From: garland@ohstpy.mps.ohio-state.edu
Newsgroups: sci.electronics
Subject: Re: Those Anti-Static Mats
Date: 11 Jan 93 16:51:26 EST
Organization: The Ohio State University, Department of Physics

In article <1993Jan11.181155.10735@phx.mcd.mot.com>, schuch@phx.mcd.mot.com (John Schuch) writes:
> In article <C0nyI5.MG2.2@cs.cmu.edu> rwc@ius4.ius.cs.cmu.edu (Randy Casciola) writes:
>>
>>I just bought one of those cheap anti-static mats at a computer
>>......
>>Well, when I held
>>the leads against the mat and pressed firmly at a distance of 
>>about 1 cm, the resistance was greater than the highest range on
>>the DVM ( >20M ohms).  
> 
> We have a special tester to determine the conductivity of the static
> mats around our plant. What you need to remember is that the mats are
> "static dissipative", not conductive. Dissipative is defined as being
> between 10E5 and 10E11 ohms per square inch. Most of the mats around
> here are about 10E7, or 100 Mohms per sq inch. The tester has two 
> conductive bars which are placed on the mat. 100 volts is applied to
> one and the current between them is measured to determine resistance.
> Remember, the charge your trying to get rid of is thousands of volts,
> not tens of volts.
> 
>>And the resistance from one end of the
>>ground cord to the other was about 1M ohm.  I assume they have
>>a 1M ohm resistor in series somewhere.
> 
> Yes, there is usually a 1 meg resistor in the ground lead. That's so
> there's no high-current ground path, which could be a real 'pain' if
> you are working a high voltages and get across the wrong wire.
> 
>>I measured the 
>>resistance of another more expensive mat (the type designed to
>>be placed on the floor for you to walk on) and at 25 cm of 
>>distance between the leads I measured about 20k ohms.
> 
> Floor mats are supposed to be at ground potential so they are
> CONDUCTIVE, not dissipative. They are usually hard grounded,
> without the 1 Mohm current limiting resistor. They should be
> used with 'shoe straps'. These basically ground your ankel to 
> the floor. The strap has the 1 Mohm resistor and has to be tested
> often. Around here, you are supposed to test you straps every
> time you enter the production area.
> 
> 
> 
> John
> 
> "ESD dosen't really exist. It was invented by the 3M marketing
>  department as a way to move conductive plastics." :-)
> 
> 
> -- 
> +--------------------------------------------------------------------------+
> | John R. Schuch  -  Motorola Computer Group  -  Manufacturing Engineering |
> | N7XVS - schuch@phx.mcd.mot.com - (602) 438-3008 - CompuServe: 70733.3330 |
> +--------------------------------------------------------------------------+

Interestingly, if one measures the resistance of a large (i.e., floor type)
antistatic mat with an ohmmeter, the value of the resistance is independent of
the spacing of the ohmmeter leads (so long as the spacing is small compared to
the dimensions of the mat). The general result is that conductivity and 
conductance (alternately, resistivity and resistance) are the same in two 
dimensions; it is approximately true for a conductor whose thickness is
negligible compared to its transverse dimensions.  Just thought you'd want to
know.

Jim Garland
Newsgroups: sci.electronics