Mitigating Electrostatic Effects on Measurement Accuracy

Transcription

1 Mitigating Electrostatic Effects on Measurement Accuracy Author: Greg Gumkowski NRD LLC 2937 Alt Boulevard Grand Island, NY USA Phone: (716) , FAX: (716) , Author: Arnold Steinman, M.S.E.E. Electronics Workshop 1321 Walnut Street, Berkeley CA USA Phone: (510) , FAX: (510) , July 2014

2 ABSTRACT 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. 1. INTRODUCTION Work place activities typically involve the contact and separation of materials. Whether it is the transfer of materials from one place to another, walking across the floor, or movement of the parts of equipment, contact and separation of materials is constantly occurring. This is the primary cause of triboelectric charge generation. When dissimilar materials are in contact and then separated, a charge exchange occurs between the two materials, leaving one of them positively charged and the other material negatively charged. [1] The presence of charge on materials gives rise to electric fields on the materials. These electric fields can interact with other objects in the environment, primarily by exerting forces on them. The forces of attraction or repulsion often result in uncontrolled movement of these objects, which can be problematic. Unwanted sticking of materials to surfaces, or unexpected movement or deflection of lightweight materials are the most obvious results. When these electrostatic driven phenomena occur in precision weighing or other instrument applications, the obvious result is measurement inaccuracies. Electrostatic forces can interact 1 P a g e

3 with the lightweight mechanisms of scales designed to make measurements in micrograms and cause errors. Lightweight substances being weighed may be moved by attraction or repulsion forces, causing them to adhere in places where the result is both measurement errors and loss of the material being weighed. Particularly in pharmaceutical applications, this can be a serious problem due the possibility of mis-dosing. Electrostatic problems are well known in many industries, and techniques have been developed to mitigate these problems. This paper will attempt to describe electrostatic phenomena and the problems they cause in weighing operations, as well as some of the methods being used to eliminate these problems. 2. TRIBOELECTRIC CHARGING Triboelectric charging occurs whenever two materials with different affinities for electrons are in contact and then separated. At transfer of charge (i.e., electrons) occurs between the two materials, leaving one positively charged (loses electrons) and one negatively charge (gains electrons), although there has been no change in the net charge of the two materials. Any material, solid, liquid, or gas may become charged. Whether it remains charged depends primarily on whether it is a conductor or an insulator. If a conductor is isolated from ground, it will remain charged. However, by definition charge flows easily within a conductor, and will be removed if the conductor is connected to ground. This leaves us with the other type of material, the insulator. Charge generated triboelectrically on an insulator resides on its surface. Charge cannot flow easily through an insulator, so the charge stays on the surface where it is generated and connecting the insulator to ground has no affect on the surface charge. There are a number of factors that affect the nature of triboelectric charging. The most important is the nature of the two materials. Experimenters have arranged materials in what is called the triboelectric series to describe whether a material will charge positively or negatively when it contacts another material. Any subset of the triboelectric series is shown in Figure 1. Note that a given material may charge either positively or negatively depending on what other material it contacts. For example, silicon coming into contact with water or nylon will charge negatively, while silicon coming into contact with Teflon will charge positively. Other factors affect the magnitude of the charge being generated. These include: Intimacy of contact smooth surfaces produce higher charge Contact pressure higher pressure produces higher charge Motion rubbing or friction produces higher charge. Rapid separation of materials produces higher charge Humidity lower humidity produces higher charge Looking at the above issues we can see some mechanical means to reduce the levels of static charge generation. Rough surfaces, low contact pressure, and less rubbing or other friction will reduce charging. Controlling the humidity in work areas can also reduce the level of static charging. However, the nature of the work process, and the variability of humidity conditions 2 P a g e

4 often makes these mechanical means difficult to apply. We will be discussing more efficient means of controlling static charge below. It should also be noted that many weighing operations take place in cleanrooms where the humidity has been reduced to very low levels to protect product quality. The result is extremely high levels of static charge generation in and around the weighing operation. Figure 1 Triboelectric charging One more method of creating electrostatic charge needs to be mentioned as well. When an object becomes triboelectrically charged, it is capable of creating charge on nearby conductive objects. This is known as induction charging. In summary then, we have two means of creating electrostatic charge. One results from the contact and separation of dissimilar maters, and the other results from bringing an isolated conductive object near another object that has already been charged. We will see examples of the problems caused by these two methods of charge generation below. [2] 3. ELECTROSTATIC PROBLEMS IN PRECISION WEIGHING Static electricity exerts mechanical forces on objects that can be easily detected by precision weighing equipment. This static electricity may be present on the object being weighed, the person using the balance, on parts of the weighing chamber, or on the sample carriers. The source of the charge may also be any other insulating materials in the measurement area including table tops and personnel clothing. Charge generation is greater in low humidity areas, such as most cleanrooms. A typical example of this problem occurs when charged objects are placed in the weighing chamber or near the weighing sensor result in a change in zero levels and otherwise affect accuracy. Electrostatic forces act on lightweight measurement components causing errors as high as 3%. Charges on multiple objects may either attract or repel, also affecting measurement accuracy. A sample or its carrier may be charged to one polarity, while the components of the weighing chamber, draft shield, or enclosure may be charged to the same or opposite polarity. The resulting electrostatic forces can alter the measurement in unknown ways. Figure 2 shows an example of the effect on the zero setting when a charged object is introduced into the weighing chamber. 3 P a g e

5 Figure 2. Charged Object in the weighing chamber. Powders and other lightweight materials, such as filter media, adhere to weighing station components and sample carriers making them both hard to handle and causing weighing errors. Small quantities of the samples may be lost from the transfer of these lightweight materials to charged objects in the weighing area. Charged vials or other transport containers attract lightweight materials, removing them from the measurement process. Conversely, the charged vials and containers may attract contaminating particles from the environment, both degrading the sample material and altering its measurement. 4. STATIC MITIGATION METHODS Control of static charge has been actively pursued in the electronics industry since the late 1970 s. As electronic devices have increased in complexity, they have become steadily more sensitive to the effects of static charge, primarily to electrostatic discharge (ESD). In addition, as the sizes of the features of the integrated circuits have shrunk into the nanometer range, the electrostatic attraction (ESA) of small particles during manufacturing has become a more important problem to solve. A number of methods are used, both in the design and the manufacturing and handling of integrated circuits, to eliminate the electrostatic hazards. [3] The first method always employed is grounding. Anything that is conductive, including the personnel, is provided with a connection to ground. This assures that any charge generated, by triboelectric or induction charging, is rapidly removed. In the context of the weighing operations, the following issues need to be addressed: An electrostatic ground point needs to be established in the weighing area. This is usually a connection to the AC power ground. All grounding of static control materials should be done to this common point ground. Personnel performing weighing operations need to be connected to the ground point using a wrist strap while performing sample transfer and weighing operations. This prevents charge on the person s body from interacting with the measurement equipment or causing unwanted transfers of the sample materials. An example of a wrist strap is shown in Figure 3. 4 P a g e

6 Personnel should wear clothing made of natural fibers which tend to absorb moisture and conduct static electricity to ground. Alternatively, personnel can wear static dissipative smocks during weighing operations. Note that static dissipative cleanroom garments are available, but not always used. Figure 3. Examples of wrist strap and grounding connection point Equipment parts in weighing instruments should be conductive if possible and have a connection to the equipment AC ground. Portable instruments should have a way to connect their ground system to the common point ground. This eliminates induction charging of the weighing instrument parts and reduces the possibility of static-related measurement errors. If possible, weighing operations should be performed on a conductive work area (e.g., a stainless steel table) that is grounded. In some instances this may be impractical due to electrical safety issues that must be considered. An alternative is discussed below. A second method of static control is to eliminate all unnecessary insulators from the weighing operation. This requires a review of every aspect of the weighing process to determine which insulators are essential and which are non-essential. Essential insulators may include the samples themselves, parts of the measuring equipment, work surfaces, sample trays, transfer carriers, and handling tools. Non-essential insulators include drinking cups, plastic envelopes for paperwork, employee clothing (sweaters, synthetics), and personal cooling fans. Whenever possible, essential insulators should be replaced with a static dissipative equivalent. This is a material that retains the desirable insulator properties (e.g., mechanical, thermal or chemical) while still having a lower resistance than an insulator. This is achieved by adding carbon, metals, or other additives to an insulating material. The result is a material that may be connected to ground to prevent charge generation. Sample trays and carriers and weighing instrument enclosures are examples of insulating materials for which there may be a static dissipative alternative. Non-essential insulators should be removed and prohibited from the weighing area. Personnel can wear cotton or specially designed static-dissipative garments to prevent charge on their clothing from interacting with the weighing process. Finally, static mitigation techniques exist for insulators that are essential and cannot be removed from the weighing process. The most important example of this is usually the material that is to be weighed. More often than not, it will be insulative and easily charged during its handling. 5 P a g e

7 This means that unwanted movement of the materials will occur, sticking to some surfaces and transferring to others. The primary method used to eliminate charge on insulators is air ionization. For surfaces that do not come into contact with the material or the weighing process, there are also antistatic chemicals that may be applied and renewed periodically. These typically work by depositing a conductive chemical on the surface, or by increasing the attraction of moisture from the air to the insulative surfaces. Humidity is often used in an attempt to control static charging. If relative humidity can be maintained above 60%, there will be a significant reduction in charge generation. But maintaining humidity at this level usually involves a significant expense. Uncontrolled humidity can easily range over a 10-80% range, with serious static problems occurring in the winter months when heating drives the moisture out of the interior air. In cleanrooms, where many precision weighing operations occur, process requirements often dictate humidity levels of 40% RH or less. At these levels humidity is an unreliable and ineffective static control method. 5. MITIGATION WITH IONIZERS Using only the air surrounding weighing process, often of cleanroom quality, ionizers are the most effective method to dissipate static charge on both insulators and isolated conductors. Air ions are molecules of the gases in air (nitrogen, oxygen, water vapor, and carbon dioxide) that have lost (positive ion) or gained (negative ion) an electron. Ions are present in normal outside air but are removed when air is subjected to filtration and air conditioning. Ionization systems work by flooding the atmosphere with positive and negative ions. When ionized air comes in contact with a charged surface, the charged surface attracts ions of the opposite polarity. As a result, the static electricity that has built up on products, equipment and surfaces is neutralized. This is shown in Figure 4. The most common methods of producing air ions artificially are alpha ionization and high voltage corona discharge. [4] Bipolar ionization (both positive and negative ions) has become the accepted standard for controlling static charge in all industries. There is, however, no single "best" method of creating bipolar ionization for all situations. Deciding which method is best for an application depends on the environment, the problem to be solved, and the nature of the work performed in the area. Ions are moved by electrostatic fields and by airflow, and the effectiveness of a system depends on various outside conditions. A number of alpha and corona ionization methods have been developed to generate ions. The following is a short description of the differences between these ionization technologies and a few examples of where each technology has been applied. 6 P a g e

8 Charged Air Molecules Insulator Figure 4. Neutralizing Surface Charges with Bipolar Air Ionization Alpha Ionization - Utilizes alpha emission from a Polonium-210 source without the need for an electrical power connection. Alpha particles cannot penetrate into live human tissue or through a thin sheet of paper, but efficiently ionize the air. Alpha ionizers are ideal for in-tool applications because they do not produce electric fields that might interact with instrument components. Without electrical power, they are useful in applications where flammable chemicals are present. Alpha technology is available in spot sources, bars and blowers. AC (Alternating Current) - High voltage is applied to a number of closely spaced emitter points, which cycle alternately, negative and positive, at the power line frequency. A high electric field is produced at the points, and produces ions of the corresponding polarity. AC technology is used in ionizing bars and blowers, and blowoff guns. Steady-state DC - High voltage is continually applied to pairs of positive and negative emitter points which create electric fields to generate ions continuously. Steady-state DC is more efficient than AC and can be used in both high and low airflow conditions. Like AC technology Steady State DC ionizers are found in bars, blowers and blowoff guns. Pulsed DC - High voltage at positive and negative emitter points is alternately turned on and off, creating clouds of positive and negative ions. Pulsed DC ion emitters can be used to neutralize static charge in whole rooms, even with low airflow, and is the most common type of ionizer in cleanrooms and laminar flow hoods. 6. IONIZER APPLICATIONS IN PRECISION WEIGHING A common ionizer used in precision weighing applications, is actually an ionizer designed to keep dust from collecting on LP records. Known as the ZeroStat it uses two piezoelectric crystals to produce positive and negative ions. Although simple to operate (squeeze and release the gun trigger) the performance leaves something to be desired. Typically, squeezing the trigger creates one polarity of ion (e.g., positive) and releasing the trigger creates the opposite polarity (e.g., negative). The standard industry test using a Charged Plate Monitor was done to determine the effect of the ZeroStat gun. [5] Figure 6 shows the ZeroStat gun test setup and the test results. With the gun 6 7 P a g e

9 inches from the isolated conductive plate of the instrument, note that it first charges to about 1200 volts and discharges to approximately 1000 volts until the trigger is released. Then it charges to about volts, decreasing towards volts over time. Each pull and release of the trigger produces the same result. Very little decrease in voltage occurs when the trigger is at rest in either position. In practice, for a positively charged object, pulling the trigger increases the charge on the object, while releasing the trigger recharges it negatively. For a negatively charged object, pulling the trigger decreases the charge on the object, while releasing the trigger recharges it negatively. The effect of ZeroStat gun on conductors, such as conductive parts of measuring equipment and conductive samples and sample containers is illustrated accurately by the results. The effect of the ZeroStat on insulators might not be as extreme, but it is expected to be similar. Conversely, an alpha ionizer or a small ionizing blower would reduce either polarity of charge on conductors or insulators to almost zero in less than 2 seconds under similar conditions. Figure 6. ZeroStat test setup and results. Alpha ionizers, working without additional airflow or electrical power are a convenient method for ionizing the interior of weighing instruments. Easily mounted in the weighing chamber, they assure that static charge on any object in the weighing chamber is quickly neutralized. Containing only a small amount of Polonium 210, they have only minimal regulations and maintenance consists only of replacing them periodically when the source operating level is no longer adequate. An example of an application of an alpha ionizer is shown in Figure 7. Note that this is the same situation as shown in Figure 2, except that an ionizer has been installed in the interior of the weighing chamber, and now there is no drift in the zero reading. Both alpha and corona ionization technology is used for small ionizing bars or blowers located in the measurement area. These are used to assure that the components of the weighing equipment, the sample materials, and transport containers have their static charge reduced to low levels. The corona ionizers require periodic cleaning to maintain their performance. Since they utilize high voltage to produce ions, they have to be designed in a way that prevents contact with the ionizer emitter points. Coupling the ion source with a blower increase the area of coverage, but might impart unwanted motion to extremely light materials. Its primary use would be to neutralize 8 P a g e

10 static charge on all critical surfaces before the actual sample is introduced. Examples of other types of ionizers are shown in Figure 8. Figure 7. Alpha ionizer in a weighing instrument. Figure 8. Other types of ionizers used to neutralize materials in weighing applications. 7. CONCLUSION The presence of static charge is inevitable in most workplaces due to the unavoidable contact and separation of materials. But static charge can be controlled by providing adequate grounding, control of insulating materials, and the application of air ionization to neutralize static charge on process essential insulators. In precision weighing applications it is essential to remove static charge to prevent measurement inaccuracies, loss of critical materials, and unwanted adhesion of materials in the weighing chamber. Static charge control methods from other industries, properly applied in precision weighing applications, can eliminate the problems caused by static charge. 9 P a g e

11 8. REFERENCES [1] ESD ADV Triboelectric Charge Accumulation Testing, ESD Association, [2] Static Charge: A Life Sciences Problem?, GIT Verlag Laboratory Journal (Germany), September 1999, [3] ESD TR ESD Handbook, ESD Association [4] Everything You Ever Wanted to Know About Air Ionization, Threshold Magazine, volume 18 number 2, March/April 2002, ESD Association [5] ANSI/ESD STM Ionization, ESD Association 10 P a g e