overhoff 200SB-HTO Manual

OPERATION/MAINTENANCE MANUAL
PORTABLE TRITIUM MONITOR
MODEL 200SB-HTO
OVERHOFF TECHNOLOGY CORPORATION
1160 US ROUTE 50, MILFORD, OHIO, USA

TABLE OF CONTENTS
SECTION DESCRIPTION PAGE
1.0. Introduction 1
1.1. Physical Description
2.0. Technical Specifications 2
3.0. Circuit Description 3
3.1. Ionization Chambers
3.2. Electrometer
3.3. Signal Processing Amplifier 4
4.0. Configuration 5
4.1. External Features
4.2. Hose Connections
5.0. Operation 6
6.0. Calibration 7
6.1. Method
6.2. Gas Calibration
6.3. Gamma Calibration 8
7.0. Maintenance 9
8.0. Service 10
9.0. Replaceable Parts 11
10.0. Warranty 12
Figure 1 – Front Panel Controls 13
Figure 2 – Hose Barb Connections 14
Figure 3 – Functional Block Diagram 15
Drawing, Model 200SB-HTO General Arrangement
Drawing, Model 200SB-HTO Chamber Position
Drawing 1020686, Ionization Chamber Can

1.0. INTRODUCTION
Model 200SB portable tritium monitor is a small, high sensitivity, hand held, battery
operated fully gamma compensated survey meter.
The instrument will measure tritium in its elemental form.
1.1. PHYSICAL DESCRIPTION
Model 200SB-HTO uses two identical ionization chambers in a side by side arrangement.
One ionization chamber, with a total volume of 200 cc, is used for measurement, the
second chamber serves for gamma compensation.
The sample stream is drawn through the ionization chamber by means of a small
rotary vane pump which is plumbed at the outlet of the ionization chamber. Entry of dust
particulates is prevented by attaching a good quality particulate filter ahead of the
instrument sampling inlet.
A large easy to read liquid crystal digital panel meter with a range from 0.1 to 2000.0
MBq/m3is used for measurement display. Other units of measurement, such as MPCa,
μCi/m3, μSv/hr, or others may be specified by the user at time of an requesting a unit. The
instrument exhibits a basic sensitivity of the order of 0.1 MBq/m3, which it is able to attain
due to the fact that it is immune to response to both radon and cosmic ray noise.
Power is supplied by a pair of “D” size batteries. While it is recommended that Alkaline cells
be used, the instrument will also operate with NiMH or even Carbon Zinc batteries,
although operating duration will be shorter. Onset of battery depletion is signaled by
illumination of an LED located next to the meter face.
A nine position alarm level stepped attenuator, adjustable over partial scale (.01 - 5 %) is
located on the front panel. A steady tone is emitted by an acoustic signaler if the
measurement exceeds the set point. A steady tone is heard if the sample air flow has been
interrupted. The alarms are non-latching.
The instrument case is constructed of light weight aluminum. A handle is attached for hand
held survey use.
Gas flow connections are made externally to the instrument by appropriate attachment of
flexible plastic hose. See Figure 2 .
A cartridge filled with disposable desiccant can be interposed between the measurement
and compensation chambers for measurement of HTO where other radioactive gases may
also be present.
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2.0. TECHNICAL SPECIFICATIONS
MEASUREMENT DISPLAY 4 1/2 digit LCD
0.1 - 1999.9 MBq/m3
GAMMA COMPENSATION two chambers in a side by side arrangement
RESPONSE RATE 20 seconds to reach 90 % of final reading,
NOISE LEVEL +0.1 MBq/m3 , 1 S.D.
(5 second electronic time constant)
ZERO STABILITY after 1 minute (or less) warm up, the zero drift
to less than 0.1 MBq/m3
ALARM (ACOUSTIC) 1. nine position stepped attenuator set point for
signal of 0.2 - 100 MBq/m3, steady tone
2. low flow produces a steady tone
ALARM (VISUAL) signal level: red LED
low flow: yellow LED,
low battery: red LED
DUST FILTER in line disposable cartridge, Pall No. 12082
SAMPLING SYSTEM 6 hose barb ports are located on the front panel
IONIZATION effective volume: 200 cm3
CHAMBER VOLUME port to port volume: 220 cm3
PUMP special high volume internal pump for a flow rate
from 2-3 LPM
POWER two “D” size batteries alkaline, carbon-zinc or NiCd
ENVIRONMENTAL -20°C to +40°C, 0 - 98 % RH
CASE light weight aluminum
SIZE AND WEIGHT 7.6" L, 5.2" W, 4.4" H excluding handle,
5 lbs (2.3 kg)
ACCESSORIES
•2 “D” size batteries
•sniffer hose
•dust filter
•transit case (optional, on request)
•two desiccant cartridges for HTO only
measurement
•container of replacement desiccant
•power converter, 100-240 VAC, 50/60 Hz,
.25A to 3.3 Vdc @1.2A,
5.5 mm O.D. x 2.1 mm I.D. plug,
center pin is positive
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3.0. CIRCUIT DESCRIPTION
CAUTION: This instrument has not been designed for indiscriminate opening
or disassembly of the internal parts. It contains highly sensitive
semiconductors which are destroyed by even the slightest electrostatic
discharge.
3.1. IONIZATION CHAMBERS
In its simplest form, an ionization chamber is an enclosed volume with two electrodes.
Voltage is applied between the electrodes, generating an electric field which will segregate
and collect electric charges which are created by nuclear events occurring inside the
chambers. Nuclear events may consist of ionization of air molecules by external or internal
alpha, beta or gamma radiation.
Model 200SB monitors are designed to measure tritium. Activity of tritium decay is such
that a concentration of 1 μCi/m3in a volume of one liter will generate an ionization current
of about 0.95 x 10-15 amperes. This is a very weak current.
Alpha pulses from naturally occurring radon, are much more energetic, they can produce
short current bursts of up to 10-13 amperes during decay, and therefore appear as large
noise "spikes" which can seriously impair tritium measurement.
Gamma radiation also has a strong effect. In practice, a gamma radiation field of
1 mR/hr will create the same amount of ionization as 90 μCi/m3(3.3 MBq/m3) of tritium.
A tritium monitor, in order to measure to low concentrations, must be able to respond only
to tritium and be immune to alpha or gamma radiation. For this purpose, a second
ionization chamber system has been included to balance out any ionization current
contribution from external gamma radiation.
In the 200SB instruments, two ionization chambers are arrayed in a side by side
ensuring good gamma compensation in all directions.
The ionization chamber polarizing voltage is supplied by a set of dry batteries with a long
life. The surfaces of the ionization chambers themselves are bare, and, to avoid damage to
the electrometer, must NOT be touched by hand.
3.2. ELECTROMETER
Also known as a transimpedance amplifier, it serves the purpose of converting the
extremely feeble ionization current into a voltage suitable for further signal processing and
measurement display.
The heart of the electrometer consists of a specially selected ultra high impedance
semiconductor device which has been chosen both for ultra low internal current leakage as
well as long term d.c. stability. The semiconductors used in Model 200SB instruments are
suitable for measurement of currents as low as10-16 amperes.
In Model 200SB, the electrometer is directly attached to the ionization chamber assembly,
which is protected by a rectangular metal cover.
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3.3. SIGNAL PROCESSING AMPLIFIER
A single printed circuit board attached directly to the front face of the instrument
contains all power supply and signal processing electronics.
Proprietary circuitry is used for the recognition and elimination of transient signals due to
radon or high energy cosmic ray pulses. Model 200SB instruments, with digital display,
use a dedicated internal circuit to disable the pulse rejection circuit when the measured
signal reaches approximately 30 μCi/m3(1.1 MBq/m3).
A front panel control has been provided for adjustment of the set point (level) at which the
acoustic alarm is desired to sound. The acoustic signaler has the second function of
alerting the user that sample gas flow is impeded.
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4.0. CONFIGURATION
4.1. EXTERNAL FEATURES
The front panel features include:
1. the digital panel meter, 0.1 - 1999.9 MBq/m3
2. function control knob
3. alarm level control knob, 0.2 - 100 MBq/m3
4. low battery LED
5. signal level alarm LED
6. low flow alarm LED
7. acoustic signaler
8. calibration potentiometer (under phillips screwhead)
9. offset potentiometer knob
10. sample IN/OUT hose barbs
11. battery compartments
12. jack for external power supply, 3Vdc (never exceed 3.5V)
Side features include:
13. snap holder for dust filter
14. target for gamma source calibration
4.2. HOSE CONNECTIONS
The instrument may be operated in either of two modes. In the first mode, the instrument
will respond to any radioactive gas passing through the instrument as well as tritium. In the
second mode, it will respond only to HTO, even in the presence of other radioactive gases.
The external plumbing (hose attachments) is selected to suit the mode in use.
FIRST MODE (refer to Figure 2)
A sniffer hose is attached to a small in line dust filter, which is directly attached to the “IN”
hose barb. The other measurement chamber hose barb is routed to the inlet of the pump
by means of a short piece of hose. Connecting a short loop of hose to each hose barb
closes off the compensation chambers hose connections.
SECOND MODE (refer to Figure 2)
In this mode, the exhaust from the measurement chambers is connected to a desiccant
cartridge. The exhaust end of the desiccant cartridge is connected to a second dust filter
and then to the inlet of the compensation chamber. The other compensation chamber hose
barb is routed to the inlet of the pump by means of a short piece of hose. In this mode, the
sample stream passes first through the measuring (upstream) chambers, and then through
the desiccant cartridge, it continues through the compensation (downstream) chambers
and finally exits via the pump.
NOTE 1: NEVER OPERATE THE INSTRUMENT WITHOUT A DUST FILTER IN THE
SAMPLE STREAM
NOTE 2: THE INSTRUMENT MUST BE IN THERMAL EQUILIBRIUM WITH ITS
SURROUNDINGS.
NOTE 3: TO AVOID ERRATIC RESPONSE, THE PUMP MUST ALWAYS BE PLACED
DOWNSTREAM OF THE LAST IONIZATION CHAMBER IN THE SAMPLE
PATH.
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5.0. OPERATION
Ensure that a dust filter is connected in line ahead of this instrument flow inlet in use.
The following steps are necessary and sufficient to operate the instrument:
1. Set measurement alarm level to desired value.
2. Rotate mode switch to the MEASURE position.
Allow one minute or less for the instrument to stabilize.
The instrument is now ready for use. In this mode the ionization chambers are active,
but the pump is not. The instrument is in a so called “standby mode” ready to sample
the instant the mode switch is advanced to the next position.
3. Rotate the mode switch to the SAMPLE position.
NOTE: If the LOW FLOW LED is illuminated, the sampling hoses are obstructed, or, for
whatever reason, sample flow through the chambers has ceased.
It is IMPERATIVE that the sample stream be free from dust, dirt or moisture. Not only will
the instrument show erratic behavior, but it may cease to function entirely. If moisture is
ingested, then continued pumping to evaporate and expel the moisture can be attempted. If
this fails, the instrument must be returned to the factory for service.
Condensation can occur if an instrument is brought from a cold environment into warmer
surroundings.
Furthermore, temperature changes to the instrument, both lower to higher as well as higher
to lower will create transient currents in the electrometer which can appear as large
phantom measurement signals.
The instrument must be allowed to thermally equilibrate to its surroundings prior to use.
If there is an OFFSET due to thermal disequilibrium, use the following procedure:
OFFSET COMPENSATION:
1. Switch the instrument into the measure mode
2. After approximately three minutes. The instrument should indicate 0000 on the digital
panel meter. An offset of 4 - 6 μCi/m3(0.1 - 0.2 MBq/m3) is typical for situations due to
temperature changes. This offset should disappear as thermal equilibrium is attained.
3. Adjust the “offset” compensation potentiometer knob as required. The location is
shown in Figure 1.
NOTE: The rotation direction for the adjustment is clockwise for change in a
positive direction.
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6.0. CALIBRATION
6.1. METHOD
Tritium monitors employing ionization chambers, such as Model 200SB may be calibrated
with either of two methods. The first method consists of injecting a known activity of tritium
gas; the second method uses external gamma radiation of a known field strength.
To ensure trace ability to National Standards, the first method must be employed. This
method is time consuming and quite difficult to perform with precision. The first method is,
however, useful as a “type” test, and can serve as a basic accurate calibration from which
the gamma response (the second method) can be cross correlated.
The second method uses an external gamma field. In this instance, the ionization chamber
must be shielded with a lead brick.
6.2. GAS CALIBRATION
Since the instrument is essentially linear, a relatively high concentration can be used for
most accurate results. Values between 4 MBq/m3to 40 MBq/m3are convenient, but any
other values from 1.0 MBq/m3to 200 MBq/m3can be used.
Instructions for the use of gas calibrators are generally provided by the manufacturer of the
gas calibrator, and these should be followed.
Some general hints can be given.
It is important that the calibration sample be well circulated through the entire calibration
system loop. The calibration loop should include the measurement chambers only.
Adequate time should be allowed for the system pressure and temperature to come
to equilibrium, and that no excess pressure is built up.
The inclusion of a previously calibrated "master" or "reference" tritium monitor in the
sampling loop is highly recommended.
The calibration can actually be repeated for several levels of tritium activity. This is not
done to verify the linearity of the tritium monitor (which is highly linear) but to ensure that
the calibration process itself is free from subtle errors.
If calibration is performed, and the instrument response is somewhat different from the
expected value, then small adjustments can be made by turning the calibration
potentiometer with a small screw driver. The calibration potentiometer is accessed by
removing the phillips head screw.
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6.3. GAMMA CALIBRATION
If the unit has previously been calibrated with tritium gas, then it is sufficient to use a small
check source to any type of gamma radiation to produce a response when placed at a
specified location relative to the instrument under test. If the check source is long lived, no
chronological correction is needed. To verify calibration, the original check source must be
used. Records must be kept to identify relative location of the check source and the
expected result. Use a lead brick to shield the compensation chamber as necessary.
If calibration by either of these methods is performed, and the instrument response is
somewhat different from the expected value, then small adjustments can be made by
turning the calibration potentiometer with a small screw driver. The calibration
potentiometer is accessed by removing the small phillips screw.
Large changes in calibration are evidence of malfunction. The factory should be consulted
at once at Tel. [phone] or by FAX [phone].
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7.0. MAINTENANCE
Very little maintenance is required for Model 200SB tritium monitors, but some periodic
attention may be necessary, especially if the instrument is to be used in adverse
environments.
The batteries should be replaced within half an hour when the low battery light illuminates.
Access to the batteries is made by twisting off the caps located on the front panel of the
instrument.
Pump life is in excess of 1000 hours of actual use, its life is preserved by ensuring that the
instrument is operated only with dust filters in line.
When not in use, the monitor should be stored in a cool dry environment.
Any battery of the proper shape and voltage can be used. NiCd cells, while having smaller
ampere hour capacity at room temperature, will outperform dry batteries below about 0°C.
NiCd batteries will operate at any temperature, down to -50°C.
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8.0. SERVICE
This instrument contains components that are easily destroyed if the case is opened
and handled without proper precaution.
Overnight service is provided by the factory. Special training can be given to
qualified technical personnel who are entrusted with instrument service and repair
responsibility.
Warranty is void if maintenance or repair (other than that which is listed in this
manual) is performed by an unauthorized repair facility.
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9.0. REPLACEABLE PARTS
The following parts and components are disposable items and may be obtained from
Overhoff Technology Corporation or from any original supplier:
Batteries, primary power "D” size, alkaline, EN95
Batteries, polarizing 45 V, No. 415
Dust filter Pall No. 12082
Dust Filter Clip Clic No. 51
Ionization Chamber P/N 1020686
Pump P/N 50085
Hose Barb, sample in Brass, P/N 22BH-4-2
Hose Barb, sample out Brass, P/N 230-4-2
Panel Meter P/N DMO-742
Desiccant Column Drierite P/N 26800
Desiccant, Drierite P/N 23005
five pound jar
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10.0. WARRANTY
All instruments built by Overhoff Technology Corporation are warranted to perform as
claimed.
Defective components or workmanship of the instrument will be corrected free of charge for
parts or labor within a period of one year from delivery. Nonperformance of the instrument
as a result of negligence on behalf of the customer is not covered by this warranty.
Should it appear to be necessary to return the instrument to our factory, authorization for
the return must be obtained from Overhoff Technology Corporation prior to shipping. In-
freight charges will be borne by the customer.
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