Model 400AC Tritium Monitor


Model 400AC Tritium Detector by Overhoff

For measurements as low as 2 μCi/m3

Range: 1 – 19,999 μCi/m3

The Model 400AC tritium monitor is a small, lightweight ergonomic design.  The unit is sensitive to the uCi/m3. It includes rechargeable battery, full gamma-compensation, RS232 serial data output, and user recalibration features. The RS-232 port is easily converted to USB.

Additional Features of the Model 400AC

(not available on the Model 400SBDyC)


The 400AC is useful for measurements as low as 2 μCi/m3. The Overhoff electrometer, which measures to below 10-16 amperes, combines low noise and
high zero stability.

Radon Interference, Noise Response

For an unambiguous measurement of very low tritium a monitor must be able to ignore response to ambient radon. The 400AC incorporates this capability and
therefore produces accurate, fast and drift free measurements to nearly +1 μCi/m3.

Total Gamma Compensation

Cruciform ionization chamber geometry provides nearly perfect gamma compensation regardless of photon energy, flux gradient or flux direction.
Gamma compensation of the 400AC is typically three orders of magnitude better than instruments using nested or side by side ionization chambers.
Gamma compensation can be disabled in cases when not required.

Fast Response

Its exceptionally rapid response is primarily due to its abiltiy to ignore radon. The electronic time constant is only 10 seconds, the pneumatic time constant of
about 12 seconds, for an overall time constant of only 15 seconds. Meter readings will reach 90% of final value within 30 seconds to a step response of
aspirated tritium.

Fast Warm-up, No zero Drift

After applying power, the initial transient “warm up” drift effects take less than a minute. Long term drifts have been eliminated and manual zero adjustments
are no longer required.

Automatic Calibration

The 400AC features the ability to perform a fully automatic gamma calibration by using the provided calibration software. Calibration is started with a single
mouse click and requires no intervention. Calibration consists of 3 stages, taking 2 minutes each, for a total of 6 minutes. During the calibration the compensation
ionization chambers are disabled automatically.


Range 1 – 19,999 μCi/m3, basic sensitivity of the order of 2 μCi/m3
Display 0 – 19,999 digits, LCD panel meter
Accuracy ±10 % of reading, ±2 μCi /m3, whichever is greater
Noise Level ± 1μCi/m3, 1 S.D. (10 second electronic time constant)
Zero Stability after 30 seconds (or less) warm up, zero drift less than ± 1μCi/m3
Gamma Compensation chambers in a side by side pattern reduce errors due to external gamma radiation.
Alpha Pulse Suppression a circuit provides recognition and cancellation of undesirable noise spikes attributed to
airborne radon
Response Rate 30 seconds to reach 90% of final reading
Alarm - Acoustic 1. Ten position stepped attenuator set point for signal alarm
2 - 1,000 μCi/m3, steady tone. OFF position is included.
2. Low flow produces an intermittent tone
3. Mute switch silences audible tone
Alarm - Visual signal level: red LED
low flow: yellow LED, flashing
low battery: red LED
Output RS232 serial data output for tritium measurement, level alarm status and calibration
Ionization effective volume: 400 cm3
Chamber volume port to port volume: 440 cm3
Dust Filter external in-line disposable cartridge type
Pump internal rotary vane pump
Flow Rate nominally 1.5 - 2 LPM
Environmental 0° C to +40° C, 20 - 90 % relative humidity non-condensing
Batteries two “D” size NiMH batteries
external jack for supplementary power input and charging
Power Converter 100-240 VAC, 50/60 Hz, .25 A to 3.3 Vdc @ 1.2 A
5.5 mm O.D. x 2.1 mm I.D. Plug, center pin is positive
Size and Weight 7.6" [193mm] L x 5.2" [132mm] W x 6.9" [175mm] H excluding handle, 6.5 lbs (3 kg)
Calibration Automatic Calibration using pc-based software (included)
Manual Calibration by adjusting a calibration potentiometer