Charge Decay Measurements

A common misconception is that surface resistivity measurements alone provide accurate information of the charge decay properties of a material. Even though for some materials there seems to be a correlation between the rate of charge dissipation and its surface resistance properties, there are too many exceptions and numerous uncontrollable factors for them to be related theoretically.

Therefore, measurements of the surface resistance alone cannot truly identify a material as being static dissipative, charge decay measurements must also be taken. The EMPL has a unique instrument capable of not only depositing a fixed amount of charge to a surface repeatably, its simultaneously measures the rate of charge decay.

JCI 155v4 Charge Decay Test Unit

The JCI 155v4 Charge Decay Test Unit uses a high voltage corona discharge (+ or - polarity) to deposit charge on the surface of the material to be tested. Once the charge is deposited, a fast response electrostatic fieldmeter is exposed and observes the voltage generated by this charge. As the charge mitigates away, the voltage decay is recorded providing a direct measurement of the static dissipation properties. Charge decay curves are classified as the charge remaining per unit time Q(t) by the decay constant t given by the equation Q(t) = Qo exp(-t/t) if Ohm's Law is obeyed. The JCI 155v4 can give the value for the initial charge deposited Qo as well as the decay constant t for each test and the data can be displayed and printed from a computer. This unit is capable of testing almost any material including fabrics, plastics, polymers, liquids and powders. Below is an example of voltage decay curves for several materials.

Since insulators are complicate materials, none of the curves above obey the simple Ohm's decay Law and instead possess nonlinear decay behavior. Thus, the JCI155v4 provides experimental data that cannot be anticipated theoretically. Notice that most of the materials tested would fail the KSC acceptance criteria since the voltage on the surfaces exceeds 350 volts after five seconds. However, the anti-static coated Teflon now becomes an ESD safe material.

The EMPL has other standard instruments used to apply charge to a surface and measure its static dissipative properties.

ETS Static Decay Meter

PDT-740B Static Decay Timer

JCI 176 Electrostatic Pan Model 210

HS Q/m Test System

JCI 147 Faraday Pail

PFC-721 Faraday Cup Assembly

ETS Faraday Cup

Large ME Faraday Cup 

ETS Electrostatic Voltmeter Model 105