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MODEL
510
MEGOHMMETER
AND ACCESSORIES
KEITHLEY INSTRUMENTS, INC.
CLEVELAND, OHIO
CONTENTS
SECTION
INTRODUCTORY
*,.,...,,*......****.I.*..,..........*...* "I
SPECIFICATIONSANDDESCRIPTION........................ II
OPERATION....,.....'..................,................ III
MAINTENANCE'
~,......,,................,................. Iv
General
Circuit schematic
voltage and resistance diagram
Replaceable parts list
APPENDIX
Accessories..................,...,...,....,...,.,. i
Model 51.036 Test Leads
Model 5101 Component Adapter
Model 5102 Volume Reoistivity Adapter
Models 51024 and 51060 Cables
Measuring technique ..,.,.,............... . ..* . . . . . ii
Introduction
Speed of indication
Component measurements
Volume resistivity
Special shielded enclosures
Use of other than standard test potentids
ASTMReprint *.*....*.*... . . . . . . . . . ..*.......*.*...... Rear Cover
510
If58
SECTIONI INTRODUCTION
The Keithley Model 510 Megohmmeter utilizes a unique logarithmic circuit
to present six decades of resistance on a six-inch mirror-scale panel
meter. With so great a dynamic range, range switching is not necessary,
and the speed and ease of measurement are greatly increased.
Because of the logarithmic ocale, the usual high-end scale compression of
a conventiona, ohmmeter is eliminated. An accuracy within l$ of the scale
length is maintained at every point on the scale; this is equivalent to an
accuracy of about lO$ of the reGiotance being measured. It is limited only
by the meter accuracy, and is about as good a6 conventional megohmmeter
circuits at mid-scale, but superior to conventional circuits at other than
mid-scale meter readings.
Test potentials of 5, 50 and 500 volts are furniohed. They are useful in
measuring voltage coefficients, and offer freedom in selecting a safe
potential for all test samples.
I -1
5'0
1158
SECTIONII
MODEL510 SPKIE'ICATIONS
L_ _
Range and Test Potentials:
Test Potential.
_-- Resistance Span
5 volts 107 to 1013 ohms
50 volts 108 to 1Ol't ohms
500 volts 10'3 to 1015 ohms
ACCURACY: within 15 of scale length, uniform over the entire scale.
After a 15-minute warmup, calibration drift is negligible over an
eight-hour period.
REGULATED TEST VOLTAGES: Regulation within 0.01s for line voltage
changes from 100 to 130 volts.
OPERATING CONTROLS the Test Potential-Calibrate
are Switch and a'three-
position lever switch, Test-Charge-Discharge. The lever switch allOW
discharge of capucitors, quick charge, and pro-electrification when
desired.
PR0TECTIVEFEATURNS: All test potentials are removed when the operating
switch is in the discharge position. Similarly, a switch is included in
the test fixtures (Models 5101 and 5102) that automatically grounds al.1
electrodes when the cover is raised. The instrument cannot be damaged
by short circuits.
ACCESSORIES SUPPLIED: Model 51036 Test Leads, 36 inches long, separate
, Hi and ground leads terminated in alligator clips. The leads are useful
bout 1012 ohms with a 5-volt potential, 1013 ohms with
;yv"dl;zy z; gift ohms with 500 volts. For higher readings, the 5101 or
5102 adapters should be used.
ACCESSORIES AVAILABLE: Models 5101 and 5102 Test Adapters; Models 51024
and 51060 Cables.
CABINNT is aluminum, 6,-5/o" x lO$' high x 12$' deep. Net weight, 164 pounds.
II - 1
510
11'58
CMCUIT DESCRIPTION
The Model 510 Megohmmeter consists of a regulated voltage supply which
provides 5, 50 and 500 volt test potentials, and a logarithmic micro-micro-
/ ammeter which meters the sample current. The panel meter reads ohms directly.
Figure 510-l shows a simplified diagram
of a resistance measurement using the 510.
The positive terminal of the test potential
is grounded and the negative terminal is ap-
plied to the unknown specimen through the
micro-microammeter. The voltage drop across
the input of the micro-microammeter is small
T FIGURE
IlO-! oompared to the test potential. Using the
negative terminal of the voltage supply as a
shield around all high impedance input leads to the micro-microammeter assures
negligible error due to leakage across the input terminal insulation. The
method is called guarding and the negative terminal of the supply is referred
to as the guard potential.
Figure 510-2 is a simplified diagram of the micro-microammeter used in
the Model 510.
The following circuit description refers to DR 11504-C in MAINTENANCE,
Section IV.
The power supply comprises a Sola regulating transformer, a rectifier-
filter system and a two-stage electronic regulator. Selenium rectifiers SR5
through SR9 provide a half wave rectified output which is filtered by R401,
c402 and C403. R&Q and X403 are used to equalize the voltage across the
series capacitors.
Following the filter is an electronic regulator employing ~6 as a series
tube and V7 and Vg as a two stage amplifier. Vg acts as the error detector.
The output of the 500 volt supply, divided dolm by R415, R411, and R410, is
compared to voltage reference tube ~8. R411 is used to set the test poten-
tial at 500 volts. 50 and 5 volt test potentials are obtained from the
divider consisting of R415, R414, X413 and R412.
The micro-microammeter consists of shunt diode, Vl, and an electrometer
amplifier which measures the potential across diode Vl. This potential is
proportionalto the logarithm of the current through the diode. The grid
and filament of Vl act as the diode. The electrometer amplifier consists of
two 5886 tubes, V2 and V3, operated as pentodes followed by V4, a l2AU7
twin triode differential amplifier. Local feedback from the triode cathodes
to the pentode screens stabilizes the operating point of the electrometer
II - 2
tubes. Overall negative feedback from the 6C4 output cathode follower, V5,
to the electrometer tube filaments stabilizes the aNplifier gain. The
power supply for the micro-microammeter is derived from the Sola trano-
former through transformer T2 and utilizes conventional rectifier-filter
systems.
The megohmmeter is calibrated by means of ~1.28and Rl2g on SW3. With
107 ohms in the circuit (R127), the meter is set to 107 ohms by adjusting
diode bias with R106. Subsequently the meter is Bet to 1011 ohms by adjust-
ing amplifier gain with H124. Occasionally amplifier balance should be
checked by turning SW3to lx, 10x or 100x position with the lever switch
in DISCHARGE. Under these conditions the panel meter should read 107 ohms.
If adjustment is necessary, R115 is used.
SECTIONIII OPERATION
The Model 510 Megohmmeter is shipped with the Model 51036 Test Leads. To
use this combination for measuring resistances, the following procedure
should be followed:
a. Plug the power cord into 110 volts 60 cps ac. A Sola constant volt-
age power transformer is used in the Model 510, and ite proper functioning
depends upon a constant power line frequency of 60 cps. A 50 cps model is
available.
b, Turn the Test-Charge-Discharge switch to Discharge.
c!. Turn the Calibrate-Test Potential switch to Calibrate, 107 ohms.
a. Turn the power switch to ON, and wait a minute or two for the tubes
to reach operating temperature.
e. Set the Cal 107 potentiometer located on the panel under the meter,
BO the meter reads exactly 10-f.
f. Turn the Calibrate-Test Potential switch to Calibrate 1011 ohms.
Adjust the Cal 1011 potentiometer 80 that the meter reads exactly
10ILg*
h. Connect the test leads to the Megohmmeter by fastening the connector
end to the input connector on the front panel. Clip the free ends to the
unknown resistance. The small, flexible wire ie at ground potential. The
larger coaxial cable is the guarded HI lead.~
i. Turn the Calibrate-Test Potential switch to apply the desired po-
tential to the unknown resistance.
3. Switch the Test-Charge-Discharge switch to Charge, and then to Test.
The nature of the unknown determines the length of time to remain in the
Charge pooition. If there is little capacitance (0.1 mfd or less) acroBB
the unknown resistance, a second or SO is long enough; greater capacitance
III - 1
will require a longer wait. The Model 51036 Test Leads employ a capacitor
to reduce the effects of 60 cps pickup, which slows the response appreciably
/
when measuring reaiBtance8 above about 1Ol-l ohms. Some rcsitance specifi-
cations require an "electrification time", which is the interval between
applying the test potential and reading the resistance. The ASTMpamphlet,
included in this Manual, discusseo this in detail. The electrification time
is measured from the time the Test-Charge-Discharge switch is moved from
Discharge to Charge.
k. After meaaming, return the Test-Charge-Discharge switch to Discharge.
The instrument is now ready for the next test specimen. With the switch in
the Discharge position, all testing conductor8 are at ground potential, 80
that specimens can be chcangedwithout danger of shock to the operator.
After the instrument has been connected, turned on, warmed up, cali-
brated, and the test voltage selected, the operating procedure for measur-
ing a number of resistances is very simple. Just connect the unknown to the
test clip8; turn the Test-Charge-Discharge switch to TEST; read the meter;
return the switch to DISCRARGE: and then change to a new unknown resistor.
1. After the instrument ha8 been operating for about five minutes, it
should be realigned to eliminate the effects of warmup drift. After this
ha8 been done, no more calibrations should be necessary during the following
eight hour period.
m. To rebalance the amplifier, eet the Calibrate-Test Potential switch
to 5 volts, end the Test-Charge-Discharge switch to Discharge. Then adjust
the Amp. Bal. potentiometer, on the left side of the cabinet, 80 that meter
reads 107. This adjustment is required only every month or 80.
The operating procedure is the same for test leads, fixtures, or the
Keithley 5101 or 5102 Adapters. A wide variety of specialized setups is
discussed in the Appendix.
When designing special testing fixtures, leads, or electrodes, provision
must be made for operating the relay in the Model 510 to remove the short
circuit at the input of the micro-microammeter. Pins 2 and 3 of the connector
carrying the cable from the unknown to the Model 510 input can be jumpered
in the connector, or a switch to connect them can be made a part of the
special fixture, following the design of the switch in the Modela 5101 and
5102 adapters.
Reading the Meter: Fig. 510-3 is an en1 rged
sketch of one decade of the meter scale. 108 end
109 are the major division marks. The integer8
between them are also marked, with the 5 line made
longer and heavier for quick identification. The
other decade8 are read in a similar manner.
III - 2
510
l/58
-,-.--_-- IV MAINTENANCE
SECTION
A. General
The Keithley Model 510 Megohmmeter has been designed to give long,
trouble-free service. High quality components have been used throughout.
DR 115044, page IV-2, is the detailed circuit schematic diagram of
the Model 510. The circuit operation W&Bdiscussed in Section II,
DESCRIPTION.
Maintenance Adjustment is R&11, which sets the 500 volt test potential.
As disxssed in GfiE?c small variations from.500 volts do not affect
the calibration of the Model 510; but for proper functioning of the power
eupply, it should be set within about 5% of 500 volts. TO readjust
the test potential with R411, connect a high impedance voltmeter (20,000
ohms per volt) from Guard to Ground, set the Calibrate-Test Potential
switch to 500 volts and the Test switch to TEST. Adjust Rkll for 500 volts
test potential.
Resetting R411 i8 not necessary under normal circumstances, even though
the tubes in the regulator have been changed, but it should be checked after
a tube change.
The tubes used in the,V2 and V3,positions are selected, matched and
labelled ~~-5886-5 and ~~-5886-6 respectively. Replacements are sold in
pairs only. The tube used at Vl is not selected.
When inspecting or replacing the electrometer tubes, the glass base
should not be touched with the hands, because the dirt and moisture will
cauu8eleakage from the grid to the other electrodes. Also, when soldering
high impedance conductors to the teflon standoffs, care should be taken to
keep the teflan clean,
All vacuum tubes, other than the 5886'~ are conventional, and selection
of replacements is not necessary.
Calibrate - Test Potential Switch and RN1 should be inspected period-
ically, and any dust which has accumulated on the insulation should be re-
moved by brushing with a camel's hair brush.
Servicing. DR 11698-C, page IV-3, shows the tube layout. Included
are the voltage and resistance measurements which may be made at each
tube element under specified conditions.
IV - 1
-.
Y
5
i
--T
VOLTAGE AND RESISTANCE WART
c
REX'LACEP&E
PARTSLIST - MODEL510
---r- -- --
circuit Part
Desig.
--.- Description - .-.-.-A".
Cl01 Capacitor, electrolytic, tubular, 1000 nffd, 12V. Cll-1000
Cl02 Capacitor, ceramic disc, .02 ma., 600~. c22-.02
c401 Capacitor, oil., 1.0 tid. (supplied with Tl) .
c402 Capacitor, tubular, electrolytic, 30 mfd, 450V c8-30
c403 Same as C402 -
c404 same as Cl02 -
c405 Capacitor, molded paper, 0.1 mfd, 600~ c2-0.1
~406 Capacitor, tubular, electrolytic, 40 mfd, 250V C27-40
c407 Same as cho6
C408 Same as ~406 .
c409 Capacitor, tubular, electrolytic, 150 mfd, 15OV. C23-1.50
C42.a: Same as ~406 -
c411 Same as C405
C412 Capacitor, tubular, electrolytic, 4 mf'd, 600~. c35-4
Fl Fuse, 1.5 amp. 3AG FU 8
M Panel meter, O-200 microamp ME9
RlOl Resistor, composition, 22K, 16, $7 Rl-22K
R102 Resistor, deposited carbon, 50 ohm, lgb, $I R12-50
B103 Resistor, deposited carbon, 150 ohm, R12-150
RlO4 Resistor, deposited carbon, 2K, l$, ;W R12-2K
R105 Resistor, deposited carbon, 300 ohm, l$, -$W ~12-300
R106 Potentiometer, composition, 1K RI'13-1K
R107 Resistor, composition, 22M, 10% Rl-22M
R108 Resistor, deposited carbon, 250 ohm, l$, &J R12-250
RlOg Resistor, deposited carbon, lOM, l$, 1W R13-10M
RllO Resistor, wirewound, l2.5K, 3'$, '7W ~7-12.5~
Iv-4
lU?PLACEABLI?
PARTSLIST - MODEL510
Circuit
--r----7--
r*rt
Des!.g.
_-._--. Description - -__Iio.
Rlll Same as RllO --t--
R112 Same as R109
R113 Resistor, deposited carbon, 200 ohm, l$, $J R12-200
R114 Same as RI.13
R115 Potentiometer, composition, 200 ohm RP13-200
R116 Resistor, deposited carbon, 4,3K, l$, $QJ R12-4.3K
R117 Same as X104
~1.18 Resistor, deposited carbon, 60K, I$, & ~1.2-609
Rll9 Same as R11.8
R120 Resistor, clepositerl carbon, 1.5M, l$, $W R12-1.5M
R121 Resistor, deposited carbon, 4.45M l$, & R12-4..45>
R122 Resistor, deposited carbon, 200K, l$, 31 R12-200K
R123 Resistor, deposited carbon, 70K, l$, $J X12-70K
~124 Same as RlO6
R125 Resistor, deposited carbon, 5K, l$, $J R12-5K
~126 Resistor, deposited carbon, 250 ohm, J+$, 4W ~12-250
R127 Resistor, deposited carbon, 1:.7X, l$, $J R12-47K
R128 Same as Rl.09
Rl29 Resistor, hi-meg, 1011 ohms R20-1011
R130 Resistor, deposited carbon, lK, 1%) $+J R12-1K
R131 Resistor, deposited carbon, )
"OK, l$, 1W Rl3..50K
X132 Same as Xl15
x401 Resistor, composition, 680 ohm, lO$, +W Rl-680
R402 Resistor, composition, 22OK, lO$, 1W R2-220K
R403 Same as ~402
__--___--_---____-l___ -___
IV - 5
510
l/58
RXPLACYXBLE
PARTSLIST - MODEL510
.
-
/ Circuit part
Desig.
-- Description -.__ No.
R404 Resistor, deposited carbon, 200K, l%, $I R12-200K
p405 Same a8 R404
R406 Resistor, wirewound, WOK,l$, LOW R30-80K
R407 Resistor, depoa-i.ted carbon, 470K, I.$, SW RI2 -470X
Rho8 Same *a R407
R'k09 Resistor, dcpositcd carbon, 60K, l$, 31 R12-6OK
R4LO Resistor, deposited carbon, 390K, l$, $4 X12-390X
R4ll Potentiometer, composition, 1OOK RP12-100
R412 Resistor, deposited carbon, lK, l$, 1W R13-1K
R413 Resistor, deposited carbon, PK, l$, 1W Rl.3-9K
R414. Resistor, deposited carbon, 1OK, I.$, 1W R13-10K
i R415 Same s.s R406
~416 Resistor, composition, 470 ohm, lO$, & m-470
RUT same .a8 RJ+16
R418 Real&or, composition, I.00 ohm, lO$, $W RI-LOO
R419 Resistor, wirewound, 1250 ohm, 5$, 5W X4-1250
R420 same as ~416
R421 Same as X419
R422 Resistor, composition, l.OOK; lO$, 2W R3-100K
la1 Relay, special RLLM
SRl Rectifier, selenium bridge m6
SR2-SR9 Rectifier, selenium, 130~, 65 ma printed circuit style RF10
SRlO Rectifier, selenium, 13Ov, 65 ma m'8
SRll Silicon alloy diode, Pacific Semiconductor #~~695 RF13
SW1 SPSTBat Randle toggle switch SW-l.4
-
PJ@LACJZABLE
PARTSLIST - MODEL510
/ c -
---.- - Description
i SW2 DPDT Rs.t Handle toggle switch (center off)
' SW3 3 pole, 5 position rotsxy switch, special
Tl Power transformer, Sola 71354 (includes CJ+Ol)
T2 Power transformer Stancor ~~841.6
Vl VWUUDI tube, type 5886
v2 vacuum tube, type sold 8,s matched pair w-5886..
v3 vacuum tube, type 5886 ~~-5886..
v4 Vacuum tube, type 12AV7 EV-12AU7
v5 vacuum tube, type 6~4 ~-6~4
v6 vacuum tube, type 12~4.1'. EV-l'B4A
W Vacuum tube, type 12AX7 EV-12AX7
V8 Vacuum tube, type OG3 EV-OG3
(, same as v7
v9
Input connector, 5 conductor, Cannon RSK-U4-3lSL
Pilot lamp, #51 (two per instrument)
--- -
IV - 7
I APPENDIX
Section i Accessories
__ _-.--_
ModeI 51036 Test Leads consist of a 36 inch double shielded cable,
--__
a 36 inch ground lead and a connector which mates with the Model. >lO
input connector. The leads are terminated with alli.gator clips.
Use of the Model. 51036 is described in Section ii, Component Measure-
merits.
Model. 5101. Component Adapter is a shielded enclosure to be used with
-Y-M
the Model. 510 when tosti~esis-tars, capacitors and other small
parts. It is designed to attach directly to the Model. 310 Megohm-
meter, See Sectiou ii, Component Measurements.
Model 3102 ._.-.. _ Hesistivity -I_
_- Volume
. Adapter is a shielded enclosure, exter-
nally similar to Model 5101. It has a set of electrodes which facili-
tate making volume resistivity measurements in Section ii, Volume Resistivity.
Models 51024 and 51060 Cables are used with either of the above
_-A-
adaptors when it ie desiraa`d to separate the adapter from the
Model 510. The cnbles are 24 and 36 inches long, respectively.
Section ii Measuring Technique
INTRODUCTION
-----
The American Society for Testing Materials has prepared a booklet,
D257-5&T, "Tentative Methods of Test for Electrical Resistance of
Insulating Materials," which describes the procedures for measuring
the important parameters of insulating materials. A reprint is
included as a part of this instruction book.
The Model 510 with its accessories is an excellent instrument for
making the tests, and the following paragraphs will describe testing
procedures in detail, with special emphasis on the precautions neces-
sary to obtain accurate measurements of the very high resistances
which the extreme sensitivity of the Model 510 permits.
Three major difficulties are encountered in measuring extremely hi.gh
resistances. They are: a) 60 cps pickup by the high impedance con-
ductors; b) spurious current leakages; and c) charges and currents
generated by sliding or deforming the insulating sample. "
60 cps pickup is eliminated by shielding the input, test circuit con-
auctors, and the specimen, as in the Models 5101 and 5102 adapters;
or by using a capacitor across the micro-microammeter as in the special
case of the Model 51036 Test Leads.
Spurious current leakage is avoided by using excellent insulation ma-
terial, such as teflon or polyethylene, in the test leads and fixtures,
and by guarding all leakage paths.
The spurious charges, or "static electricity," can be avoided by handling
the test sample as little as possible before measuring it. A 200 foot
piece of pol ethylene coaxial cable, for example, can lose charges at the
rate of 10-Y ampere for.several hours after it has been flexed moderately.
Electrification time, voltage and temperature coefficients, and moisture
or solvent content are also important consiclerations when measuring insu-
lation resistance.
SPEEDOF INDICATION
The measurement of the leakage resistance of capacitors with the Model 510
involves long times when the product of the resistance and capacitance is
large. The capacitor is charged quickly to the test potential by the
Test-Charge-Discharge switch, bringing the reading on scale, but reading
equilibrium is often a slow process. The following empirical expression
gives T, the time in seconds for the 510 to read within lO'$ of the final
resistance value:
TX W
RC where R is the leakage resistance in ohms
C is the capacitance in farads
E is the test potential in volts
a-l
Faster response can be obtained by using a battery to furnish the test
potential, and Keithley Models 200B and 2008 combination to read the
leakage current. Using the 2OOBon the 0.008 volt range permits the
lowest practical shuntresistor in the 2008, which is substantially
smaller than the effective diode resistance in the Modal 510 at equi-
librium.
As t-m example, consider the measurement of a 5000 mmf capacitor with a
leakage resistance of 1013 ohms. The 510 with 50 volts applied to the
sample will read within lO$ of 1013 in about 4 minutes. The 20013, using
the .008 volt range, with a 109 ohm shunt and 50 volt supply will read
within 10% of final current in about 12 seconds.
COWOWNTMJZRSUiXMENTS
- _~-_
Measuring with Modal 51036 Teat Lea&s: The Mod01 510 is shipped. with
the Model 51036 %st Leads for general purpose use. Tnsse leads are
intended for measuring unshielded objects such as resistors, insulating
terminals, and <.he coil-to-coil and coil-to-frame insulation of motors
of transformcrr: ~
The TestC1iars.e -Dizharge switch should be in the Discharge position when
changing specimen;, otherwise the test potential appears across the alli-
gator clips. ThiJ is particularly dangerous when testing capacitors at
500 volts.
When measuring capacitors, the twitch should remain in the Charge position
long enough to assure t'nat the capacitor is completely charged before mov-
ing the switch to the Test position.
The effects of 60 cps pickup by the specimen ars eliminated by a 500 mmf
capacitor located in the connector housing and connected across the input
of the micro-microammeter. Because of the capacitor, however, the response
of tho amplifier and meter are slowed. $.or instance, to read gO$ of f!!;ial.
value in 2.5 secon3.8, one may measure lo*--- ohms using 5 volts ted pot-,;,.-
tial, ,1012 ohms with 50 volts, and lo12 ohms with 500 volts. jIi &--;a 1`6; _
r 0.d
ings require proportionately longer times. For fast measurement of high
resistances, the object being measured must be shielded from 60 cps pick-
up, and the micro-microammeter not slowed with a capacitor.
The detailed steps of lining-up and making measurements with the Model 510
were given in Section III, OPERATION.
Measuring with the Model 5101 Component Adapter: The Keithlcy Model 5101
-.---- -.-
Component Adapterisaenient shielded enclosure for holdin,g electronic
components, such as resistors or capacitors, while their. resistance is being
maasured. It connects directly to the Model 510. The circuit schematic
diagram is given on DR 11504-C, the main diagram for the FFor most components, the most convenient connections are two spring clips
on banana plugs (Grayhill@-l), one plugged into GNDand the other into
HI. The component leads slip easily into the springs which hold the com-
ponent from touching the box or the relatively low resistance insulation.
Two clips are furnished with each 5101 Adapter.
a .- 2
Closing the lid of the box operates switch SWl. One section ungrounds
the Guard circuit, and the other operates relay Rl in the Megohmmeter,
which opens the input to the micro-raicrosuumeter.
,
Opening and closing the lid performs all the necessary switching when
changing samples.
mi% RXSISTIVITY:
Volume resistivity is determined by measuring the resistance of an insula-
tion sample, as described in the ASTMSpecification Section 4 (b), and
reducing the geometry to an equivalent cube, as shown in Section 9 of the
Specification. Fig. 4 shows the most popular electrode configuration
whose dimensions are usually measured in centimeters, giving the volume
resistivity in ohm-centimeters.
r-i CND r-----l
The Keithley Model 5102 Volume Resistivity Adapter provides the desired
electrode configuration in a shielded box which can be connected directly
to the Model 510. Graphite, or aquadag, or silver conducting paint elect-
rode patterns should be painted or stencilled on the surfaces, as described
in the ASTMSnecification. The electrode dimensions of the Model 5102 are
given below.
The conversion of the resistance read-
ing of a sample into resistivity units
is given by
where P 5 resistivity in ohm-centimeters
- DE center electrode diameter in
inches
t 5 sample thickness in inches
R. resistance reading in ohms
The electrode structure in the Model
5102 is designed to be used with a center
electrode diameter of two inches. For
this case,
Pr 8 R ohm-centimeters
t
a-3
While the diameter of the specimen is set by elec-trade structure, any
thiCkncS8 of sample up to i$ inch may be tested.
Fig. 510-6 showo a holder for
large rigid ~lpecimeno which are
not suited to the Model 51.02.
A spring in used on the end of
a rod to make the connections.
Note that the hi&h impedance
oonductor is well guarded.
If the specimen has a resistance
greater than about 109 or lOlo
ohm, the test fixture should be
inside a grounded enclosure to
prevent 60 cps pickup by the HI conductor
The Keithley Model 5101 Component Adapter makes a convenient encloser for
measuring amal. rigid samples. A table can be plugged into the ground jack,
and springs which contact the HI and Guard electrodes can be connected to
plug8 which are inserted into the HI and Guard jacks.
Fig. 510-T 18 a dia-
gram of the Model
5101 used in this
fashion.
Measuring Volume Resistlvity using the Model 5101 Component Adapter.
If temperature coefficients of resistance or the high temperatLn?e perform-
ance of insulating material are of interest, the specimen and holder are
mounted in an oven.
For temperatures up to about l2O'?F, the Model 5101 or 5102 adapters are
suitable. Acceasoryfcables Models 51024 or 51060 are available for con-
necting between the Models 510 and the adapter.
Above l20%, a special fixture is required, deeigned to
tithetand the heat. Fig. 51.0-8 shows such a fixture in
an oven connected to the Model 510. The insulation in
the fixture is ceramic or tefl.on, and the connecting
cable usee teflon insulation, because polyethylene
softens at elevated temperatures.
a-4
510
1158
The Guard and HI electrodes must be mounted 80 that at operating temperatures
they are in the same plane.
Fig. 510-8 shows the connecting cables passing directly through holeo in the
oven walls. If it io desired to mount connector8 in the oven walls, Fig.
510-g shows the details of a typical installation. Teflon insulated con-
nectors, such ae MIL Type 0-239A should be used. The shell of the HI: lead
connector which ia operated at Guard potential must be insulated electrical-
ly from the oven wall which should be at Ground. Someprotection must be
provided for the operator, because the exposed parts of the plug can be
500 volts below ground.
The metal housing of an oven generally makes a good shield against 60 cps
pickup, and should be connected to ground. Difficulties may arise through
pickup of the 60 CPR voltages from exposed conductors or heating coils and
from door hinges which are not conducting and tend to insulate the door
from the cabinet. In most c&see, no shielding in addition to the grounded
metal oven and shelves if! necensary.
One of the constituents of teflon is fluorine. At room temperature, teflon
io chermically stable, but when elevated above about 500