How Does Static Electricity Affect Your Analytical Weighing?

How Static Electricity Affects Your Analytical (Precision) Balance and How to Overcome These Effects

There are various options available for eliminating static electricity during precision weighing, ranging from simple, low-cost measures. However, due to existing metrology and practical limitations, many of these measures are difficult to implement, time-consuming, and generally impractical. On the other hand, there are methods that are both powerful and space-saving, especially when directly integrated into your balance. Static electricity is a physical phenomenon commonly observed in many areas of our daily lives. It holds great significance in industry and in all types of research and development laboratories. Events such as electrostatic charging of materials occurring particularly during industrial processes and production procedures, or during material analysis, create adverse effects. Electrostatic discharges can damage electronic equipment and components. Spark discharges can easily ignite flammable materials in the vicinity, which could lead to serious accidents. For this reason, millions of dollars worldwide are spent on efforts to eliminate electrostatic charges and their associated adverse effects.

What is Static Electricity?

Static electricity results from friction between two objects (bodies). This friction process transfers electrons from a donor to recipient objects, which results in ion formation (see Figure 1). An atom with excess electrons carries a negative charge, while an atom without electrons acquires a positive charge. However, this is only a temporary change in charge, because any excess electrons become neutralized when the material has certain conductivity or when it is grounded. Friction can occur within the sample, between the sample and container, or between tare containers. For example, during convection in a drying oven, air friction generates a charge on glass containers, and the internal friction of powders and liquids transferred between containers generates a charge on particles within that sample. In practice, it is impossible to avoid friction during material processing or transport. Thus, electrostatic charging almost always occurs. Destructive electrostatic forces in the area around the balance can also be generated by people moving around the balance.

Direct Effects on the Balance

All balance manufacturers must respond to problems of electrostatic loading of materials with appropriate technological solutions. Static electricity can have an adverse effect on the weighing process itself or on its results. For this reason, eliminating these effects requires time-consuming material selection or material transport procedures. In some cases, weighing a material becomes impossible due to the accumulation of electrostatic charges during transport. Furthermore, the electrostatic properties of some materials can change as ambient humidity increases and decreases, which makes weighing even more difficult. Most often, electrostatic phenomena are worse when relative humidity drops below 45% – this is typically observed during winter at European latitudes or in air-conditioned rooms. For this reason, balance users will experience different conditions from one weighing to another or between weighings taken at different times of day, compared to weighings taken on another day, which will affect the reproducibility of their results. Electrostatic charging of materials can occur under the following conditions: • In solids, when the material's surface resistance Rs > 10 GΩ (according to IEC93) • In liquids, when conductivity is < 10 nS/m • In conductive materials, when they are not grounded During the weighing process, the interaction between electric charges generated on the weighed material and the fixed parts of the balance that are not conductively connected to the pan causes this electrostatic force. Thus, an electric field is created between the weighed material and the fixed parts of the balance. Examples of fixed balance parts include components such as the enclosure or balance plate. The resulting electrostatic forces can cause changes in load values of up to one gram (displayed values). Serious drift in weighing readings and poor reproducibility of results are also serious problems. The generated charges flow slowly through the weighing pan, so the resulting forces do not remain constant over time, causing drift in weighing and poor reproducibility. Depending on the polarity carried by the charge, the interaction can be either repulsive or attractive, meaning weight results can deviate in both the negative and positive directions. A repulsive interaction occurs when the charge on the sample and the environmental charge have the same polarity (both + or both -) (see Figure 2). The weighed material appears heavier than it actually is. An attractive interaction occurs when the charge on the sample and the environmental charge have opposite polarities (one + and one -). An attractive interaction will therefore cause the weighed material to appear lighter than it actually is (see Figure 3). [caption id="attachment_131723" align="aligncenter"] Figure 2: Repulsive interaction during weighing. The force resulting from both the weighing pan and the environment being negatively charged is directed downward (yellow arrow). This causes the sample to appear heavier.[/caption] [caption id="attachment_131724" align="aligncenter"] Figure 3: Attractive interaction. When the weighing pan and the environment have opposite charges, the resulting force is directed upward (yellow arrow). This causes the sample to appear lighter.[/caption]

Neutralizing Electrostatic Charges

To eliminate the effects of static electricity on your weighing, you need to discharge both your sample and the surrounding area of charges. One method that produces excellent results for protecting your weighing room and weighing pan from electrostatic fields is the use of a completely transparent conductive coating on all glass elements of the balance enclosure. Another solution is to use ionizers and anti-static pens beside your balance (see Figure 4). This solution works according to the principle of surface neutralization through ion bombardment. In most cases, surface neutralization is very effective in reducing charge formation when it helps eliminate electrostatic charges on containers and samples in the external environment of the balance. [caption id="attachment_131725" align="aligncenter"] Figure 4: Ionizer and anti-static pen[/caption] Your balance's bottom weighing feature can be used to weigh bulky materials such as plastic blocks. The sample is fixed using a suspension below the weighing pan to take advantage of the proportional reduction in electrostatic force resulting from the square of the distance between the 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 effect of electrostatic forces on the weighing results can be reduced because the weighing pan provides effective shielding. However, in the opposite case, electrostatic charges will still affect your weighing process. Sometimes it is sufficient to place an object between the sample and the weighing pan, which reduces the forces to a point where they have no visible effect on the weighing result. For some applications, it is also sufficient to increase the shielding effect of the weighing pan. For this purpose, special pans (Figure 5) with larger diameter than standard pans can be used. [caption id="attachment_131726" align="aligncenter"] Figure 5: Anti-static weighing pan for better protection from electrostatic charges from samples. Designed as a perforated disk to reduce its weight, this pan is primarily used for weighing filter materials.[/caption] Figure 6 shows an example of a specialized balance for weighing filters that uses a Faraday cage (a grounded metal shield) to solve the problem of electrostatic charges. During weighing, the weighing pan and a conductive cover attached to the pan completely protect the filters. This filter weighing balance is typically used in the automotive industry or in environmental institutes to determine particulate matter in emission measurements. [caption id="attachment_131727" align="aligncenter"] Figure 6: Micro balance with special enclosure for weighing filters up to 90 mm in size[/caption]

What Type 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 surrounding the balance. Balances available on the market have internal systems (located within the balance) that neutralize static electric charge containing these characteristics. [caption id="attachment_131728" align="aligncenter"] Figure 7: An ionizer balance enclosure with four jet nozzles for effective elimination 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 current flows from a high-voltage electrode into the air. The electrical field strength around the very fine needle is so high that air molecules become ionized and a plasma region forms around the electrode. The generated ions transfer the charge to areas with lower potential. After recombining with free charges, they again form neutral gas molecules. The use of four nozzles makes charge elimination very effective. By using opposite polarities of the nozzles, a kind of focusing effect is created in the weighing pan area. This makes the neutralization of electrostatic charges from sample containers and substances such as powders very effective without disrupting air flows. Thus, it prevents errors in weight measurements resulting from electrostatic forces. Additionally, the completely transparent conductive layer on the cabinet glass panels provides additional protection against electrostatic fields in the immediate vicinity of the balance. This also provides balanced and accurate weighing results independent of electrostatic charges. [caption id="attachment_131729" align="aligncenter"] Supports different applications where elimination of electrostatic charges is necessary to measure very small amounts of particulates in filters (here special holder for filter diameters up to 150 mm)[/caption]    

Mustafa Yetim Sales Manager Sartonet Seperasyon Teknolojileri A.Ş.