Ludlum Measurements 3-98 User manual

LUDLUM MODEL 3-98
SURVEY METER
June 2020
Serial No. 346285 and Succeeding
Serial Numbers

LUDLUM MODEL 3-98
SURVEY METER
June 2020
Serial No. 346285 and Succeeding
Serial Numbers

Model 3-98 Survey Meter
June 2020
STATEMENT OF WARRANTY
Ludlum Measurements, Inc. warrants the products covered in this manual to be free
of defects due to workmanship, material, and design for a period of twelve months
from the date of delivery. The calibration of a product is warranted to be within its
specified accuracy limits at the time of shipment. In the event of instrument failure,
notify Ludlum Measurements to determine if repair, recalibration, or replacement is
required.
This warranty excludes the replacement of photomultiplier tubes, G-M and
proportional tubes, and scintillation crystals which are broken due to excessive
physical abuse or used for purposes other than intended.
There are no warranties, express or implied, including without limitation any
implied warranty of merchantability or fitness, which extend beyond the description
of the face there of. If the product does not perform as warranted herein,
purchaser’s sole remedy shall be repair or replacement, at the option of Ludlum
Measurements. In no event will Ludlum Measurements be liable for damages, lost
revenue, lost wages, or any other incidental or consequential damages, arising from
the purchase, use, or inability to use product.
RETURN OF GOODS TO MANUFACTURER
If equipment needs to be returned to Ludlum Measurements, Inc. for repair or calibration, please
send to the address below. All shipments should include documentation containing return shipping
address, customer name, telephone number, description of service requested, and all other necessary
information. Your cooperation will expedite the return of your equipment.
LUDLUM MEASUREMENTS, INC.
ATTN: REPAIR DEPARTMENT
501 OAK STREET
SWEETWATER, TX 79556
800-622-0828 325-235-5494
FAX [phone]

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MODEL 3-98
SURVEY METER
LUDLUM MEASUREMENTS,INC.
SWEETWATER, TEXAS
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M3-97 Survey Meter
page ii
TABLE OF CONTENTS
1.GENERAL.........................................................................................................................................................................1
2.SPECIFICATIONS ..........................................................................................................................................................1
3.DESCRIPTION OF CONTROLS AND FUNCTIONS...............................................................................................2
4.OPERATING PROCEDURES.......................................................................................................................................3
5.CALIBRATION................................................................................................................................................................4
5.1 Detector Operating Point......................................................................................................................................4
5.2 Calibrating CPM Meter Dial................................................................................................................................4
6.MAINTENANCE..............................................................................................................................................................5
7.THEORY OF OPERATION...........................................................................................................................................6
7.1 Input.........................................................................................................................................................................6
7.2 Amplifier..................................................................................................................................................................6
7.3 Discriminator..........................................................................................................................................................6
7.4 Audio........................................................................................................................................................................6
7.5 Digital Analog Convertor......................................................................................................................................6
7.6 Scale Ranging..........................................................................................................................................................6
7.7 Meter Drive.............................................................................................................................................................7
7.8 Meter Compensation..............................................................................................................................................7
7.9 Fast/Slow Time Constant.......................................................................................................................................7
7.10 Low Voltage Supply.............................................................................................................................................7
7.11 Low Voltage Reference........................................................................................................................................7
7.12 High Voltage Supply............................................................................................................................................7
PARTS LIST ........................................................................................................................................................................8
Model 3-97 Survey Meter............................................................................................................................................8
Main Circuit Board, Drawing 464 X 633...........................................................................................................8
Calibration Board, Drawing 363 X 528..............................................................................................................9
Chassis Wiring Diagram, Drawing 363 X 582...................................................................................................9
DRAWINGS AND DIAGRAMS......................................................................................................................................11

Model 3-98 Survey Meter
June 2020
page 1
1. GENERAL
The Model 3-98 is a portable survey instrument
with four linear ranges up to 500,000 cpm. The
instrument features an internal pancake tube
along with the capability of utilizing an
external probe by a switch on the front-panel
labeled INT/EXT. Separate high-voltage
adjustments for the internal and external probes
are provided. The high-voltage power supply is
regulated and adjustable from 200 to 1000 V.
The unit body is made of cast aluminum,
including the meter housing. The can is 0.23
cm (0.090 in.) aluminum. Other operating
features of the instrument include a unimorph
speaker mounted to the instrument can with an
audio ON-OFF capability, fast-slow meter
response, meter reset button, and a six-position
switch for selecting battery check or scale
multiples of X0.1, X1, X10 and X100.
Each range multiplier has its own calibration
potentiometer.
Most Ludlum Measurements GM probes will
operate on this unit, as will many of the
scintillation detectors. The external high
voltage is factory set for 900 V. For special
requirements, it may be adjusted for operation
with any GM or scintillator tube between 400
and 1500 V.
The unit is operated with two "D" cell
flashlight batteries for operation from
approximately 0 to 65 °C (32 to 150 oF). For
temperature operation to -18 °C (0 oF), either
very fresh alkaline or rechargeable NiCd
batteries may be used. Battery drain averages
17 milliamperes.
2. SPECIFICATIONS
RANGES: four linear range multiples of X0.1, X1,
X10, and X100; typically used in combination with
the 0-5k cpm meter dial, 0-5000,000 cpm is
achieved with the range multiplier
METER: 1mA, 6.4 cm (2.5 in.) scale, with pivot-
and-jewel suspension
METER COMPENSATION: temperature
compensation is provided by thermistors on the
main board
INPUT SENSITIVITY: 30 mV, (±10 mV)
HIGH VOLTAGE: adjustable 400-1500 V
RESPONSE: 4 or 22 seconds for 90% of final
meter reading
LINEARITY: ±5% of full scale
INTERNAL DETECTOR: pancake style GM
detector with 1.7 ±0.3 mg/cm2window
WINDOW AREA: Active: 15.51 cm2;
Open: 12.26 cm2
PROTECTIVE SCREEN: stainless steel, 79%
open
POWER: two standard "D" size batteries
BATTERY LIFE: exceeds 600 hours with a fresh
set of alkaline "D" cell batteries
BATTERY DEPENDANCE: instrument
calibration changes less than 3% within battery
check limits on meter
AUDIO: built-in unimorph speaker with an ON-
OFF switch
CONNECTOR: series "C," 706 U/G; BNC or
MHV may also be provided
SIZE: 22.9 x 8.9 x 21.6 cm (9 x 3.5 x 8.5 in.) (H x
W x L), with handle
WEIGHT: 1.4 kg (3 lb), less detector and batteries
FINISH: drawn-and-cast aluminum, with
computer-beige polyurethane enamel and silk-
screened nomenclature

Model 3-98 Survey Meter
June 2020
page 2
3. DESCRIPTION OF CONTROLS AND FUNCTIONS
Range Multiplier Selector Switch: a
six-position switch marked OFF, BAT,
X100, X10, X1, X0.1. Turning the
range selector switch from OFF to BAT
position provides the operator a battery
check of the instrument. A BAT check
scale on the meter provides a visual
means of checking the battery status.
Moving the range selector switch to
one of the range multiplier positions
(X0.1, X1, X10, X100) provides the
operator with typical overall ranges of
0-500 kcpm (for a typical meter dial it’s
0-5 kcpm). Multiply the scale reading
by the multiplier for determining the
actual reading.
AUD ON-OFF Toggle Switch: In the
ON position, the switch energizes the
unimorph speaker, located on the left
side of the instrument. The frequency
of the clicks is relative to the rate of the
incoming pulses. The higher the rate,
the higher the audio frequency. The
audio should be turned OFF when not
required to reduce battery drain.
F-S Toggle Switch: Provides meter
response. Selecting the fast, "F"
position, of the toggle switch provides
90% of the final meter reading in four
seconds. In slow, "S" position, 90% of
the final meter reading takes 22
seconds. In the "F" position, there is a
fast response and large meter deviation.
In the "S" position, there is a slow
response and damped meter deviation.
RESET Pushbutton: When depressed,
provides a rapid means to drive the
meter to zero.
INT/EXT Switch: a two-position
switch used to alternate between the
internal pancake tube and the external
probe. In the INT position, the internal
tube is utilized. In the EXT position,
the external probe is utilized.
H.V. Adjustment: provides a means to
vary the high voltage from 400 to 1500
volts. The high voltage setting may be
checked at the probe connector with an
appropriate voltmeter.
Range Calibration Adjustments:
recessed potentiometers located under
the calibration cover, on the right side
of the front panel. These adjustment
controls allow individual calibration for
each range multiplier.

Model 3-98 Survey Meter
June 2020
page 3
4. OPERATING PROCEDURES
Caution: Verify instrument voltage input
rating before connecting to a power converter.
If the wrong power converter is used, the
instrument and/or power converter could be
damaged.
Note: To open the Battery Lid, twist the lid
button counterclockwise a quarter-turn. To
close, twist clockwise a quarater-turn.
Open the Battery Lid and install two "D"
size batteries. Note (+) (-) marks on the
inside of the lid. Match the battery polarity
to these marks.
Note: The center post of flashlight battery is
positive. Close the battery box lid.
Switch the range switch to BAT. The meter
should deflect to the battery check portion
of the meter scale. If the meter does not
respond, recheck that the batteries have
proper polarity.
Turn the INT/EXT switch to EXT.
Connect a detector to the Model 3-98.
Turn the instrument range switch to X100.
Expose the detector to a check source. The
speaker should click with the AUDIO ON-
OFF switched to ON.
Move the range switch to the lower scales
until a meter reading is indicated. The
toggle switch labeled F-S should have fast
response in "F"and slow response in "S".
Depress the RES switch. The meter should
zero.
Turn the INT/EXT switch to INT and hold
the check source to the end of the can until
a meter reading is indicated.
Note: To assure proper operation of the
instrument between calibrations, an instrument
operational check should be performed prior to
use. A reference reading with a check source
should be obtained at the time of initial
calibration or as soon as possible afterwards,
for confirming correct operation. Confirm the
proper reading on each scale.
If the instrument does not read within 20% of
the proper reading, it should be sent in to a
calibration facility for recalibration.

Model 3-98 Survey Meter
June 2020
page 4
5. CALIBRATION
5.1 Detector Operating Point
CAUTION: The detector operating voltage is
connected to both detectors at all times,
regardless of which position the detector
INT/EXT switch is set. The maximum
operating voltage for the internal GM detector
is 1000 VDC; therefore, the external detector
operating voltage must be kept below 1000
VDC.
Switch the INT/EXT switch to the INT
position and adjust the HVE control for
900 volts at the instrument connector.
NOTE: Measure high voltage with a Model
500 pulser on a high-impedance voltmeter with
a high meg probe. If one of these is not
available, use a voltmeter with a minimum of
1000 megohm input resistance.
For external GM detector applications, the
power supply may be adjusted for 450-550
volt and 900 volt GM tubes. Follow the
above procedure, only set the supply at the
new operating voltage.
For scintillation counters, connect the
scintillator. Expose the unit to a source and
develop an operating voltage versus count-
rate plot. Set the operating voltage at the
flattest portion of this curve.
5.2 Calibration CPM Meter Dial
To calibrate the cpm scale, a Ludlum Model
500 pulse generator is required.
Connect the pulse generator to the
instrument and adjust the pulse
frequency to provide a 4/5-scale
deflection on the X100 range
(400,000 cpm). Adjust the X100
range calibration potentiometer as
required.
Adjust the pulse frequency to a 1/5
scale deflection on the X100 range
(100,000 cpm) and confirm that the
meter indicates ±10% of incoming
pulses. Decrease the pulse
frequency by decades and adjust
each range calibration
potentiometer accordingly.

Model 3-98 Survey Meter
June 2020
page 5
6. MAINTENANCE
Instrument maintenance consists of keeping the
instrument clean and periodically checking the
batteries and the calibration.
An instrument operational check should be
performed prior to each use by exposing both
the internal and external detectors to a known
source and confirming the proper reading on
each scale.
Recalibration should be accomplished after any
maintenance or adjustment of any kind has
been performed on the instrument. Battery
replacements are not considered to be
maintenance and do not normally require the
instrument to be recalibrated.
Ludlum Measurements recommends
recalibration at intervals no greater than one
year. Check the appropriate regulations to
determine required recalibration intervals.
The batteries should be removed and the
battery contacts cleaned of any corrosion at
least every three months. If the instrument has
been exposed to a very dusty or corrosive
atmosphere, more frequent battery servicing
should be used.
Use a spanner wrench to unscrew the battery
contact insulators, exposing the internal
contacts and battery springs. Removing the
handle will facilitate access to these contacts.
Note: Never store the instrument over 30 days
without removing batteries. Although this
instrument will operate at very high ambient
temperatures, battery seal failure can occur at
temperatures as low as 38 °C (100 °F).