Laboratory Products
Electrostatic Charges and Their Effects on Weighing METTLER-TOLEDO How to deal with static samples to achieve fast and accurate results
Electrostatically charged samples or containers can be diffi cult to weigh. Problems with balance stability or measurement drift can often be caused by static charges. This application note explains measures that can be taken to prevent electrostatic charges building up, but more importantly what can be done to dissipate these charges once they have been generated. Understanding electrostatic charges and the effects they can have on samples and containers are critical to avoid disruption to weighing processes.
What is Static
Static is the accumulation of electrical charges on the surface of a non-conductive material. The phenomenon of static electricity requires the separation of positive and negative charges. When two different materials are in contact with one another, electrons can move from one material to the other, which leaves a positive charge on one material, and an equal negative charge on the other.
What Causes Electrostatic Charges
Friction is the most common way for electrostatic charges to be generated on an object. Everyone remembers the experiment in physics class, where a glass rod is rubbed with a cloth and will then attract small pieces of paper or lift the hair away from your head. Typical laboratory examples include:
Drying a glass beaker with a cloth
• Handling a volumetric fl ask with disposable gloves • Unpacking a laboratory vessel from a plastic bag • Filling a vessel with loose materials (bulk)
Charge separations are caused by rubbing, for example, when tare containers are grasped or lifted. This is fostered by materials that have a strong electrical insulation effect, such as PTFE or plastic materials (like polypropylene, polycarbonate or polystyrene), but also glass. In addition, unfavourable external conditions, such as a dry atmosphere, poorly grounded fl oors, or the use of plastic gloves, can contribute to the formation of charge separation and, thus, to electrostatic charging.
However, charges can also arise without the involvement of friction. Merely separating two different materials, such as lifting a glass fl ask off of a plastic surface is suffi cient to generate a strong static charge.
The Effects on Weighing and Weighing Accuracy
Electrostatic charges can cause sample handling diffi culties, errors in weighing results, and longer weighing times. The effect on the accuracy and reproducibility of the measured value can be very signifi cant.
The presence of static is often recognisable by: • Drifting measurement readings • Non-repeatability of measurement results • Balance instability (or stabilisation time longer than usual)
Drift and non-repeatability of results, and balance instability is usually due to the dissipation of electrostatic charges. These observations are due to the vertical component of the Coulomb force continually changing, which makes it diffi cult to obtain a precise measurement diffi cult.
Electrostatic charges also disrupt the weighing process if the sample or opening of the tare container is charged.
When this happens, powder can ‘jump’ from the spatula onto the tare container, causing precision working to become an ordeal (as illustrated in Figure 1). Dry powders are very susceptible to static infl uences and can be troublesome to weigh. Weighing a small quantity into a large glass or polymer vessel represents the classic use case where an electrostatic charge could signifi cantly increase the error of the weighing result.
Figure 1. ‘Jumping’ powder during the weighing-in step caused by electrostatic charges.
The cause of these issues is a net charging of the weighing sample that, without additional measures, is not reduced or is reduced only slowly. If weighing vessels are charged, polar opposite charges on the metal surfaces of the weighing chamber are attracted to them (see Figure 2). This attraction between the charged vessel to the weighing pan and the charges on the weighing chamber produce an additional force that the balance measures as supposed extra weight. Typically, under such infl uences, balances take a long time to stabilise, and the measurement is inaccurate due to the additional disruptive force.
Figure 2. A negative charge on the measuring fl ask and a positive charge on the balance housing cause a force to be exerted between the balance and the vessel. The vertical component of this force adds to the weight of the fl ask and infl uences the weighing result.
INTERNATIONAL LABMATE - FEBRUARY 2021
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