Model 78 and 78-1
Technical Manual
Section 1
Ludlum Measurements, Inc.
Page 1-1
March 2021
Introduction
he Ludlum Model 78 and 78-1 Digital/Analog Stretch Scope features two radiation
radiation detectors mounted on a 3.7 m (12 ft) extendable pole. The two energy-
compensated, Geiger-Mueller (GM) detectors measure gamma radiation from 0.1 mR/hr
mR/hr to 1000 R/hr (1µSv/h – 10,000mSv/h). The Model 78 measures in R (Roentgen)
(Roentgen) units while the 78-1 measures in Sieverts.
Both an analog meter and a backlit 4-digit LCD (liquid crystal display) are utilized for display. The
4-decade logarithmic meter face spans from 0.1 to 1000. The digital readout displays the level of
radiation in either mR/hr or R/hr (µSv/h), depending upon the detector selected. The LCD also
has indicators for low battery, alert, alarm, overflow, and detector overload.
Other instrument features include Dead Time Correction (DTC) to compensate for detector dead
time; audible click-per-event with programmable 1, 10, 100, and 1000 divide-by; LCD backlight
with programmable "on" time; programmable fixed or variable response time; and a count
overflow visual alarm.
Warning
There is a reduced response in pulsed fields due to the detector dead time.
The Model 78 incorporates independent adjustable alarms for both detectors. There are two alarm
indications. The first-level alarm is indicated by display of
ALERT
on the LCD. The second-level
alarm is indicated by display of
ALARM
and a continuous audible tone. The audible alarm can be
silenced (acknowledged) by depressing the
RESET
switch.
The electronics front panel is equipped with switches for control of detector selection, audio
on/off, fast or slow response times, LCD backlight, display reset, and power on/off. A headphone
jack is also available. Two “D” cells located in the handle provide power to the electronics. Battery
life is approximately 250 hours.
The stretch scope is made of stainless steel tubing that retracts to an overall length of 105.9 cm
(41.7 in.). The handle can be removed for storage, reducing the length to 92 cm (36.2 in.). The
electronics are housed in an aluminum body with a beige powder-coat finish and subsurface-
printed Lexan front panel. Front-panel switches are sealed with rubber “boots” for resistance to
moisture.
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