Portable Instruments for Measuring Tritium in Air Concentration
Overhoff manufactures a wide variety of portable tritium air monitors which vary in measurement range, sensitivity, response time, HTO discrimination, data output, and other features. Each customer application is slightly different, and for this reason, we offer over 12 different portable tritium air monitors so that you can choose the monitor that best suits your needs.
Our portable tritium monitors are generally grouped into three categories depending on the size and number of ionization chambers used. The model name corresponds to the size of the ionization chamber volume, which correlates to the sensitivity.
200 Series: Two 200cc ionization chambers, coaxial
400 Series: Four 200cc ionization chambers, cruciform geometry
1400 Series: Two 1400cc ionization chambers, coaxial
All of our portable tritium monitors share the same basic, essential features for operation:
- A high quality in-line particulate filter is used at the sampling inlet to prevent the entry of dust particulates.
- Sample stream is drawn through the ionization chambers by a pump plumbed at the outlet of the furthest downstream ionization chamber.
- Ionization chamber and electrometer assembly for measuring tritium (and external gamma radiation if applicable).
- Signal processing amplifier with alpha pulse suppression circuitry to recognize and eliminate transient signals due to radon or high-energy cosmic ray pulses.
- A large, easy to read liquid crystal digital panel meter is used for measurement display.
- Power is supplied by “D” size alkaline or NiMH rechargeable batteries with an AC power adapter if applicable.
- A ten-position knob for setting the alarm setpoint, adjustable over partial scale, including an OFF position. An acoustic signaler emits a steady audible tone if the measurement exceeds the alarm setpoint. An intermittent tone is heard if the sample air flow has been interrupted causing the low flow alarm. The acoustic alarm is silenced by a MUTE pushbutton. The alarms are non-latching. Includes visual LED alarm indicators for alarm level, flow, low battery, and high-voltage power supply malfunction (applicable to 400 series only).
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 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.
Activity of tritium decay is such that a concentration of 1 μCi/m3 in 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, producing short current bursts of up to 10-13 coulombs during decay, and appearing as large noise “spikes” which can seriously impair tritium measurement.
An electrometer converts the extremely feeble ionization current into a voltage suitable for further signal processing and display. Overhoff’s electrometer is unparalleled in performance and can accurately measure currents as low as 10-16 amperes. A single PCB 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. A dedicated internal circuit is used to disable the pulse rejection circuit when the measured signal reaches approximately 60 μCi/m3. An OFFSET control is furnished in order to adjust the reading to zero in case of offsets caused by tritium contamination of the chambers or otherwise.
Gamma radiation also has a strong effect on tritium measurement. In practice, a gamma radiation field of 1 mR/hr will create the same amount of ionization as 90 μCi/m3 of tritium. Therefore, in order to measure to low concentrations, a tritium monitor must be able to respond only to tritium and be immune to alpha and gamma radiation. For this purpose, a second ionization chamber of identical volume is typically included to balance out any ionization current contribution from external gamma radiation. In the 200 and 1400 series, the two ionization chambers are arranged coaxially. In the 400 series instruments, four ionization chambers are arrayed in a cruciform pattern, ensuring almost perfect gamma compensation in all directions, even for high gradient nonuniform fields.
Our portable instruments can also be configured to measure HTO (tritium oxide) only, even in the presence of other radioactive gases such as noble gases often encountered in nuclear power plant environments (see Models 200SB-HTO and RS400-HTO). These monitors include 6 hose connections and an external desiccant cartridge so that the monitor can be configured to measure either total tritium (HT + HTO) or HTO only. In HTO only mode, the external desiccant column is interposed between the measurement and compensation chambers which absorbs and removes the HTO to provide a differential measurement that is proportional to HTO only [HTO + other radionuclide – (other radionuclide) = HTO].
The Model 200 series uses two coaxial 200cc ionization chambers. A high-volume pump sampling at 2-3 LPM combined with a 3 second electronic time constant yields an overall response time of under 5 seconds for the Model 200SB. This series is low-cost with medium sensitivity and a very fast response time. Useful as a field or emergency tritium monitor due to the compact size and rapid warmup and response. Note: data output is not available on the 200 series (see 400 series).
Model 200SB: One chamber is used for measurement and the second for gamma compensation.
Model 200SB-HTO: 200SB with an external desiccant cartridge to measure HTO only, even in the presence of other radioactive gases.
Measurement Range: 10 to 199,990 µCi/m3, 0.1 to 1,999.9 MBq/m3, 1 to 19,999 DAC or MPCa
Sensitivity: 10 µCi/m3 (0.4 MBq/m3)
Response Time: 5 seconds to reach 90% of final value
Model 200SS: Both 200cc chambers are used for measurement to increase sensitivity to tritium (not gamma compensated). If gamma compensation is not required, then this is an excellent low-cost option for measuring low-levels of tritium with a fast response time.
Measurement Range: 1 to 19,999 µCi/m3, 0.1 to 1,999.9 MBq/m3 or DAC, 1 to 19,999 µSv/h
Sensitivity: 3 µCi/m3 (0.1 MBq/m3)
Response Time: 15 seconds to reach 90% of final value
The Model 400 series uses four 200cc ionization chambers arranged in a cruciform geometry to provide better sensitivity and stability. Typically, two chambers are used for measurement and two for gamma compensation. The cruciform geometry provides nearly perfect omnidirectional gamma compensation regardless of photon energy, flux gradient, or flux direction, with virtually no offset in gamma fields up to 10 mR/h.
In general, the 400 series has the following specifications:
Measurement range: 1 to 19,999 µCi/m3, 0.1 to 1,999.9 MBq/m3 or DAC, 1 to 19,999 µSv/h
Sensitivity: 2 µCi/m3 (0.1 MBq/m3)
Response time: 30 seconds to reach 90% of final value
Special features described below include:
- HTO discrimination: Adds an external desiccant cartridge to allow for HTO only measurement even in the presence of other radioactive gases.
- RS-232 data output: Transmit data to any PC via a standard RS-232 link.
- Gamma compensation ON/OFF front panel toggle switch: Allows user to quickly turn off gamma compensation to allow for easy calibration via a gamma source.
- Circuit self-test function: Tests the unit to ensure it is working correctly prior to operation.
- Water/Splash Proof Enclosure: Seals all front panel components to prevent the entry of water.
400SBDyC: Standard configuration of the 400 series.
RS400: The RS400 includes RS-232 data output for sending the data to a remote PC.
RS400-HTO: Includes RS-232 data output and HTO discrimination. HTO discrimination (noble gas compensation) is accomplished by interposing a desiccant between the measurement and compensation chambers to provide a differential measurement that is proportional to HTO only and ignores other radioactive gases present. Useful for applications where other radioactive gases may be present.
FP400: Fast purge version, uses four smaller 110cc ionization chambers and a high-volume pump of 2-4 LPM to provide quicker response time, requiring just 12-15 seconds to reach 90% of the final value. Useful for applications which require low-level sensitivity, but with a fast and stable response. Sensitivity: 5 µCi/m3 (0.2 MBq/m3)
400AC: The 400AC is an upgraded version of the 400SBDyC. Main upgraded features include: front panel gamma compensation on/off switch for easy gamma calibration, automatic recalibration software, RS-232 data output, and NiMH rechargeable batteries.
400AC-M: The 400AC-M is a ruggedized version of the 400AC tailored for military, outdoor, or rugged use. Features include: water/splash proof front panel components with hinged gasketed cover to further protect the unit when not in use, circuit self-test function to ensure the unit is working prior to operation, and designed to operate in extreme conditions (low/high temperatures, ruggedness, salt, fog, wind, rain).
400AC-WP: Model 400AC that is water/splash proof. All front panel components are sealed to prevent the entry of water and includes a bright LED display that is readable even in sunlight or low-light conditions.
2×200-LD: Two-channel tritium leak detector to measure both elemental tritium (HT) and tritium oxide (HTO) in real-time. A front panel toggle switch allows you to select if HT or HTO is displayed. Elemental tritium will readily combine with oxygen to form tritium oxide and will also replace hydrogen atoms in compounds. Therefore, the presence of elemental tritium is indicative of a leak in the process system or container. The 2×200-LD can be used to rapidly locate the elemental tritium leak and secure it.
There is only one model in the 1400 series, the Model SP1400DD. This is our most sensitive portable tritium monitor and offers unparalleled sensitivity and stability through the use of two 1400cc ionization chambers, one for measurement and one for gamma compensation. The chambers were selected to be of a manageable size and weight while achieving maximum performance. Includes a shoulder strap for enhanced portability and freeing both hands for directing the sniffer hose. A heavy-duty internal pump provides up to 4 volume changes per minute to ensure a fast overall response.
Measurement range: 1 to 19,999 µCi/m3, 0.01 to 199.99 MBq/m3
Sensitivity: 1 µCi/m3 (0.04 MBq/m3)
Response time: 60 seconds to reach 90% of final value
See below for more information about maintenance and calibration of our tritium monitors.
Overhoff tritium monitors are designed for many years of trouble-free operation with very little maintenance required.
Pump life is typically in excess of 1,000 hours of actual use (2,000 hours on SP1400DD). Pumps are replaced when failure occurs.
General operator maintenance involves:
- Inspecting the dust filter for excessive dust buildup. Check flow rate. Does the pump have sufficient flow such that the Low Flow Alarm is not triggered when 10 feet of sniffer hose is connected to the inlet of the dust filter?
- Gamma check: A low intensity gamma radiation source can be used as a quick verification of monitor performance. On the side of the instrument case towards the front is the defined location for “GAMMA CHECK”. When using the identical gamma check source, at the defined spot, it should always produce the same instrument response, provided, of course, temperature and pressure variations are taken into account. This source check may be performed at a frequency of your choice, it could be daily, weekly or monthly. We recommend a low intensity gamma check source of the type which is commonly intended for G-M counters or other survey instruments. For example, a 10 µCi Cs-137 check source should be sufficient for a monitor reading of 100-200 µCi/m3.
- Battery recharge (NiMH rechargeable) or replacement (alkaline) within an hour after the low battery light illuminates.
Calibration can be performed by using either of the two methods listed below.
- Tritium Gas Calibration
The first method consists of injecting a known activity of tritium gas. To ensure traceability to National Standards (i.e., NIST), the first method must be employed. This method is time consuming, and can be difficult to perform with precision. It 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.
- Gamma Calibration
The second method uses an external gamma field of known field strength. In this instance, the polarization of the compensation ionization chambers is reversed to coincide with that of the measurement ionization chambers. In this condition, the effect of external gamma radiation now adds rather than cancels, and a known gamma field should produce a predetermined measurement indication.
Customers can send their tritium monitor(s) back to the Overhoff factory for annual calibration. Overhoff will calibrate the monitor(s) and provide a calibration certificate traceable to NIST. Alternatively, we also sell tritium calibration gas and tritium gas calibrators to allow you to calibrate instruments on-site.
Warranty, Support, Repair:
All new Overhoff instruments are covered by a 1-year warranty 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.
If repair or service is required, customers can send their Overhoff instrument(s) to our factory. We are also happy to provide support over the phone in case the problem can be fixed on-site.