The ± ZERO Electret is designed to "zero" your SPER-1 or 1E (Surface Potential Electret Reader). The stainless steel disc in the centre of the electret has no surface potential; when used to check your reader, the voltage displayed should be within ± 3 volts of 0.
The ± ZERO Electret is an integral part of your E-PERM QA/QC program and helps ensure the accuracy of your radon measurements.