04 Feb

How to measure ohms of ESD Chair back rest

Q: Hi, I am a fresh ESD engineer. I need to perform a testing on ESD chair by using 5 pound Megohmmeter. I encountered a problem when I was trying to test on the backrest and rear of backrest. The backrest is a vertical plate, how to put a 5 pound electrode perpendicular to that surface? Even ANSI/ESD STM 12.1 2006 also not mentioned clearly about the testing on the backrest. Can I hold the 5 pound electrode by my hands so that it is contact with surface of the backrest? Or is there any other proper way?”

A: Hello,  I understand your frustration with some of these tests.

It sounds like you’ve got all the proper testing equipment and you’re following the best procedures for this.  The main concern is to see that the seat, seat back and arms are making good contact to the personnel, that personnel is wearing the proper clothing so as to enable the combination of personnel to chair to esd flooring system to electrical ground are enabling the tribocharge to recombine to earth.  If you have an ESD chair that includes a static conductive/dissipative seat, back, arms, casters, etc. we can make these resistance measurements to the chair’s groundable point, to a plate sitting underneath a caster, and ultimately, all the way across the floor to earth ground.

It could fairly easily be verified that the resistance from the seat back to the seat is less than 1.0E9 Ω or even better, around 1.0E6 Ω; but there is a requirement to measure a groundable point to spots 1, 2,3, 4, and 5 on the chair back as per ANSI/ESD STM12.1-2006 Seating-Resistance Measurement Figure 2 page 6.  This can be done with the chair laying down so the seat back is horizontal instead of vertical.  There’s also a provision to measure the rear of the back rest in three places 1, 2, and 3.

I consult the ESD TR20.20-Handbook and take into account the resistance of the personnel with proper clothing (this could require the personnel to wear an ESD smock, or in my case, I wear blue jeans and a cotton shirt which actually works well with ESD chairs!) through the ESD flooring system to earth ground with my feet/ esd shoes up and off of the floor; the 5 lb weight is in the palm of my hand and the other lead is in electrical ground.  I don’t see a specific standard value for it, but I would expect that combination resistance to be less than 1.0E9 Ω and a good typical value is usually less than 3.5E7 Ω, which would remind us of the requirements set forward in ANSI/ESD STM97.1-2006.

I hope this has given you some ideas and have a great day!

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