overhoff 357RM Manual

OPERATION/MAINTENANCE MANUAL
TRITIUM MONITOR
MODEL 357RM
OVERHOFF TECHNOLOGY CORPORATION
1160 US ROUTE 50, MILFORD, OHIO, USA

Operation and Maintenance Manual, Model 357RM Tritium Monitor, Serial No. 5075-76
REVISION INDEX
Rev Date Section/Page Details Appvd By
- 02-20-2019 Original Document DW

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Tritium Monitor BETATEC Model 357 with dual 2 liter ionization chamber, single range, digital
display, radon alpha pulse suppression, single alarm system
TABLE OF CONTENTS
FIGURE 1, FRONT AND REAR PANELS........................................................................................1
1.0 GENERAL INFORMATION.................................................................................................2
1.1. INTRODUCTION
1.2. AVAILABLE CONFIGURATIONS
1.3. FEATURES .............................................................................................................3
1.4. GENERAL DESCRIPTION
1.5. RESPONSE OF IONIZATION CHAMBERS TO RADIATION
1.5.1. SPECIFIC IONIZATION CURRENT
1.5.2. THE WALL EFFECT........................................................................................4
1.5.3. RECOMBINATION EFFECTS
1.5.4. DISCRIMINATION AGAINST ALPHA PULSES..............................................5
1.6. CONTAMINATION (PLATE OUT) OF IONIZATION CHAMBERS
1.7. TECHNICAL SPECIFICATIONS
1.8. PERFORMANCE SPECIFICATIONS
1.8.1. MEASURMENT .............................................................................................6
1.8.2. ALARM SYSTEM
1.8.3. IONIZATION CHAMBER VOLUME
1.8.4. FLOWMETER................................................................................................7
1.8.5. DUST FILTER AND ELECTROSTATIC FILTER
1.8.6. PUMP
1.8.7. ENVIRONMENTAL TEMPERATURE
1.8.8. REAR PANEL INTERFACE
1.8.9. POWER
1.8.10. PHYSICAL
FIGURE 2, SAMPLE HOSE CONNECTIONS .................................................................................8
2.0. EQUIPMENT AND INSTALLATION....................................................................................9
2.1. EQUIPMENT SUPPLIED
2.2. INSTALLATION
2.2.1. PRECAUTION, SAMPLE FLOW SYSTEM .................................................10
2.3. OPERATION
2.3.1. INSTALLATION OF REMOTE OR ANCILLARY SYSTEMS
FIGURE 3, FUNCTIONAL BLOCK DIAGRAM...............................................................................11
3.0. INSTRUMENT ARCHITECTURE, DESCRIPTION...........................................................12
3.1. GENERAL
3.2. PENUMATIC
3.2.1. DUST FILTER
3.2.2. ION TRAP
3.2.3. IONIZATION CHAMBER
3.2.4. PUMP
3.3. CIRCUIT DESCRIPTION ......................................................................................13
3.3.1. IONIZATION CHAMBERS
3.3.2. ELECTROMETER
3.3.3. SIGNAL PROCESSING AMPLIFIER...........................................................14
3.3.4. ALARM CIRCUITS
3.3.5. POWER SUPPLIES

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4.0. CALIBRATION…………...............................................…………………………......…………15
4.1. FIRST METHOD, TRITIUM GAS CALIBRATION…………………………………..16
4.2. SECOND METHOD, GAMMA CALIBRATION
4.2.1. CALIBRATION VERIFICATION USING A SMALL GAMMA CHECK
SOURCE......................................................................................................17
4.3. THIRD MEHTOD - ELECTRICAL EQUIVALENCE METHOD
4.4 GAS METHOD, PRACTICAL PROCEDURE
4.4.1. DIRECT GAMMA CALIBRATION................................................................18
4.4.2. GAMMA VERIFICATION, USE OF CHECK SOURCE (PREFERRED
METHOD)
4.5. ELECTRICAL CALIBRATION, PRACTICAL PROCEDURE
FIGURE 4, CALIBRATION ADJUSTMENT………………................…………………………………19
4.6. EVIDENCE OF MALFUNCTION, PERIODIC CALIBRATION ..............................20
5.0 REPAIR AND TROUBLESHOOTING...............................................................................21
5.1. ELECTRICAL
5.2. MECHANICAL............................................................................................................22
6.0 MAINTENANCE ................................................................................................................23
6.1. OPERATOR MAINTENANCE
6.2. SUPERVISORY MAINTENANCE
6.3. FACTORY MAINTENANCE
7.0 STORAGE.........................................................................................................................24
8.0 WARRANTY......................................................................................................................25
9.0 DRAWINGS/DIAGRAMS...................................................................................................26
APPENDIX 1, MANUFACTURER’S DATA SHEETS
MEDO LINEAR POWERED DIAPHRAGM PUMP VCO-201
SOLBERG HEPA FILTER CARTRIDGE HE04
SELCO PANEL METER MODEL A9111-1

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1.0. GENERAL INFORMATION
(see Figure 1, Front Panel)
1.1. INTRODUCTION
A tritium monitor is an instrument designed to determine the presence and level of radioactive gas
(tritium) in air or other gas streams. These monitors may be used to detect radioactive gases in
many applications like:
room air
stacks, hoods, or other effluent passages
process piping
glove boxes, and similar.
These monitors are generally calibrated in terms of (micro) Curies per cubic meter though other
units can be used as requested (Bequerel, pCi/cm3, etc.).
The principle of measurement is based on collecting the current that is generated by the radioactive
decay (of tritium) inside an ionization chamber. The ionization current is proportional to the
concentration of the gas radioactivity, as well as to the specific activity of the radionuclide (tritium)
being detected.
A tritium monitor consists of the following parts:
1. An ionization chamber to collect the ionization current.
2. A sampling system to circulate the sample (air) through the ionization chamber.
3. An electrometer to amplify the very weak ionization current.
4. All other associated electronics to process and display the signal.
Ionization chambers respond not only to the airborne radioisotope which circulates through the
ionization chamber, but also respond to the presence of external high energy radiation capable of
ionizing the air inside. Therefore, ionization chambers will respond to X-rays and gamma radiation
as well.
To overcome this effect, Overhoff Technology Corporation (OTC) tritium monitors can be supplied
with compensating ionization chambers. Here a second ionization chamber is used to cancel the
effects of external radiation upon the measuring ionization chamber. Additional gamma radiation
suppression can be accomplished by using lead shielding. OTC tritium monitors are equipped with
special circuitry to identify and reject ionization currents that are produced by decaying radon, or
other airborne alpha emitting radioisotopes.
1.2. AVAILABLE CONFIGURATIONS
OTC tritium monitors are available in a number of different configurations. Form and size depend
upon the application and use.
Portable instruments are battery powered and light weight.
Instruments used for fixed applications employ 19" rackmount enclosures which carry power
supplies and signal processing circuits. They may also contain the ionization chamber and
electrometer.
Ionization chambers and electrometers are available for remote installation where the "display"
enclosure is connected via a shielded ten conductor cable.

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1.3. FEATURES
While the basic purpose of the OTC tritium monitor is to measure the presence and level of tritium
(or other airborne radioisotopes) the monitors may be supplied with a number of user selected
special features.
Your particular instrument, which is described in this manual, has these special features.
1. Ionization chambers with nominal volume of 2 liters each.
2. Single measurement range over four decades, with digital display.
3. Pump and flowmeter for sample transport.
1.4. GENERAL DESCRIPTION
This monitor consists of dual 2 liter ionization chamber with an integral electrometer coupled to the
electronic circuits in the display cabinet. The chambers are mounted internally in the cabinet. The
cabinet contains all signal processing, alarm and external interface circuits, read out, and all
required power supplies. The measurement ionization chamber serves to collect the current
produced as tritium decays radioactively. The second ionization chamber is identically constructed,
but sealed. It serves to cancel the effects of external gamma fields. The electrometer serves to
transform this current into a form and magnitude suitable for display, alarm, and external uses, as
the ionization current itself is very weak.
The signal processing circuits serve to reject unwanted signals and to translate the electrometer
signal voltage into a form and magnitude suitable for display, alarm and external uses as well.
The alarm circuits provide an acoustic signal to denote that a preset level of measurement has
been exceeded.
The purpose of the power supply needs no special comment.
1.5. RESPONSE OF IONIZATION CHAMBERS TO RADIATION
1.5.1. SPECIFIC IONIZATION CURRENT
The current generated in an ionization chamber is the result of collecting electrons generated from
ionization of gas caused by occurrence of a nuclear event in the gas inside.
The number of ions (magnitude of the current) is influenced by numerous factors like the energy,
physical nature and particle range. As a good rule of thumb for beta particles in air one secondary
electron (and one positive ion) is formed for every 34 electron volts of energy lost by the primary
beta particle as it travels its path.
The Curie is defined as 3.71 x 1010 nuclear decay events per second. The mean energy of tritium
decay is 5.69 kev. Therefore it is calculated that 1 Curie of tritium produces an ion current very
close to
1 x 10-6 Amperes
A concentration of 1 μCi/m3of tritium in a chamber of a volume of one liter will thus produce a
current of
1 x 10-15 Amperes

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It must be remembered that the ionization chamber responds to the quantity of tritium present
inside. This is to say that effects due to temperature and pressure may need to be accounted for.
Even if a sample of gas is known to contain tritium at a certain concentration, i.e., parts per million
or other, it must be remembered that the activity (amount per unit volume) is dependent upon
temperature and pressure. The ionization chamber only responds to the quantity of radiation
inside.
1.5.2. THE WALL EFFECT
Ionization chambers also exhibit several other peculiarities. The wall effect can be a problem if the
track length of the decaying particle is appreciable when compared to the dimensions of the
chamber.
For ionization chambers with small linear dimension, and if the track of ionized particles is
comparatively long (the mean free path), an appreciable part of the energy of the primary particle
is simply dissipated in the wall of the ionization chamber. This effect increases as the chamber
dimensions shrink, and decreases as the chamber dimensions increase.
In air, atmospheric pressure, the maximum mean free path of a tritium beta particle is of the order
of five millimeters, and for chambers with linear dimensions of ten centimeters or greater this “wall”
effect becomes negligible.
1.5.3. RECOMBINATION EFFECTS
At high concentrations, another effect takes place.
When the ion population density is high, some of these positive and negative ions will recombine
and are lost to the measurement electrode.
This is known as saturation or stagnation since the effect is more pronounced in corners of the
ionization chamber where the potential field gradient is low. The effect can be reduced by
increasing the ionization chamber voltage.
For measurements of tritium at very high concentrations, such as are required when working with
pure T2, special chamber geometries are employed, long slender ionization chambers, with
relatively large internal ion collecting electrodes and short spacing between the chamber elements
enhance field gradients. With even moderate polarization potentials of 100 V or so, such chamber
geometries show linear response even to pure tritium streams.

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1.5.4. DISCRIMINATION AGAINST ALPHA PULSES
Since the energy of an alpha decay is at least 10,000 times more active than that of a tritium beta
event, suppression of alpha pulses needed in order to distinguish the presence of tritium at low
levels. Stable and accurate measurements of tritium for values below 5μCi/m3can only be obtained
with means to suppress response to alpha decay.
Alpha decay events, as detected in an ionization chamber, are not instantaneous. The special
circuitry which recognizes the alpha pulses requires some amount of time to suppress the event.
During pulse suppression, the instrument analog circuitry is placed in a “holding” mode, response
is frozen during the interval associated with the alpha event. The holding intervals occur at random,
but effectively add to the apparent time constant of the electronics. The instrument response
becomes slower with increasing radon or gamma noise background. For large background the
instrument will even freeze completely, the alpha pulse light will be permanently illuminated.
1.6. CONTAMINATION (PLATE OUT) OF IONIZATION CHAMBERS
Tritium gas will combine with the oxygen and the moisture in the air to form oxide of tritium (HTO
or T2O). Chemically indistinguishable from normal water or watervapor, the tritium oxide will attach
itself to the walls of an ionization chamber and bond both physically as well as chemically.
1.7. TECHNICAL SPECIFICATIONS
The instrument described in this manual has been designed and constructed for your
particular application.
It has been built and tested to the specifications listed on the next pages.
Circuit diagrams and interconnections between parts of the monitor, as well as those leading to
user selected remote devices or interfaces, are given at the end of this manual.
Consult the factory for further information, or for application engineering
1160 US Route 50, P. O. Box 182
Milford, OH 45150-9705, USA
Telephone [phone]; Facsimile [phone]
[email protected]; su[email protected]
www.Overhoff.com

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1.8. PERFORMANCE SPECIFICATIONS
The following specifications will apply when this instrument is used for the measurement of tritium
1.8.1. MEASUREMENT
RANGES Tritium 1 – 19,999 μCi/m3
DISPLAY Digital Meter, 4 ½” digit LED
ACCURACY ±10 % of reading, ±1 μCi/m3, whichever is greater
STABILITY AND ±1 μCi/m3, ambient temperature, after warm-up
DRIFT LONG TERM
NOISE ±1μCi/m3, 2 sigma, with 20 second time constant
GAMMA second ionization chamber of equal volume, coaxially
COMPENSATION mounted, serves to cancel effects of external gamma fields
RESPONSE RATE two linear time constants
ELECTRONIC 20 seconds for measurements below approximately 80 μCi/m3
3 seconds for measurements above 80 μCi/m3
WARM-UP TIME less than 5 minutes
1.8.2. ALARM SYSTEM single alarm, with set point adjustable from 1 to 1,000 μCi/m3
INDICATORS acoustic signaller, red LED
MODE a. ON. Alarm will sound and can be reset
if the signal level recedes below the set point.
b. OFF. Alarm is inactive
c. RESET.
1.8.3. IONIZATION CHAMBER
VOLUME measuring: 1,600 cm3
total wetted: 2,000 cm3
ION TRAP Kanne Type, coaxial integral
ELECTRODES Screw-on barrel and 1/8” diameter rod with Teflon insulator
GASKETS silicone rubber
PRESSURE 0.1 to 2 atmospheres
PORTS 1/4" NPT, fitted with hose barb fittings for 3/16" I.D. vinyl tubing
MATERIALS OF all wetted surfaces, stainless steel
CONSTRUCTION