Q: Why does humidity change anti-static zipper test results?
Most anti-static behaviour depends on a thin film of moisture on the surface, so resistance falls sharply as relative humidity rises. A tape that reads 10 to the 11 ohms at 30 percent relative humidity can read 10 to the 9 at 65 percent. Testing without controlling humidity produces numbers that look good in summer and fail in a dry winter warehouse.
Condition the specimen in the standard atmosphere for the time the method requires, and measure in that same atmosphere. Record the temperature and relative humidity with every result. For products used in dry or cold conditions, test at the low-humidity condition the customer actually faces, not only at the standard laboratory condition. Electrode contact and electrification time also affect the reading, so refer to the standard text.
Acceptance should be defined at a stated humidity, with a rule for the lowest humidity at which the product must still perform. If a supplier reports a resistance without stating humidity, treat the number as indicative only and ask for a retest under a controlled atmosphere.