In 8.3.1 of S20.20-2021, there are two options for process qualification and ongoing compliance verification. I have a question regarding option 1 (area where ESDS items are handled must measure < 125 volts/inch).
Static field meters typically have a stipulated measurement distance of 1”. As I understand, for such a meter, the voltage reading on the display can be taken as the measurement value.
What should be done in case the stipulated measurement distance is not 1”? We have meter that has two different measurement ranges, each with a different stipulated distance. -5000V to +5000V at 1/2", and -30kV to +30kV at 4".
As I understand the distance to volt relationship is not linear, so I’m wondering if there’s a conversion factor we can use for our meter.
Hafridi,
I will try to answer this question but I am not completely sure since I have no experience with field meters with a stipulated distance other than 1 inch. If someone else with experience wants to chime in, please do so.
The field meter is reading in volts but the measurement it makes is the electric field which is in volts/distance (inch or meter). The basic formula is V = E * d, where E is the electric field and d is the distance from the field. In the case of the meter that has a stipulated distance of 4 inches, the voltage the meter is reporting is E (the electric field) times the 4 inches. To get the electric field (E = V/d), you would take the voltage reading and divide by the 4 inches. Similarly for the 1/2 inch stipulated distance field meter you would divide the voltage reading by 1/2 (or multiply by 2) to get the electric field
This topic has been depated quite a bit in committee. The general rule is that when measuring insulators directly, the reading at the stipluated distance is not a field but a potential. This is simple math. For example, if there is an insulator that you are concerned about and want to measure the potential, then a meter that measure volts/in held 1 in from the insulator and the reading is 100 volts/in, the potential would be 100 volts on the insulator. Notice that is what is in the second way of insulators or the only way in the 2014 version. The interesting part here is that even though electic fields will change per the square of the distance, voltage or potential changes lineraly with distance.
Now if you are measuring the field there is no stipulated distace. In this case, the field that the meter reads in volts/distance would be the requirement. In this case the stipulated distance is not a factor. In this case of your meter, 5000 would be the requirement no matter which scale was choosen.
Thank you for the replies, Andy and John.
John, I am a little confused about your 2nd paragraph. First, I assume the value 5000 you stated refers to 5000V/m (or 125V/in). If so, then wouldn’t the stipulated distance still matter? If the limit for a field measurement is to be <125V/in, a reading of <125 in my view would be acceptable for a meter with a stipulated distance of 1". But, wouldn’t a meter with a stipulated distance of 1/2" (or 4") have to adjust this limit since that meter would be reading V/0.5in (or V/4in)?
Just bumping this thread to check if there are thoughts to my previous post from the experts.