John Chubb

John Chubb Instrumentation,

Unit 30, Lansdown Industrial Estate, Gloucester Road, Cheltenham, GL51 8PL, UK.

Tel: +44 (0)1242 573347 Fax: +44 (0)1242 251388 email: jchubb@jci.co.uk

ABSTRACT: An approach is described for assessing the general prospect of electrostatic risks from materials by simultaneous measurement of the quantity of charge transferred, the initial peak voltage generated and the subsequent rate of decay of surface voltage. The quantity of charge transferred divided by the peak voltage is, effectively, a ‘capacitance’. It is proposed that this parameter, in conjunction with the charge decay time performance, indicates the prospect of risks from static charge retained on materials.

The ability of materials to avoid problems from static electricity has traditionally been judged from measurements of surface or volume resistivity. When one thinks a bit more carefully about the real requirements to avoid static problems one appreciates that resistivity measurements do not, and cannot, provide easy, simple or reliable guidance. This is particularly true for newer materials - such as the fabrics used in cleanroom garments which include conductive threads.

The present paper considers the features relevant to how problems may arise with static electricity. Electrostatic problems usually arise with tribocharging of materials. The results of tribocharging studies are usually difficult to reproduce and understand. The present paper describes a method of measurement which brings consistency to tribocharging studies and provides a basis for a fair and reliable way to assess materials for certain basic risk features.

Static electricity arises as the separation of positive and negative charges at the interface between two dissimilar surfaces. If electrical charge can move only slowly on one or other of the surfaces charge then the charge may be described as 'static' - and so is available to influence situations and devices nearby. Plastics and composite materials are used in many areas of life. Such materials, in conventional terms, are ‘insulators’ - they do not allow easy charge migration.

Risks from static arise in many industrial and practical situations:

The types of problems and risks caused by static relate not just to the quantity of charge on the surface of materials but particularly to:

How should the risk of problems involving static electricity from materials be assessed? Let us start by considering the objective for controlling risks from static. In most practical electrostatic risk situations static charge is transferred between surfaces by a fairly short duration mechanical action. This may be a person getting up from a chair, an article sliding down a chute, surfaces being rubbed together or torn apart in handling, etc. A quantity of charge is transferred between the materials by these actions. A basic question for both surfaces is how quickly can this charge move over the surfaces of these materials in relation to the timescale in which the surfaces separate. Charge will tend to remain in regions where charge mobility is low, but move quickly away over the material where the mobility is high. If you measure the ‘current’ flow to, or from, the material you measure just the moving component of charge - the high mobility component. (This is the traditional ‘resistivity’ measurement - which can be very misleading). From the static risk and problem points of view, it is primarily the charge retained on the material itself that is relevant. What is significant about this retained charge maybe the surface charge density, maybe the surface voltage or maybe the charge or energy which can be accessed by an electrical discharge. The feature of significance depends upon the nature of the risk or problem. Where a material rubs another, such as clothing rubbing a car seat, the 'risk' is associated with the charge taken away with the clothing and this may not directly depend on whether charge is retained on the car seat material. Hence no one single method is able to assess materials for all types of risk.

A simple concept, which has wide applicability, is that if electrostatic charges can move easily over and through a material, and there is somewhere for the charge to move to, then there can be no problems from static electricity held on the material itself.

The two immediate questions that arise in relation to charge movement are:

- how do you measure how quickly charge can move away (decay or dissipate)?

- what is a suitable target time for charge decay?

As noted above, a 'resistivity' measurement will not, and cannot, provide a generalised way to assess materials because it can only show the motion of charge and cannot show the retention of charge.

Probably the method best known for charge decay measurement is Federal Test Standard 101C Method 4046 [1]. This basically involves holding a sample by conducting clamps, raising the clamps to 5000V, checking with an electrostatic fieldmeter that the sample has reached 5000V and then observing with the fieldmeter how quickly the voltage of the middle of the sample falls when the clamps are earthed. While this method can work with homogeneous and isotropic materials it can give very erroneous results with many normal materials [2]. The problem with the method is basically that it responds to the fastest route available for charge migration in the material and does not provide any information on the real charge retention capability of the material. In practice it is also not known if materials are really homogeneous - and if not then what is to be done.

A basic assessment of materials by charge decay might be made after charging materials by rubbing - tribocharging. Tribocharging is a notoriously inconsistent approach and requires some experimental skill. However, instrumentation has been developed which provides quick, easy and reliable assessment of the charge decay capabilities of materials. This instrumentation uses a high voltage corona discharge to put a local patch of charge on to the surface of the material to be tested and observes, with a fast response field mill type fieldmeter, how quickly the deposited charge moves away [3,4]. This method of measurement is included in a recent British Standard [5] and a recent draft IEC document [6]. Studies have shown that observations of corona charge decay match well the decay of triboelectrically generated charge [7], that the use of corona does not significantly affect the material [7]. Studies have also shown there is no relationship between charge decay and resistivity [8].

Charge decay measurements with corona charging are now being used for judging materials in a wide variety of industries - for instance: lingerie fabrics, hosiery, cleanroom garments and their laundering, spectacle lens coatings, powder coating paint, photocopying paper, pharmaceuticals, optical fibre coatings, coatings for plastic film materials, margarine tub materials and microelectronic packaging.

The above discussion may suggest that charge decay measurement can be used as an unequivocal way to judge whether materials will present static risks. This is too simplistic a view. A short charge decay time may indeed ensure avoidance of static problems on the material itself. However, if the decay time is long it may be that there are other ways by which risks and problems can be avoided.

Conductive threads are often included in materials - for example in the fabrics for ‘workwear’ and ‘protective’ clothing and in carpets. These threads may, perhaps, help avoid static risks:

- by capacitance limitation on the surface potential

If the conductivity of the threads is too high then (for example for metallic threads) then certain risks (for example ignition risks) may be increased in certain situations.

It is suggested that no single test will assess all aspects of the electrostatic performance of materials for all applications. However, it seems plausible that by measuring how high an initial voltage occurs when the material is rubbed, how much charge has been transferred and also how quickly the charge (or surface voltage) decays one will have a broader and fairer basic assessment of the likelihood of risks from charge retained on materials. These measurements will only show whether risks are avoided by static charge held in the material itself. They cannot answer the following questions:

The above points need separate test approaches. The direct assessment of risks of igniting flammable gases by discharges to fabrics is a difficult and specialist activity - as is the direct assessment of various other electrostatic risks, such as fabric cling and damage to semiconductor devices. The present paper is concerned with problems and risks associated with static charge retained on materials.

The two basic questions to be addressed in relation to assessing whether surface voltages created on material by tribocharging will create risks are:

- what is the surface voltage created?

- how quickly does the initial charge, and associated surface voltage, dissipate?

In real practical situations materials become charged triboelectrically. Hence, measurements must be based on, or relate to, triboelectric charging. Tribocharging is very susceptible to the way it is done and does not usually give consistent values at repeated testing. An approach has been devised which gives the prospect of bringing order to such studies [9]. The approach (described more fully in Annex 1) is to measure both the surface voltage generated and the charge transferred at individual tribocharging events – and also to record how quickly the initial surface voltage decreases as the transferred charge dissipates. It seemed likely that the initial peak voltage might vary directly with the quantity of charge transferred – and that the ratio of these, effectively a capacitance, might be a parameter which would be useful for characterising materials.

If a high surface voltage is generated when a particular, and practically likely, quantity of charge is transferred (low capacitance) then a significant electrostatic risk may arise. If, however, only a low surface voltage arises for the same quantity of charge (high capacitance) then risks are not likely to occur.

The above approach, as so far investigated, is an experimental method to obtain basic experimental data and understanding the behaviour of materials. It does not answer all questions about electrostatic risks from materials but it indicates how a wide range of risks an problems can be avoided. The approach is suitable to get basic information about materials by skilled workers, but may well not be suitable as an industrial test. Corona charging provides the basis for compact and easy to use instrumentation in an industrial test situation [3,4,5,6]. If there is good correlation between charge transfer, initial surface voltages and decay times obtained in tribocharging studies and with corona charging then there will be opportunity to use corona charge based instrumentation for the simple, consistent and fair assessment of materials in practical situations.

The basic approach for experimental measurement has been:

Full details of the test procedure used are described in Annex 1. The test arrangement is shown in Figure 1.

A fairly long PTFE rod (25mm diameter 300mm long) is used for sample charging so the sensitivity of the Faraday Pail is not affected by bringing earthed items (such as a rod support handle) nearby. The rod is held at one end. The rod is made charge neutral by rotating the rod surface near, but not touching, the side of a candle flame. It was found necessary to neutralise most of the length of the rod a) to reduce influence of residual charge on the fieldmeter observing the potential on the sample, and b) so that a reliable measurement of the separated charge left on its rubbing end can be made by insertion (without further rubbing contact) into a Faraday Pail.

The action in making a measurement was to hold the tip of the PTFE rod about 100mm away from the target point in the middle of the sample area, to check everything was ready - then to make a quick bouncing scuff contact with the target area and swing the PTFE rod well away and hold this well clear of the test set-up while charge decay observations were made.

The charge transferred to the tip of the PTFE rod was measured using a JCI 147 Faraday Pail. This has the ability to measure charge to 1pC. The tip of the rod, which had contacted the sample, was lowered vertically through the shield aperture and into the pail without contacting any surfaces. Readings were recorded manually directly as nC.

The voltage of the middle of the sample was observed with a JCI 140C Static Monitor with 100mm separation between the surface and the sensing aperture. The sensitivity of this instrument is 1mV output for 1V of surface at 100mm when observing a large plane conducting surface. Because the area charged is so small in the present test situation the output gives an underestimate of the actual local peak surface potential, but a fair indication of the electric field influence of the surface charge at nearby items.

Observations of surface voltage were displayed and recorded by linking the JCI 140C to a microcomputer running the software program JCILOG. This was operated to give recording of readings at 0.25s intervals. Measurements were also made with observations recorded using a ‘Picoscope’ digital storage oscilloscope linked to a microcomputer. The response time of the fieldmeter is less than 0.1s. Although a faster response and recording time might seem desirable these times were sensible in relation to the likely timescale for manual movement of the PTFE rod away from the scuffing action.

Values of peak voltage and decay rates were derived by loading the numerical data obtained from the data recording into Spreadsheet software (Works or Excel).

Care was taken when making measurements to try to make scuffing contact with the middle of the sample surface, so that all instances of charge transfer had a similar coupling to the JCI 140 fieldmeter. It would be desirable for future studies if travel of the rubbing rod could be mechanised and direct measurement made of the mechanical ‘action’ involved in individual charging events. It is anticipated that the quantity of charge transferred for a particular material may relate to this mechanical ‘action’.

The values of temperature and humidity during the period of testing were measured with a Casella 'whirling hygrometer'. Care was taken to avoid hand contact with the area for testing and to avoid breathing on to, or in the direction of, the samples in preparation for and during the conduct of tests.

Figure 2 shows an example of the surface voltage variation associated with an individual charging event.

Measurements were made on a variety of materials, including simple fabrics and fabrics (such as those used in cleanroom garments) which include conductive threads.

1 Pale blue polyester

2 White lingerie fabric

3 White lingerie fabric - washed 5 times

4 Handkerchief (cotton and other fibres)

5 Polyethylene bag

Pattern Spacing Conductor Treatment

10 Grid 5mm Tri-lobe core

11 Grid 5mm Tri-lobe core

12 Grid 5mm Surface

13 Grid 5mm Two edge surface

14 Grid 5mm Circular core

15 Grid 7.5mm Surface

16 Grid 10mm Surface unwashed

17 Grid 10mm Surface washed

18 Stripe 5mm Tri-lobe core washed

19 Stripe 5mm Tri-lobe core

20 Stripe 20mm Tri-lobe core washed

Measurements with a number of simple materials (1-6 above) show that the initial peak voltage generated when individual materials are rubbed is broadly proportional to the quantity of charge transferred. The constant of proportionality is an ‘effective capacitance’ for the charge. The variation of the effective capacitance with quantity of charge transferred for simple fabrics is shown in Figure 3. Some of the fabrics which include conductive threads also show a fairly constant value of capacitance with quantity of charge, but others show quite a strong increase with quantity of charge. The results for these fabrics (10-20) are shown in Figures 4, 5 & 6. Figure 4 deals with fabrics with 5mm grids of surface and core conductive threads, Figure 5 with fabrics with grids of surface conductive threads of different spacing and Figure 6 compares grid and stripe patterns of core conductive threads.

For many of the studies carried out, the rise of surface voltage at rubbing was small, only a few tens of volts, and the rate of decay was quite quick, to below 1s. Although the response time of the fieldmeter was below 0.1s measurements of initial peak voltage were not particularly accurate. When observations of the fast charge decays were recorded with JCILOG only a small number of points were recorded with the 0.25s time steps used. In this situation the decay times were obtained by plotting ln(V) versus t, and estimating the best fit ‘time to 1/e’ from the initial peak voltage. Values of decay times calculated for a number of the faster decay materials studied without conducting threads are shown in Figure 7.

Initial peak surface voltages up to about 100V were observed, as shown in Figure 7, even when the decay time was as short as 0.2s. With decay times around 0.7 - 1s the initial surface voltage could be as high as 500V. These results show that charge decay times below 0.2s are needed to avoid the occurrence of initial peak voltages below 100V. This is appreciably less than the time of 0.5s suggested previously as an acceptance criterion [4,5].

The values of initial peak voltages observed may well have been less than the true local instantaneous values because of the time taken to move away the PTFE rod with the influence of its counter-charge. None the less, these studies did involve manual charging actions and hence represent something of a worst case for manual tribocharging. It is noted that shorter decay times may well be needed to control static risks where there are high speeds of mechanised charging action [3].

The initial peak voltage observed on many materials seems to vary generally in proportion to the quantity of charge transferred. This leads to the concept of an 'effective capacitance' for

the charge on the material. This concept arose in earlier studies with corona charging of materials [8].

The effective capacitance exhibited by fabrics which include conductive threads is much larger than for those which do not. The effective capacitance is higher for the closer spacing of the conductive threads and higher for close spaced conductive threads with surface conductivity compared to core conductivity.

Appreciably larger quantities of charge are transferred for the cotton and lingerie fabrics than for the polyester fabrics and for polyethylene. This is shown in Figure 3. Although the pressures and speeds of manual rubbing were not controlled or measured, repeated measurements gave comparable values of charge transfer and the intensity of rubbing could be judged to an extent by ‘feel’. The variation of maximum charge between the simple fabric materials seems to be in line with expectations from triboelectric series information [10].

Charge transfer by manual rubbing seems to be limited by the charge retained on the rubbing material – at least when this is PTFE. This is shown by the observation that little additional charge is transferred when materials are rubbed with a multiple scuffing action compared to a single scuff action. Little charge is transferred from a PTFE rod left charged from a previous test. This indicates that charge transfer may be limited by the voltage difference, or by the local electric field, at the point of rubbing. Larger quantities of charge are certainly transferred with higher pressures and higher speeds of rubbing action. These values of charge transferred provide a baseline for comparative studies with corona charging.

It would be useful to examine in further studies whether the apparent limitation on maximum charge levels with the fabrics including conductive threads relates to triboelectric series differences between rubber and rubbed surfaces and whether behaviour is affected by humidity. It would also be interesting to examine if the quantities of charge related to the amount of mechanical work involved in the rubbing action.

The quantities of charge transferred in the present tribocharging studies are up to 10nC for simple fabrics and up to about 2.2nC for fabrics which include conductive threads. The present values of charge are much less than the quantities used in earlier corona charging studies [8], so direct comparison of results is unwise. Preliminary results from corona charging studies [11] indicate that for the same materials and similar quantities of charge transfer similar values of effective capacitance are observed. At larger quantities of charge the effective capacitance increases and seems to become proportional to the quantity of charge - with certain limiting surface voltages relating to fabric construction.

If charge movement in the conductive threads is affecting the observed values of initial surface voltages then this influence is likely to happen very quickly. Because charge movement to, or away from, the area rubbed will be into a much larger area of material its effect will be much diluted. Hence, with brief scuff charging it seems unlikely there will be any significant difference whether the outer boundary of the sample is earthed or not.

The main outcome of the present studies is that meaningful measurements of peak surface voltage, charge transfer and charge decay in manual tribocharging actions can be made in direct experimental studies with fairly simple test arrangements. The experimental arrangement seems a good model of many practical tribocharging situations. The practical relevance of the values measured should thus be convincing.

The initial peak voltage observed on many materials seems to vary generally in proportion to the quantity of charge transferred. This leads to the concept of an 'effective capacitance' for the charge on the material.

The ‘effective capacitance’ experienced by the charge (in addition to charge decay time) is useful to characterise the prospect of electrostatic risks from materials. A high capacitance material is less likely to be associated with surface voltages at which risks can occur. If a figure can be agreed for maximum likely local charge transfer by rubbing then the effective capacitance of a material will be a guide to the surface voltage risks presented by retained charge.

Fabrics which include conductive threads have higher values of effective capacitance than those which do not. Fabrics which do not include conductive threads give low values of effective capacitance which do not seem to vary too much with the quantity of charge transferred. The capacitance of fabrics which include conductive threads is higher for a closer spacing of the threads and appears to be higher for surface conducting threads than for encapsulated conductive threads. The relationship between capacitance and fabric constructional features needs fuller investigation.

Studies are now needed to examine whether similar values of effective capacitance arise with corona charging as with tribocharging. If it is shown that the effective capacitance with corona charge is indeed similar to that with tribocharging [11] then the opportunity is available for easy to use instrumentation to assess practical materials in a much fairer way than has been possible till now. In prospect, assessment will involve charging samples using a short pulse of corona charge deposition with measurement of the quantity of charge transferred, the initial peak voltage achieved and a record of the form of the charge decay curve. A material will be considered 'good', or acceptable, either:

a) if the charge decay curve lies fully below a decay curve corresponding to an appropriate exponential decay (for example 0.2s)

b) if the effective capacitance for a charge transfer of 1-10nC is greater than a value appropriate to the application (which for ignition risks might be 20pF and for semiconductor manufacturing operations might be 100pF).

Appreciable surface voltages (over 100V) can occur with low ‘effective capacitance’ materials when the charge decay times (initial peak voltage to 1/e of this) is ½ second. It is concluded that charge decay times below about 0.2s are needed to avoid the risk of occurrence of significant surface voltages in manual tribocharging actions.

It is hoped that notice will be taken, in discussions of formal Standards, of the approach, results and conclusions described in the present paper.

[1] US Federal Test Standard 101C Method 4046 Oct 8, 1982

[2] J. N. Chubb; P. Malinverni "Experimental comparison of methods of charge decay measurements for a variety of materials" EOS/ESD Symposium, Dallas, 1992 p5 A.5.1

[3] J. N. Chubb "Instrumentation and standards for testing static control materials"

IEEE Trans Ind Appl 26 (6) Nov/Dec 1990 p1182.

[4] J. N. Chubb "Instrumentation to assess the static charge dissipation capabilities of materials" 13th Int Symp Contamination Control, The Hague 16-20 Sept, 1996

[5] "Methods for measurements in electrostatics"

British Standard BS 7506: 1996: Part 2

[6] "Principles of electrostatic phenomena" Draft IEC 61340-1, 1998

[7] J. N. Chubb "Dependence of charge decay characteristics on charging parameters" Intl Conference Electrostatics 1995, Inst Phys, York April 3-5, 1995

[8] J. N. Chubb "Corona charging of practical materials for charge decay measurements" J. Electrostatics 37 1996 p53

[9] J. N. Chubb "Instrumentation and measurements to assess eletcrostatic conditions and risks in cleanroom situations" Cleanroom Technology Expo, Frankfurt, 6-7 May, 1998 (Angel Business Communications Ltd, London, EC1V 1LR)

[10] D. M. Taylor, P. E. Secker "Industrial electrostatics: Fundamentals and measurements" Research Studies Press, 1994

[11] J. N. Chubb "Charging and charge decay measurements on a variety of materials" Paper for Inst Phys 'Electrostatics 1999 Conference', University of Cambridge, Mar 1999

Note: Details of the features of proprietary JCI instruments (JCI 140 Static Monitor & JCI 147 Faraday Pail) and software (JCILOG) are available on the Website http://www.jci.co.uk

2.1 Electrostatic fieldmeter for remote non-contact surface voltage measurement with ability to measure to 1V or less and response time 0.1s or less

2.2 Labstand support

2.3 Frame for support of stretched sample over open backing

2.4 Fast digital recorder (e.g. digital storage oscilloscope)

2.5 Faraday Pail with ability to measure charge to 10pC

2.6 Candle or other naked flame or radioactive charge neutraliser

2.7 PTFE charging rod 20-25mm dia 300-400mm long

2.8 Temperature and humidity measuring instrumentation

Three test areas are needed for fabric materials, garments or test samples. The test areas need to be about 300x300mm. Avoid cutting fabric samples within 50mm of selvage edge.

Condition specimens for a minimum of 16 hours in standard atmospheres of 23C and 50%RH and 23C and 12%RH and carry out tests within the conditioning atmosphere.

5.1. Mount sample support frame so sample will be held secure and steady during charging actions and so that rubbing rod can be moved easily and quickly to contact middle of test sample area and then moved well away.

5.2 Position Faraday Pail within reach so that charge can easily be measured on the charging rod before and after a test without its charge influencing observations of surface potential on the test surface.

5.3 Position charge neutralising flame or radioactive source within easy reach but well away from the test surface and the Faraday Pail so there will be no unintended neutralisation of charge on the sample or on the rubbing rod.

5.4 Mount the fieldmeter for surface voltage measurement with its sensing aperture 100mm above the centre of the surface of the sample area

5.5. Connect the fieldmeter to a digital storage oscilloscope and switch on. Check that the test surface voltage reading is very close to zero.

5.6. Stretch the sample over the earthed support frame and secure under tension. There should be no earthed surfaces within 50mm of the reverse side of the sample. Check if static charges generated by handling the sample are giving a significant surface voltage reading. If so then allow time for the reading to fall to a low value.

5.7. Switch on the Faraday Pail measurement circuit. Earth the pail and check zero and reading are stable.

5.8. Measure the ambient temperature and humidity

5.9. Light the candle or other flame if this is the charge neutraliser approach used.

5.10. Select one end of the PTFE rod for rubbing and the other to be used for hand support. Hold the PTFE rod near the neutraliser. If this is a naked candle flame hold the rod by the side of the flame but avoid contact of the flame with the rod surface. Rotate the rod and move it so that all surfaces of the rod (apart from that which will be directly used for manual support) are neutralised. Present the rod under the fieldmeter and check for any reading near the tip or over the surface of the rubbing rod. Alternatively, hold the rod vertically downwards from the end support and lower the rod down into the Faraday Pail without touching any surface. Check if there is any residual charge - anything greater than say 5pC. If there is then repeat the charge neutralising procedure until this is removed.

5.11. Make a trial test by a brief ‘scuff’ of the PTFE rod on the centre of the sample area directly under the sensing aperture of the fieldmeter by swinging the tip of the rod to contact the sample surface and then moving it quickly well away from the test area. Observe the variation of surface voltage recorded and displayed on the microcomputer. This usually shows a sharp initial voltage peak (probably negative) followed by a voltage swing to the opposite polarity from which the voltage will then decay away. Check that surface voltage changes polarity as rubber moves away.

5.12 Measure charge on PTFE rod ‘rubber’ without contact in Faraday Pail - the polarity of this charge should be opposite to that of the second voltage swing from which the voltage decay was observed.

6.1 The basic measurement procedure is:

- neutralise charge on PTFE rod

- zero Faraday Pail

- check the reading of the fieldmeter measuring sample surface voltage is very near zero

- note the time

- make a brief ‘scuff’ of the PTFE rod on the centre of the sample area directly under the sensing aperture of the JCI 140C by swinging the tip of the rod to contact the sample surface and then moving quickly well away from the test area. The rubbing rod must move fully out of view of the fieldmeter (to at least 500mm away) in less than ½s - and preferably less than ¼s. Hold the PTFE rod clear of contact with anything prior to measurement of its charge.

- measure and note charge on the rubbing rod when introduced into the Faraday Pail without contacting any surfaces (the polarity of this charge should be opposite to that of the second voltage swing from which the voltage decay was observed).

- observe and record the variation of surface voltage. Fieldmeter readings shall be recorded continuously from before making a test to such time after rubbing that a good estimate can be made of the rate of charge decay. The fieldmeter observations are used:

- note the value of the peak voltage swing from which charge decay is observed

6.2. At least three, and preferably five, measurements shall be made on each test sample. These measurements shall be made with different pressures of ‘scuffing’ or rubbing so that different amounts of charge transfer can be expected. If the decay time is short then it will be useful to retest at the same position to show consistency of results and lack of influence of the test on the sample surface. For this the surface voltage should be allowed to fall to a low level before retesting - for example less than +5V.

6.3. With garments measurements should be made on areas near the hem and near the waistline.

From observations recorded the values are obtained of peak initial voltage, the quantity of charge transferred and the rates of charge decay.

Where the initial decay rate is fast it is appropriate to estimate the initial peak voltage which would have been observed at the end of the rubbing period. If care has been taken not to rotate the PTFE rod during charging then the time of the end of rubbing is the time of the very start of the first voltage excursion which is when the rod is lifted from the test surface.

The results of testing shall be presented in a table which shall list for each test:

From the charge transfer and the V_pk (and V_t=0) the ‘effective capacitance’ is calculated by dividing the quantity of charge transferred to the test surface by the peak surface voltage. This is entered into the table. From the values in the table a graph is prepared showing the variation of effective ‘capacitance’ and the variation of decay time characteristics with the quantity of charge transferred.

The following information shall be recorded in the Test Report:

a) date of measurements, identity of test laboratory and name of test operator

b) description and identification of material tested and location of test area on a garment

c) history of test sample (e.g. number of washes)

d) temperature and relative humidity

e) test conditions (e.g. diameter of test area, material of ‘rubber’of corona charging voltage, duration, polarity and whether open or earthed backing

f) table of individual values of initial peak sample surface voltage, charge transferred and the time from peak voltage to 1/e of this voltage. (For computer stored data the file reference of the test). The table to include estimated values for the initial peak voltage at t=0 and the effective capacitance experienced by the transferred charge

g) graph of the variation of effective capacitance and decay characteristics with quantity of charge transferred

h) serial number of instruments used and date of most recent calibration