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Model 2501 and Model 2503
Keithley Inrtruments, Inc. Product Notes
2877s Aurora Road/Cleveland, Ohio 44139/U.S.A.
(216) 248GkWTelex: 98-5469 May 3, 1982 Rev. B
STATICCHARGEMEASUREMENTS
USINGKEITHLEYEQUIPMENT
INTRODUCTION with the Model 2501 and appropriate Electrometer, pro-
vided the system is recalibrated. That is, the Detec-
EleCCrOstetlC charge is a deficiency or exce*s Of elec- tor Head must be far enough away from the charged sur-
trons on en ungrounded surface. Charges ere readily face to avoid corona discharge to the Head end input
acquired o" poor conductors of electricity (such as cable length should be shortened.
plastics, synthetic fibers, fabrics, paper, and hydro-
carbon salids and liquids) during handling and indus-
trial processing of these materials due to friction GENERATION OF ELECTROSTATICS
with themselves, other materials, or machine parts.
The effect of static potentials is depicted in Figure It is necessary to study quantitatively the location
Once acquired, the charges are not easily dissi- and intensity of accumulered charges. The Keifhley
iced because of the law conductivity of the materia Model 250, or 2503 Static Detectors (described tn
Figures B and C) and a" appropriate Keithley Elec-
trometer are reliable, self-contained measuring systems
designed for measuring electrostatic charges a" webs
Of plastic, laminates, paper, or any relatively flat
surface where static charges are distributed.
FIGUKE A MODEL 2501 STATIC DETECTOR HEAD is
EFFECT OF STATlO POTENTIALS 3" in diameter and comes with a 10 foot
cable. IC gives a voltage divFsio" ratio
The annoyances and hazards of such static charges have of 10,OOO:l AlO% when held 3/g" away from
long bee" familiar to the plastics, paper, printing, e charged plane at least 3" in diameter.
and textile industries, and to those working with ex-
plosives, inflemnable liquids, or working in explosive "any static problems are encountered with stationery
dust or vapor atmospheres. Electrostatic f"rces cB"se films or laminates. Plastic begs clinging together
the attraction of d,,st and dirt, and cause self-attrac- or repelling each other would fall into this category.
tion of the charged material, which makes it difficult
to operate fabric looms, stack plastic bags, handle A potential may be induced o" the film or laminate
webs of plastic or paper, and many others. The co"se- for meesuring the leakage time by either of two methods.
quences of electrostatic spark discharges range from
the annoyance of personal shocks to a" explosion in a In the f,.rsL method, the charge is induced on the film
factory, arsenal, gtai" elevator, or gasoline tanker. by manual rubbing contact between the stationary film
or laminate and a rapidly moving cloth or tissue.
The Keithley gtatic Charge measuring systems consist This method is difficult to standardize and would also
of the Static Detector Model 2501 or 2503, which is tend to remove surface coatings of anti-static agents.
held "ear rhe surface to be measured, a" inter-connect- 1ts main virtues ere simplicity and speed.
ing low noise cable far the Model 2501, and a Keithley
Electrometer, used as a high impedance voltmeter, which `rhe second method of producing a static charge o" the
is calibrated to read the surface potential "ear the film or laminate is to bring it "ear a" assembly of
Detector. The Model 2501 measures static volts to 30K" needlepoints charged to a high dc potential of from
and the Model 2503 measures static volts fo 15K". Static 10 IX 50 kilovolts. The potential is set t" a known
voltages up to one megavolt ca" possibly be measured value end the film or laminate becomes charged bv the
Document Number 32426
ionized air and by induction. This method is more When the charged film "r laminate comes "ear e charged
quantitative then the first but requires cbe use of surface of the same polarity, a repulsive eleccroacatic
ecialized equipment. force will result. This force could be of sufficient
strength t" ceuse the fFlm to fly eway from e machine
part or to fly away from lower film layers during wind-
UP. Under such conditions, stacking of begs made from
plastic film could become difficult.
Static charges on plastic film will induce charges of
opposite polarity o" nearby ungrounded (electrically
neutral) machine parts. The resultant electrostetic
field will set up attractive forces between the film
and the machinery. The intensity of the electrostatic
field will depend upon the distance seperating the
machine part from the film and the magnitude of the
charges on the plastic film. This field may be strong
enough to cause sticking or janming of the film in e
processing machine. Methods of computing these forces
will be found F" the section entitled "Qusntitative
Evaluation".
The Keithley Static Charge measuring systeme are de-
MODEL 2503 STATIC DETECTOR PROBE signed to measure the static voltage on flat surfaces
Solid coaxial tube l/2" in diameter, co"- for evaluating the reduction of charge buildup.
sisting of a 3 l/2" heed, 3 l/2" coupler,
1" sdapter, and two 90" sngle adaprers Typical surfaces of interest ere sheets of psper or
which may be placed enywhere along the plastic when being used in processing. packaging or
probe. It gives e 10,000: 1 NO% voltage converting equipment. When acetic electricity prab-
division ratio whe" held l/4" away from lems ere suspected, measurements can be made on the
e charged plane et least l/2" in diameter. plastic webs or substrates es they move through the
machinery. The meesurements should be made acro(~s
the entire width of the web or substrate and a" aver-
APPLICATIONS age value recorded. Measurements et ~onee~t. points
would be made only to essess the explosion hazard of
THE EFFECTS OF STATIC ELECTRICITY the electrostatic charges in an inflammable atmosphere.
Dust end dirt will be attracted to moving or station- If the static charge of stacking end wind-up operations
ary plastic films or laminates which possess electro- of the web or substrate is to be determined, the
static charges. Thus anti-stetic measures must be used measurements should be made six inches before the
until e method is found which will reduce the potential wind-up or stack es well es on the wound-up roll or
on the film or laminate under the severest of stetic the stack. The measurements should again be made
generating conditions. ecross the entire width of the web or substrate and
en everage value recorded. These procedures will
Electrostatic charges ten cease spark discharges of provide a picture of the charge intensity before en-
sufficient energy levels to ceuse explasions in B" tering the roll or stack and on the roll or stack.
inflammable atmosphere. The minimum voltage necessary Calculating the effect of static charges on the ma-
for ignition may be 200 volts between close "eedle- chinery will be discussed later.
points and 2,500 volts for e discharge between spheres
l/2" apart. After the static problem is deffned, the effectiveness
of anti-seatic measures ce" be evelueted. St*tif
For e give" capacity between parallel plates, the en- meesurements should be made on the most convenient
ergy of the charge is proportional to the square of and represenrative area of the film or sheet in the
the voltage. (For example, the electrosraric energy processing equipment. The measuring system should be
of a plastic film charged to 1,000 volts is about 1,500 placed et the eeme spot on each rest run. so that the
ergs or 0.00015 joule; if the potential of the plastic factors which effect the formation of static will be
film is raised to 10,000 volts, then the energy rises kept the sate for each test run. Comparison runs with
to 150,000 ergs or 0.015 jwle.) If the use of a" e" snti-static measure and the control run should be
anti-static measure reduces the voltage on the film or made on the seme day so that temperature end humidtty,
laminate to one-tenth of ifs former value, the" the which affect static so much, will have the eeme effect
energy of the spark will be reduced to one-hundredth. on the static buildup on the different film samples.
If the minimum electrostatic Potential in volts re- The static charge acquired muet be measured immediately
quired to ignite a particular explosive gas is know". after charging in either cese co have meaningful re-
the Keithley Static Charge measuring system may be suits.
used to deter",ine whether a dangerous situation exists.
Anti-static measures can the" be evaluated es previ- Since the maximum ecquirable potential of films and
ously described. laminates and the leakage rate of the charge is depen-
-2-
dent on the relative irumidity and temperaeure of the high voltage connects to ground. The electrostatic
surrounding atmosphere, tbeee meaeurements should be field surrounding the needlepoints is Lhe force which
carried out under controlled humidity and temperature ionizes the surrounding air.
conditions.
An induction neutralizer oees grounded wire bristles
It is generally preferable to take readings on the or tinsel, which do not touch the film surface, and
underside of a film or sheet since this side has us"- which are fastened along metallic or wooden support
ally been in more intimate contact with the machine b***. The needles of theee neutralizers acquire e
parts. The Static Detector should be kept away from charge by inducrio" from the electrostaric field be-
the edge of the film because the electrostatic field t.,,een them and the electrostatically charged surface.
intcneity will be distorted in this area. The electric field surrounding the needlepoints is the
force which ionizes the surrounding air. The differ-
The static charges measured in a machine o" moving film ence between these neutralizers and high voltage "eu-
will vary slightly in intensity from point to paint, tralizers is the method of charging the needles. With
and the average or most consistent reading should be induction neutralizers, the higher the charge on the
used for calculations but both high and low peake surface, the greater will be the ionization of the
should also be reported. surrounding air.
The electrostatic propertiee of Stationary films or A radioactive neutralizer generally ueee either radium
laminates can be aasessed and eriti-static measures or polonium for ionizing the air. Here, alpha parti-
evaluated by making t"o static meaeuremente. The first cles are emitted from the disintegrating nuclei of
measurement serves to determine the maximum pofential radium, polonium, or other radioactive isotopes and
or voltage that the film or laminaee ca".acquire. the strike the air particles, ionizing them. The ionizing
second meas"reme"t, made at the same time as the first, range of alpha emission in air is limited to e" effec-
determines the rate at which the charge leaks away. tivc dietance of about three inches.
The charge leakage may be defined as the rime in eec-
ends required for the charge to fall to one-half of its The basic requirement for a" e"ti-static agent 18 that
original value. A plastic film or laminate can be con- if should prevent the accumulation of static charges
sidered "on-static when the leakage time falls below end leek them away rspidly when present. Many anti-
l/2 second. static agents ere selected primarily to leek static
charges from the film surface. They are effective
THE ElLMINATION OF STATIC CHARGE because they increase the electrical conductivity of
the film surface, either by increasing the concen-
The most cormonly used method for removing: saatic on tration of water on the surface through hygroscopic
film or laminates is the discharge of the static by a addition, or by splitting into ions.
conducting metal connection to ground. For example,
grounded tineel or brushes rubbing the charged surface Other anti-static agents rely on lubricating effects
are cormonly used. in order to prevent the generation of the cherges
through friction. Many agents Possess both conductive
Conductive discharge will have limited effectiveness. end lubricating qualities. 1t is generally believed
Only the charges picked up by the metal at the friction that egente which have lubricating qualities or con-
points will be removed. Conductive discharge is uee- ductive and lubricating qualities are the most useful
ful because it prevents any spark discharge which might on moving plastics.
coneritute a sefety hazard.
I" order to be comnercinlly useful en anti-static
Although air is one of the poorest electrical conduc- should:
tors, its conductivity can be markedly improved by
increasing it6 molecure content. If the air poaaesees 1. Se unaffected by normal handling.
sufficient condwti~i~y, the charges on the film thread 2. Not affect the physical properties of the film
or fiber surface will drain away rapidly. Also, when or laminate adversely.
the moisture content of the air is raised, a thin layer 3. Se relatively permanent.
of moisture may be depoeited on the film thread or 4. Be low in cost, safe to handle and if the appli-
fiber surface, making it more conductive. I" certain cation warrants, approved by the Food and Drug
cases, the application of Steam has bee" reconnnended. Adminietracio".
static neutralizers supply ionized air between the Numerous chemicals have bee" proposed as anti-stetic
grounded neutralizer end rhe charged film or laminate. agents for plastic films or laminates, but most belong
`rhe iontzed air particles are drawn o"t of the air and to one or the other of the following four classes:
neutralize all or pert of the opposite charges residing
an the film. There are three mai" types Of 8LBLiC neti- I. ~itroge" compounds, aucb ee long-chsi" amines,
tralizers in use: high voltage, induction, end radio- amides, end quaternary bases.
active. 2. Sulphonic acids and sulohonates, such as sodium
alkyl benzene sulphonate.
A high voltage neutralizer applies a high voltage to 3. Polyglycols end their derivatives, including
needlepoinrs placed along rhe neutralizer bar. These polyglycol esters of fatty acids and polYglycol
needles do not contace the plaetic web. A" alternat- aryl or alkyl ethers.
ing current of 5,000 to 15,000 volts is supplied to 4. Polyhydric alcohols end their derivatives, qs,
the needles from a conductor in the center of the bar. for example, sorbieol laurate.
The other terminal of the transformer euPPlYi*g the
OPERATION SUMMARY QUANTITATIVE EVALUATIONS
connectthe Model2501or 2503Detector to the Keithley The Keithley eeasuring *yetem mea*ure* the voltage of
electrometer, and connect a ground wire from the Elec- the surface to which it is exposed. It measure* valt-
trmneter case to a good external ground point (see age in Cead of coulombs, coulombs per cm', or electrons
figure D). Turn the Electrometer to the proper sensi- per cm3 because it is possible to build a voltmeter
tivity. with the special features needed for the task, while
direct meas"rementa of charge and/or charge density
j,ODEL 2501 OPERATION are much more cumbersome. I,, measuring and in eval-
uating the data obtained with the Keithley system it
,.,ith the slide all the way into the Detector Head, press is neceseary to keep in mind the cancepc that voltage
the INPUT SHORT button and set the Electrometer zero. is measured directly and charge and charge density are
Release the INPUT SHORT button.
position the detector Head 3/S inch from the surface
being measured, pull the slide ""t, and read the pot-
ential of the surface being measured.
A,, alternative method is to withdraw the slide irnme-
diately after zeroing, and then move the Detector Head
info posie~ion while maintaining ehe 318 inch distance.
Reliable results are obtained if the reading is made
within about fifteen seconds after removing the slide
and if the meter is not driven off scale while moving
the cup into position. The exposed target electrode
in the Detectm Head m"st not be touched.
&Q&&L 2503 OPERATION
set. the Electrometer zero by engaging the Electrometer
zero check BWLtCh.
Disengage .the Electrometer zero check switch, position
the Detector Probe l/4 tnch from the surface being
measured, end read the static potential.
The Detector-to-charged surface spacing m"sL be main-
tained accurately to within *l/32 inch, or errors in
voltage measurements can readily occur. FIGURE D
Schematic Representation of
In computing charge and charge density, further inac- Keichley Static Detection System with
curacies enter because the area of charged surface a Model 2501 Static Detection Heed
affecting the target of ehe ~ecector is not sharply
defined. Figure D shows the Keithley 2501 Detector Head and
Electrometer measuring a charged surface, giving the
Further, voltage of the charged surface after the significant electrical parameters. The Static ~etec-
Detector has been removed depends upon many capaci- LO= Head consists of B spun aluminum cup, holding a
tances which cannot be readily evaluated. So, here target electrode on a teflon insulator, The Detecting
again, the actual value cannot be known to the acc"8- Head also has a Slide, which is at ground potential,
tamed accuracy for physical meas"rements. a8 is the Conical Shield. When it is pushed into the
Shield, it places the Target in a volume which Fe free
The Keithley Static ~etecrors are carefully designed of electrical fields. The input Short switch connects
so that the ~ececeor target can be completely enclosed the Target to ground potential, when operated.
and brought to ground potential during zeroing. When
that Fs done, the complete change from zero LO the "n- Cl ta the capacbmce between the charged ~"rface and
known potential is applied BC~DBB cspacieance Cl, as the Target. C3 is the capacitance from the charged
described in Figure D. When the Electrometer ia zeroed surface Co the Conical Shield and rhe Target, wieh the
without the Model 2501 slide in place, the measurement Target connected to the Shield. C2 is the total ca-
will be the potential difference between the desired pacitance to ground of the Target, the connecting
unknown surface and some unknown arbitrary potential cable Hi conductor, and the inp"e circuit of the Meter.
affecting the detector target when the Electrometer The Meter is an Electrometer Voltmeter which has a"
Was being zeroed. input resistance greater than 1014 ohms. It* purpose
is to measure the voltage of capacitor C2. There is
Rezero fhe Electrometer when changing ranges and be- no si&ficant charging of C2 by the Electrometer
tween readings. And remember to observe correct during the time meaaorements are being made.
Detector-to-charged surface distances. Also, the
Keithley Electrometer must be used on the VOLTS position.
-4.
Quantitatively: Assume 5000 square inches is the total sres of the
sheer which is carrying charge through the potential
clE1=C2E2 where El is the potential SC~OBB Cl and difference. The total charge is:
E2 is the potential *cro88 C2. Since charge can
be defined 8s Q=CE, then the charge on Cl and C2 is q = 8.4 x 10-9 (coulombs/in2) x 5000 (sq. in.)
equal. C2 is principally cable capacitance and is
constant since a fixed length of cable is used. Inserting these values in (3) gives:
Cl is determined by the dimensione of the Target, w = 6.3 joule8
the Conical Shield, and the distance from the
charged surface to the Detecting Head and is chosen If thFs work wsa accomplished in one-tenth second, 63
to give adequate sensitivity and a Head size con- watts would be required. This is an appreciable amo"nt
;gn;;;t to use. E2 is El/lO,OOO and Cl is C2/ of power to be transferred from the mechanical syetem
2 . and put into the electrical system. Such a power
level indicares that ststlc electricity generation
Knowing the voltage of the charged surface, the total requires eubstancisl consideration in the design of
charge in the area affecting the target 1s determined equipment handling chargeable materials, in the design
by of static removing equipment, and in the desirability
Q = C3E1 Q coulombs (1) of finding some mesns to prevent the generation of
C farads static electricity in the first place.
E volts
When the Head is withdrawn from the charged surface,
with the Detector Head 3/S inch from the charged sur- which is presumed to be an excellent insulator, there
face, the area inside the cone, which is the part that 18 no addition or reduction of charge. But the ca-
affects the target, has a capacitance C3 of about 2 pscitsnce of the surface to ground~ls reduced consid-
micro-microfarads to the grounded surfaces. Assume the erably, depending on how msny other conductors st
surface potentisl El was measured to be 5000 volts. ground pqrential sre in the imediate vicinity unless
If the charged surface were backed by a ground plane, the charged surface is large enough that the change
then the effective value of c3 and the stored energy in capacity is negligible.
would be much grester. Solving (1) :
From equation (1) El = &; thus, El increases 88 C3
Q = 2 x 10-12 (farad) x 5 x 103 (volts) is -->..^^_I
Q = 10-S coulombs
The diameter of the surface inaide the cone is 3 inches;
the area is therefore 7.1 square inches.
With a tocal char%= of 10-S coulombs, the charge den-
sity is 1.4 x 10 coulombs per square inch.
charge density as measured by the Model 2501 Detector
Head is expressed:
aEm - 2.8 x 10-13 El coulombs/in2 FIGURE E
AREA MODEL 2503 STATIC DETECTOR PROBE TIP
The energy expended in moving charge through s poten- Partial curaway ahowing internal con-
tial gradient is expressed: ~truction and probe target area. Drawing
does not show the coupler or adapters
w = l/2 QE that make up the probe assembly.
W is work in joules (3)
Q is the total charge, in coulombs The voltage of a surface which has been measured with
E is the potential difference through which the the ElectromeLer, because of the reduction in C3 as
charges are moved the head is withdrawn, ranges from about twice the
reading on the meter down to exactly chat read by the
Q is obtained by taking the charge density on a web meter depending upon the mobility of the charges on
or sheer of plastic or paper as computed above, then the surface and upon the change of capacitance of the
multiplying it by the area that is affected by the surface ss it is being measured end as it is being
processing machine. used.
E is the voltage difference, and can be from zero Conducting surfaces attached to a low impedance so"rce
(assuming that no charge existed on a reel of plastic of voltage, such 88 a battery or rectifier power
before it we unwound) to the El read by the Electrom- supply, however, gain and lose charge as the externsl
eter (assuming that the voltage was measured on the capacitances at their outputs change. Thus, their
web within a short distance from where it parted from potentials are not affected by the DetecCing Head.
the rollJ. If El was measured at 30,000 volts, the
charge density would be: Greater sensitivFty can be obtained by increastng Cl
and/or decreasing C2.
Charge Density = 2.8 X 10-l' ~1 (co"lom;s/in2)
= S,4 x lo-9coulombslin (4)
-5-
Cl is determined by the Target area and its spacing The Model 2503 Static Detector Probe may be evaluated
'from the charged surface. This is determined by the quantitatively as is the Model 2501. Figure E is a
dimensions of the Conical Shield and is difficult to cutaway sketch of rhe Model 2503 showing interior
modify without extensive machining. A new Detecting details.
Head could, of course, be fashioned by the user to
meet his specific requircmencs. Contact your Keithley Sales Represenrative for a full
description sod price of each of the complete lFne of
C2 is principally the capacirance of the connecting Keithley Electrometers and both Static Detectors.
cable, and will be reduced directly as the length
of the cable is reduced. Eliminating the cable al-
together produces a substantial increase in seositiv-
ity, but makes the instrument physically awkward to use.
If, in addition to static charge measurements you also
wish to investigate other material phenomena such as
charge dissipation, consider the Keithley Volume and
Surface Resistiviey system which is comprised of our
Model 6105 Kesistivity Adapter, Model 6lOC Elecfro-
meter, end Model 240A High VoltsgpSPower Supply.
Surface resistivieies of up tp,lO ohms and volume
resistivities of up to 3 x 10 ohm-cm cm be readily
measured with this Keitbley system which is in accord
with the ASTM Standard Method of Test D257-66 for
Electrical Resistance of Insulating Maeerials. For
further information see the Keiehley Product Notes
entitled "Results and Techniques of Volume and Surface
kesisfivity Measurements Using Keithley Instruments".