Mitigating Electrostatic Effects on Measurement Accuracy

Electrostatic charge can have unwanted effects on the accuracy of instruments making precision weighing measurements. This paper will discuss electrostatic phenomena and their interaction with weighing operations. Charge is generated primarily by the contact and separation of dissimilar materials. If one of the materials is a conductor, charge can be quickly removed or even prevented by connecting the material to ground. In most cases, however, at least one of the materials will be an insulator or an isolated conductor. As contact and separation occurs throughout the weighing process, the materials involved are almost certain to be charged. A charged insulator is also capable of inducing charge on nearby isolated conductors. Examples of materials that may be charged include samples, transport media, and parts of weighing equipment or their enclosures. Once generated, the static charge affects both the instruments and the materials being weighed. Electrostatic forces interact directly with the mechanisms of weighing machines, making precise measurements in the microgram range all but impossible. Electrostatic forces of attraction and repulsion affect light weight sample materials, causing unwanted movement and losses during transfers, as well as the movement and clinging of unwanted particles to measurement surfaces. Measurement problems caused by static charge are not limited to weighing applications. Whenever small physical quantities or objects need to be measured, electrostatic forces can cause errors or unwanted movement of the object being measured. This is true for atomic force microscopes, force and mass measurements, and electrochemical measurements. Mitigation methods for static charge are well known in the electronics industry, as it is imperative to protect sensitive integrated circuits from the effects of static charge, both during manufacture and use. Grounding of conductive materials, replacing insulators with dissipative materials, and air ionization are the primary static control methods. Air ionization is of particular importance in weighing operations as equipment parts, samples or transport media are often insulators or isolated conductors. This paper discusses how static control may be applied in precision weighing operations, both in the equipment and in sample transport, to remove static charge and improve the accuracy and repeatability of measurements.