Analytical Weighing

How Static Electricity Affects Your Analytical (Precision) Weighing and How Can You Overcome These Effects? There are simple and low-cost precautions among the various options to eliminate static electricity during your precision weighing. However, due to current metrological and practical limitations, many of these precautions are difficult to use, time consuming and generally not applicable. On the other hand, there are methods that are both powerful and space-saving, especially when integrated directly into your balance. Static electricity is a common physical phenomenon in many areas of our daily lives. It is of great importance in industry and in all kinds of research and development laboratories. Events such as electrostatic charging of materials, especially in industrial processes and production processes or during material analysis, have adverse effects. lectrostatic discharges can damage electronic equipment and its components. Spark discharges can easily ignite flammable materials in the immediate vicinity, which can cause serious accidents. Therefore, millions of dollars are spent worldwide on efforts to eliminate electrostatic charges and their associated negative effects.

What is Static Electricity?

Static electricity is caused by friction between two objects (body). This friction process transfers electrons from donor to acceptor objects, resulting in ion formation (see Figure 1). An atom with an excess of electrons carries a negative charge, while an atom without an electron receives a positive charge. However, this is only a temporary change in charge since any excess electrons become neutral once they have a certain conductivity or are grounded. Friction may occur inside the sample, between the sample and the container, or between the tare container. For example, during convection in a drying oven, air friction creates a load on glass containers, and internal friction of powders and liquids transferred between containers creates a load on the particles in that sample. In practice, it is not possible to avoid friction during the processing or transport of substances. Thus, electrostatic charging almost always takes place. Destructive electrostatic forces can also be created in the area around the scale by people moving around the scale.

Direct Effect of Static Electricity on Weighing

All balance manufacturers are required to respond to electrostatically charged weighing problems of materials with appropriate technological solutions. Static electricity can have a negative effect on the weighing process itself or its results. Therefore, it requires time-consuming material selection or material handling procedures to counteract these effects. In some cases, weighing a material becomes impossible due to the build-up of electrostatic charges during transport. Also, as the ambient humidity increases and decreases, the electrostatic properties of some materials may change, making weighing even more difficult. Most of the time, electrostatic phenomena are worse when the relative humidity drops below 45%. Therefore, users of scales will experience different conditions from one weighing to another weighing, or weighing during the day versus weighing on another day, which will affect the repeatability of their results. • Electrostatic charging of materials may occur under the following conditions; • In solids, when the surface resistance of the material is Rs > 10 GΩ • In liquids, when conductivity < 10 nS/m In the case of ungrounded conductive materials during the weighing process, the interaction of the electrical charges on the weighed material and the stationary parts of the scale that are not connected to the weighing pan as a conductor cause this electrostatic force. Thus, an electric field is created between the weighed material and the stationary parts of the balance. Examples of fixed scale parts are parts such as the cabinet or the scale plate. The resulting electrostatic forces can cause charge changes (displayed values) of up to one gram. Severe drift of weighing readings and low repeatability of results are also serious problems. [caption id="attachment_132978" align="aligncenter" width="315"] Figure 1a: Schematic representation of ion formation: When two neutral atoms collide or are subjected to friction, electron exchange is shown.
Figure 1b: The lost electron forms an ion on the other side. The total charge of the atom
on the left is positive (positive ion); the total charge of the atom on the right is
negative (negative ion).[/caption] The resulting loads flow slowly over the weighing pan, so the resulting forces do not remain constant over time, causing shifting in the weighing and low repeatability. Depending on the polarity the charge carries, the interaction can be either repulsive or attractive, meaning that the weight results can deviate both in the negative direction and in the positive direction. A repulsive interaction occurs when the charge on the sample and the medium charge have the same polarity (both + or both-) (see Figure 2). The material being weighed appears heavier than it actually is. An attractive interaction occurs when the charge on the sample and the medium charge have different poles (one + and one -). An attractive interaction will therefore make the material being weighed appear lighter than it actually is (see Figure 3). [caption id="attachment_132979" align="alignright" width="687"] Figure 2: Repulsive interaction during weighing. When both the weigh bucket and media are negatively charged, the resulting force is directed downwards (yellow arrow). This makes the sample appear heavier. Figure 3: Attractive interaction. When the weigh bucket and the media have opposite charges, the resulting force is directed upwards (yellow arrow). This makes the sample look lighter.[/caption]

Neutralizing Electrostatic Charges

To eliminate the effects of static electricity on the weighing, you need to de-charge both your sample and the surrounding area. One method with excellent results to protect your weighing chamber and weighing pan from electrostatic fields is to use a completely transparent conductive coating on all glass elements of the balance cabinet. Another solution is to use ionizers and anti-static pens next to your balance. (see Figure 4). This solution works on the principle of surface neutralization by ion bombardment. In most cases, surface neutralization is very effective in reducing charge build-up as it helps to remove electrostatic charges on containers and samples from the external environment of the balance. [caption id="attachment_132980" align="aligncenter" width="695"] Figure 4: Ionizer and anti-static pen.[/caption] The bottom weighing feature of your balance can be used to weigh bulky materials such as plastic blocks. The sample is fixed using a hanger at the bottom of the weighing pan to take advantage of the proportional reduction in electrostatic force as the square of the distance between charge carriers. This method of reducing the effect of electrostatic charges can also be used when weighing on the weighing pan. If the distance between the sample and the weighing pan is significantly smaller than the distance between the sample and the fixed parts of the balance, the influence of electrostatic forces on the weighing results can be reduced, as the weighing pan provides an effective shield. However, if it is the other way around, electrostatic charges will still affect your weighing process. Sometimes it may be sufficient to place an object between the sample and the weighing pan, reducing forces to the point where they have no noticeable effect on the weighing result. For some applications, it is sufficient to increase the protective effect of the weighing pan. For this purpose, special pans with a larger diameter than the standard pans (Figure 5) can be used. [caption id="attachment_132981" align="aligncenter" width="291"] Figure 5: Anti-static weighing pan for better protection of electrostatic charges from samples. Designed as a perforated disc to reduce its weight, this pan is primarily used for weighing filter materials.[/caption] Figure 6, shows an example of a special balance for weighing filters using a Faraday cage (a grounded metal shield) to solve the problem of electrostatic charges. During weighing, the weighing pan and an electrically conductive cover attached to the pan completely protect the filters. This filter weighing scale is often used in the automotive industry or environmental institutes to determine particulate matter in emission measurements. [caption id="attachment_132982" align="aligncenter" width="300"] Figure 6: Microbalance with special cabinet for weighing filters up to 90 mm in size.[/caption]

What Kind of Equipment Can You Use to Eliminate the Effects of Electrostatic Charges?

In general, the time required to neutralize electrostatic charges depends on the material, surface and shape of the sample, and the relative humidity around the balance. The balances on the market have systems that neutralize the internal (inside the scale) static electricity charge, which includes these features. [caption id="attachment_132983" align="aligncenter" width="305"] Figure 7: An ionizing balance cabinet with four jet nozzles for effective removal of electrostatic charges.[/caption] In this system, four nozzle jets are positioned on the rear wall (see Figure 7). The physical functional principle of these nozzles is Corona discharge, a process in which a current flows through a high voltage electrode into the air. The electric field strength around the very fine needle is so high that the air molecules are ionized and form a plasma region around the electrode. The ions produced transmit the charge to areas with lower potential. After recombining with free charges, they form neutral gas molecules again. The use of four nozzles makes charge elimination very effective. By using the opposite poles of the nozzles, a kind of focusing effect is created in the area of the weighing pan. This makes the neutralization of electrostatic charges from sample containers and substances such as dust very effective without disturbing the airflows. Thus, it prevents errors caused by electrostatic forces in weight measurements. In addition, the completely transparent conductive layer on the cabinet glass panels provides additional protection against electrostatic fields in the immediate vicinity of the balance. This ensures stable and accurate weighing results regardless of electrostatic charges. Supports different applications where removal of electrostatic charges is required to measure very small amounts of particles in filters (here special holder for filter diameters up to 150 mm). Mustafa Yetim Sales Responsible Sartonet Seperasyon Teknolojileri A.Ş.