Model 357RM


  • Continuous Duty Tritium Air Monitor
  • Coaxial gamma compensation (2L volume) with virtually no offset in fields up to 20 mR/h and built-in ion-trap
  • Sensitivity: 1 µCi/m3, 1 MPCa, or 0.04 MBq/m3
  • Range: 1 to 19,999 µCi/m3, 0.1 to 1,999.9 MBq/m3, DAC, and MPCa, 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:

  • Low flow alarm with alarm indicator on the front panel and relay contacts on the rear panel
  • Plate-out proof wire grid ion chambers to reduce tritium/gamma contamination by up to 1000x
  • HTO only measurement and noble gas compensation
  • 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 357RM is our basic, low-cost, general purpose stationary tritium in air monitor designed for continuous duty.  The 357RM tritium monitor has been in service for over 30+ years and is proven to be easy to use, accurate, and reliable.  Useful for measuring airborne tritium concentration in rooms, stacks, exhausts, glove boxes, and similar environments.  Suitable for 19” rack-mount or table-top use, the Model 357RM includes the essential components required for the accurate measurement of tritium in air: dual 2L ionization chambers with coaxial gamma compensation, long life pump with HEPA filter (99.99% efficiency at 0.1 micron) and flowmeter, alpha pulse suppression, and a single adjustable alarm set-point with audible and visible alarm.  The HEPA filter component is located on the front panel for easy inspection and service.

The Model 357RM 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.  An optional low flow alarm with a front panel alarm indicator can also be added.

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.

If requested, the HTO gas ports can be installed on the compensation chamber for noble gas or other radioactive gas compensation.

The only maintenance required for the 357RM is periodic service of the pump and replacement of the dust filter.  The sensitivity, accuracy, and noise level of the Model 357RM is superior to competitive instrumentation by an order of magnitude.

Calibration of the 357RM 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 that 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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