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Question I am in the process of writing a test procedure for table ESD mats. The mats are type T2. I understand that the dissipative range is 1e5 - 1e11. If I may I need to ask several questions: When I test for RTT is there a formula that will allow me to calculate in the effect of RH? Should the RTG resistance always be less than the RTT? If so then how do I calculate how much less it should be so the person testing the mat can verify that the RTG of the mat is within the acceptable range? Examples: If I read 1e008 for RTT then I should get approx. 6e007 for RTG? If I read 5e007 for RTT then I should get approx. 10e006 for RTG? If I read 8e007 for RTT then I should get approx. 5e007 for RTG? This is my general understanding that the RTG is approx 30% - 50% less than the RTT. Is this correct? Any help is much appreciated.- Anonymous, Winooski, VT
Answer 1) When I test for RTT is there a formula that will allow me to calculate in the effect of RH? ANSWER: Not that we know of. If Mat measures less than 1.0 x 109 then the RH will have a negligible effect. 2) Should the RTG resistance always be less than the RTT? ANSWER: If the mat has a conductive layer AND if the ground snap makes good contact with that conductive layer, then yes, RTG should always be less than RTT. Additionally, if you want your mat to always have a known, defined path to ground, then definitely, the RTG should be less than the RTT. 3) If so then, how do I calculate how much less it should be so the person testing the mat can verify that the RTG of the mat is within the acceptable range? ANSWER: Generally, the RTG would be about ½ the RTT when there is a conductive layer. It doesn’t have to be exactly half and to have a known, defined path to ground, it only has to be at least 10% less than the RTT. Using an approximate of 30% is fine. Often, there is a minimum value for the RTG which is greater than or equal to 1.0 X 106 ohms. Therefore, it would make sense for the acceptance criteria to be: RTT - Greater than or equal to 1.0 X 106 and less than or equal to 1.0 x 109 ohms. RTG - minimum of 10% less than RTT but equal to or greater than 1.0 X 106 ohms. 4) Examples: If I read 1e008 for RTT then I should get approx. 6e007 for RTG? Yes; If I read 5e007 for RTT then I should get approx. 10e006 for RTG? No, ~ 2.5 X 107; If I read 8e007 for RTT then I should get approx. 5e007 for RTG? Yes; This is my general understanding that the RTG is approx 30% - 50% less than the RTT. Is this correct?Yes.
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