Model 347


  • Ultra Sensitive Continuous Duty Tritium Air Monitor
  • Superior cruciform gamma compensation (4L volume), virtually no offset in fields up to 30 mR/h
  • Sensitivity: 0.5 µCi/m3, 0.02 MBq/m3
  • Range: 1 to 19,999 µCi/m3, 0.1 to 1,999.9 µCi/m3, 0.01 to 199.99 MBq/m3
  • Single adjustable alarm set-point
  • Audible and visual alarm indicators for signal level, low flow (optional)
  • Alpha pulse suppression, radon rejection
  • Long-term zero stable, span calibration is permanently stable due to special electrometer design

Available Options:

  • Plate-out proof wire grid ion chambers to reduce tritium/gamma contamination by up to 1000x
  • HTO only measurement and noble gas compensation
  • Low flow alarm
  • Choice of one additional data output: RS-232, 4-20mA, or Ethernet
  • Remote display unit
  • Mounted on wheeled stainless-steel cart for portability


The Model 347 is an ultra-sensitive stationary tritium in air monitor designed for continuous duty.  The 347 utilizes four 2L ionization chambers for the best possible sensitivity, stability, and gamma compensation available.  The large chamber volume provides additional sensitivity and stability, even in areas with elevated or fluctuating background levels.  Two chambers are used for measurement and two for gamma compensation, arranged in a cruciform geometry to provide near perfect gamma compensation regardless of photon energy, flux gradient, or flux direction.  The pump system is mounted on a separate panel and includes an ultra-pure filter element, HEPA filter, pump, and flowmeter.  The ultra-pure filter element combined with the HEPA filter ensure the sample air is completely clean of particulates before being measured to enhance the sensitivity and accuracy.  With radon rejection, the Model 347 is fast, ultra-sensitive, and accurate.

The Model 347 includes a standard 0-10VDC analog output found on all our stationary instruments and can support one additional data output (choice of: RS-232, RS-485, 4-20mA, or Ethernet).  It has a simple alarm system consisting of a single independent signal alarm and a toggle switch with three alarm modes: on, off, and reset.  The signal alarm activates the acoustic signal and will continue to operate until manually reset, even if the displayed signal has receded below the setpoint.  A remote alarm unit can be added that includes a latching or timing circuit to easily mute and re-set the alarm.  Relay interface connections are included on a terminal block at the rear of the enclosure.

Proprietary circuitry provides recognition and cancellation of undesirable noise spikes attributed to airborne radon.  Unique plate-out proof wire-grid ion chambers are available in place of the standard solid-wall chambers which will reduce tritium contamination by three orders of magnitude by decreasing the true surface area of the chamber.

The only maintenance required for the 347 is periodic service of the pump and replacement of the dust filter.  The sensitivity, accuracy, and noise level of the Model 347 is superior to competitive instrumentation by an order of magnitude.

Calibration of the 347 tritium monitor can be accomplished by three different methods:

  1. Tritium gas calibration: Injecting tritium gas into the ionization chambers to produce an accurately predictable concentration and adjusting the calibration potentiometer accordingly. Overhoff offers factory calibration services and also provides tritium gas calibrators for the customer to calibrate on-site.
  2. Gamma calibration using a gamma source: Using a certified gamma range or source, tritium monitors can be calibrated using the relationship 1mR/hr yields the same ionization current as 90 µCi/m3 of tritium.
  3. Electrical equivalence method: All Overhoff ionization chamber – electrometer modules carry a BNC receptacle to which a precisely calibrated ultra-high resistor can be attached. Using Ohm’s law and knowing the volume of the ion chamber, you can calculate the current value and expected panel meter display and adjust the calibration potentiometer as required.  Calibration resistors are available for purchase.
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