WBJI DSM-502
PAGE 4
2.0
DESCRIPTION
The Model DSM-502 is a ruggedized, multi-purpose, state of the art, DIGITAL RATEMETER/SCALER with
an Internal Detector that can be calibrated separately in dead time compensated cpm and dose. It has an
adjustable high stability HV supply that can operate any probe requiring voltages from 300 – 1500 vdc. The
probe can have a completely independent calibration in cpm/cps dead time compensated, dose that is
automatically activated when the probe is selected from the front panel. The large easy to read digital LCD
readout is utilized to display all relevant data in Counts,
µ
R/hr, mR/hr R/hr & Sv units. The DSM-502 does
not require any external pc boards, pc downloads etc. to calibrate and operate. All of the controls
necessary to calibrate the DSM-502 with any of the probes available from Johnson are contained in the
instrument. Calibration data i.e. HV, dead time, count time, alarm setpoint & over range setpoint, fast tc,
slow tc & HV can be displayed on the LCD. Diagnostic circuitry constantly monitors the battery, high voltage
and detector condition and automatically indicates when operation is out of tolerance. A FAST/SLOW
RESPONSE switch (that is adjustable) is available on the front panel to change the full scale response from 10
- 250 seconds for FAST and 10-250 seconds for SLOW. A speaker that produces a loud AUDIBLE CLICK
for each probe pulse can be turned OFF or ON from the front panel. The ALARMS for OVERRANGE &
OPERATING ALARM have a visual ALARM indicator in the LCD and an audible indication when the alarm
has been activated. The OPERATING ALARM can be temporarily disabled from the front by toggling the
ZERO switch. Durability is enhanced by heavy duty, cast aluminum construction and the direct
interconnection of the industrial type printed circuit boards.
3.0
THEORY OF OPERATION
3.1 GENERAL
The DSM-502 utilizes the latest field proven, microprocessor circuitry to operate all of the Internal GM
probes. The instrument can function as a regular ratemeter without dead time compensation, regular
ratemeter with dead time compensation, scaler with counting times from 10 – 2550 seconds in 10 second
increments. The system changes ranges automatically and has separate non interacting controls for dose and
dead time compensated cpm or cps calibration for each input. Other adjustments with internal controls are:
HV, COUNT TIME, LCD REFRESH RATE, FAST TC & SLOW TIME CONSTANT. Operation in either
the counts/minute or dose mode for each probe is switch selectable from the front panel. The electronic
circuitry is located on 2 heavy duty industrial type printed circuit boards that are directly interconnected to
improve reliability and durability. All of the internal power is provided by highly regulated -5 vdc & +5 vdc
(
±
0.5%) low temperature coefficient power supplies. A separate high stability higher voltage power
supply is provided with a low temperature coefficient provides 300 - 1500 vdc (
±
1%) to operate probes
with different HV requirements. An OVER RANGE ALARM is available that produces a beeping audible
alarm when the usable range of the probe has been exceeded. Diagnostic circuitry constantly monitors the
power supply, high voltage supply and GM detector and indicates out of tolerance operation by indicators on
the LCD or by audible alert. An optional RS-232 data port is provided by a 9 pin sub-d connector. Data is
available at the connector when the “DATA” switch located on the front panel is depressed. .
3.2
DETECTOR SIGNAL – DIAGNOSTICS – CALIBRATION CONTROLS
A microprocessor meter operates much differently than an analog type meter. Most of the functions i.e. dead
time correction, diagnostics etc. are provided by the microprocessor programming (software). The DSM-502
contains two micro-processors. One processor controls and manipulates the data from the probes and
provides the HV and calibration support. The 2nd processor on the DISPLAY pc board controls the data to
the display and provides the RS-232 signal to the ‘DATA OUTPUT CONNECTOR”. Switch SW4 is an
internal switch on the CPU that controls the CALIBRATION FUNCTIONS of the DSM-502. SW5 is a push
button switch located by SW4 that is used to activate the calibration procedure selected and save the settings
of the potentiometers utilized to calibrate each particular function. FIGURE 2 shows SW4 and SW5 and
their position on the CPU pc board. Calibration of the ALL THE PARAMETERS IN THE SYSTEM is
completed by utilizing SW4, SW5, P1, P2 & P3. SW4 is utilized to select the function that will be calibrated or
set and P1, P2, & P3 are utilized to adjust the parameters. When a particular function has been set
(calibrated) to the desired levels SW5 is depressed to save the readings on P1, P2 & P3. Each position on SW4
represents a function or parameter that is being adjusted. As a result of this system the potentiometers P1, P2
& P3 are utilized over and over again to adjust the parameters. When the calibration procedure has been
completed SW4 IS ADJUSTED TO “0” ZERO POSITION FOR NORMAL OPERATION FIGURE 2
SHOWS THE LOCATION OF SW4, SW5, P1, P2 & P3. FIGURE 3 SHOWS THE COMPONENT LAYOUT
OF THE POWER SUPPLY PRINTED CIRCUIT BOARD AND THE LOCATION OF THE MAJOR
COMPONENTS.
