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ESD chairs

Updated: Dec 28, 2021

Testing ESD Chairs per ANSI/ESD STM12.1-2013

I am trying to ensure that my ESD chairs are working per the ANSI/ESD STM12.1- 2013 standard

The required test conditions:

a) According to the standard, Page 3, we are to clean the casters and groundable points with a mixture of 70% alcohol and 30% water.

b) The test is to be in conducted in low humidity 12 +/- 3% RH and a preconditioning of 48 hours.

c) We are also to have a planar surface with a surface resistivity of greater than 1.0 X 1012 ohms/square

Here are my Questions:

Q1) We paid thousands of dollars for a humidity system to keep the RH up to 40%- 45% during the winter months. Now I need to find a place to have each chair (quantity of about 85) sit for 48 hours prior to the test? How do other companies comply to this that may have hundreds of chairs?

Q2) Should the chair read as a dead short, i.e. 10^5 or 10^6?

Q3) Does the chair need to have ESD wheels or the drag chain or both?

Q4) If a person sitting in an ESD chair, holding the resistance bar in their hand connected to the meter and the other connection connected to the weight on the ESD floor is tested; should they read a direct short like 105 or a dissipative reading like 10^7 -10^8 ?

Q5) Is it possible to sit in an ESD chair that has passed the prescribed test and have low readings such as 105 or 106 and NOT wear a wrist strap?

A. Response

Purpose: ANSI/STM12.1-2013, is a test method for measuring the electrical resistance of seating used for controlling electrostatic charge. The purpose of the STM is to provide measurement methods for the qualification of seating prior to installation or application.

Sample Size: The required specimen (sample) size required for qualification of ESD Seating, shall be (3) three specimens. Each specimen shall be configured in the manner in which it will be used in the intended application.

Product Qualification: Product qualification results for ESD control items are collected by the end user via one of three methods;

1) Manufacturer’s product specification review (e.g., technical data sheet). A statement by the manufacturer that their product has been tested either internally or externally per the conditions of STM12.1; meeting a given resistance range or limit(s).

2) Independent laboratory evaluation (e.g., third party evaluation). Most commonly either the manufacturer or the end user commissions an independent party to evaluate the ESD control item per the required standard. Data collected is provided to the requesting party. Manufacturers typically will use this data for publication in their technical data sheets.

3) Internal laboratory evaluations (e.g., end user evaluation). The least common of all product qualification options. Here the end user evaluates the ESD control item per the required standard. For many ESD control items this requires the use of controlled environmental conditions, typically 12% RH and 50% RH at 23°C.

For situations where the controlled environmental conditions cannot be achieved, (i.e., 12% RH, 23°C), a manufacturer, independent laboratory or internal laboratory may still evaluate seating per STM12.1. The alternative environmental test conditions shall be collected and reported as part of the modified qualification data. The end user is responsible for evaluating the use of the alternative environmental conditions to ensure their minimum humidity conditions are represented.

As a practical use, the end user should perform qualification testing at the worst case environmental conditions that they experience in their factory location(s).

For your specific situation. If your humidity control levels maintain a minimum of 40% RH, it is acceptable to qualify your chairs at this worst case RH level.

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