41 The effects on the weighing accuracy could be:
• The reported value could be more or less than the real weight • Errors in the range of 1−100 mg have been observed
Because the charges generated can be negative or positive, and either attract or repel each other, the real weight may be more or less than the reported value. Errors of a few milligrams to 100 mg have been observed, which is highly signifi cant in terms of percentage error when weighing small samples.
Yet there are cases where the problem is diffi cult or indeed impossible to notice. For example, it is possible to achieve a stable weight with a net electrostatic force present, without any of the typical indicators that static is affecting the weighing result.
How Quickly do Charges Dissipate? Electrostatic charges dissipate over time:
• Depending on the conditions, charge dissipation can take a few seconds or minutes, or several hours in a controlled dry atmosphere (relative humidity ≤ 20%).
• Good insulators (e.g. borosilicate glass or laboratory grade plastics) can make charge dissipation slower.
Charges dissipate fast from poorly insulating materials (good electrical conductors), but this can be very slow with items made from a good insulator (poor electrical conductor).
Most laboratory vessels are made of borosilicate glass, which is an excellent electrical insulator. The same is true for almost all laboratory-grade plastics used in the manufacture of laboratory items. Even common window glass (sodium silicate glass) makes a good insulator under dry conditions. These good insulating materials can slow down charge dissipation. Clean glassware taken straight from the laboratory dishwasher carries and holds a large amount of charge.
The time constant of charge dissipation is infl uenced by the surface conductivity. The higher the surface conductivity of the charged body, the faster the electrostatic charges can drain away. Next to the material’s intrinsic properties, the surface conductivity also depends to a great extent on the relative air humidity and the degree of surface contamination.
Figures 3 and 4 show the electrostatic charge dissipation over a period of fi ve minutes for a glass vessel compared to a PTFE vessel. Both graphs show larger measurement errors in lower relative air humidity environments (blue lines = 20% RH). In every case, the weighing errors reduce over time, as the charge dissipates. Figure 3 shows that a glass fl ask is unable to become charged at 80% relative air humidity (green line). However, for PTFE containers, even when the air humidity is high, signifi cant weighing errors are observed over several minutes (see Figure 4).
Preventing and Eliminating Static Charges Tips to avoid generating electrostatic charges whilst weighing in the laboratory: • Use electrically conductive or anti-static treated materials whenever possible: - Plastic and glass vessels can quickly become charged and are therefore non-ideal materials. - SmartPrep weighing funnels from METTLER TOLEDO are specifi cally designed for this purpose. • Avoid contact between dissimilar materials when handling • Avoid unnecessary rubbing of the vessel (especially touching it whilst wearing protective gloves). - If possible use weighing tweezers for handling containers. • Increase air humidity in air-conditioned rooms: - Electrostatic charges frequently occur during the winter season in heated (dry) rooms. - The optimum relative humidity is 45 - 60%. • Make sure that the balance and the weighing pan are always electrically grounded. - All METTLER TOLEDO balances fi tted with three-pin plugs are automatically grounded • Avoid wearing electrically insulated footwear. Use grounded shoes and grounding straps instead.
The simplest way to avoid static charges is to use conductive materials, such as those already offered in many plastic laboratory vessels. With these vessels, the disruptive charges drain away via the weighing pan that is grounded, and the charge is eliminated. Unfortunately, it is often not possible for the user to freely select the material of the tare container for many applications.
It also helps to make sure that the user does not substantially contribute to the buildup of electrical charges by wearing insulated footwear or unnecessarily rubbing or holding the vessel with protective gloves. A very dry atmosphere also fosters the charging of weighing samples, especially in winter. A relative humidity of 45 - 60% can reduce the problem, although this alone often does not resolve the problem altogether.
Reducing the Forces Produced by Static Charges Tips to avoid generating electrostatic charges whilst weighing in the laboratory:
• Electrically shield the tare container and sample in a metal basket (which acts as a Faraday cage)
• Use a smaller tare vessel, to reduce surface area and weight, which has a direct relation to the forces generated.
• Ensure that the sample is placed centrally on the weighing pan, and ensure minimum possible protrusion beyond the rim.
• Use a light electrically conductive underlay to increase the distance between the tare vessel and the surface of the weighing area.
• Use a Faraday Cage Figure 3. Glass Vessel - Electrostatic charge dissipation over time in varying humidity conditions.
A Faraday cage is an enclosure made of a mesh of conductive material, which is used to block electrostatic and electromagnetic fi elds. The ErgoClip from METTLER TOLEDO (Figure 5) is a small metal basket that acts a Faraday cage, and is optimally designed for shielding electrostatic charges. It eliminates the disruptive infl uence of having a differently charged tare container and balance interior, and has the added advantage that it holds the tare vessel securely in a defi ned position.
Figure 4. PTFE Vessel - Electrostatic charge dissipation over time in varying humidity conditions.
Figure 5. Weighing with an ErgoClip, a small metal basket, which acts as a tare vessel holder and shields the vessel from electrostatic charges.
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