 ohms).

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Dave Platt                                    AE6EO 
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Reply to
Dave Platt

That resistor is there to protect you from discharge shocks, or electrocution if you touch something with a lethal voltage

Reply to
Michael A. Terrell

It won't work. Look at the website I posted a link to. Look at the specified resistance. Also, be careful what you clean the mat with. The wrong cleaner will make the mat worthless. It is an expensive mistake. You need to clean the mat on a regular basis to keep it in good working condition.

Reply to
Michael A. Terrell

The cleaner is $122 for a spray bottle. What's so special?

I don't see any resistance specified on that site.

That site has mats that are $10/sq ft, but most are $100/sq ft with no specifics about the difference.

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Reply to
Tom Del Rosso

You can use some other cleaners. Most of that cost is the hazmat shipping crap. Read up on the care of mats.

Do you think you need the more expensive mat? The cheaper one is fine for 99% of applications. You want to protect the surface, though. Don't drag sharp items across it, don't solder on it, and don't cut it.

Reply to
Michael A. Terrell

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