"Anti static" mats

[Nemo]

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

 

[Ecnerwal]

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".

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.

 

[Nemo]

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

Nemo

 

[gregz]

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

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

Greg

 

[George Herold]

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

   

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.

--
Dave Platt <[email protected]>                                   AE6EO
Friends of Jade Warrior home page:  http://www.radagast.org/jade-warrior
  I do _not_ wish to receive unsolicited commercial email, and I will
     boycott any company which has the gall to send me such ads!

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.

 

[Tom Del Rosso]

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.

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.