Model 451P Ion Chamber - Pressurized

Micro Roentgen Survey Meter for Gamma and X-ray

451P MicroR Ion Chamber, for gamma radiation measurement

The Fluke Model 451P Ion Chamber is used in a wide range of medical and health physics applications. The Model 451P Ion Chamber was designed to measure leakage and scatter around diagnostic X-ray and radiation therapy suites. Furthermore, the Model 451P is ideal for site surveys and is regularly used by X-ray manufacturers, radiation therapy, government agencies, state inspectors, research labs, biomedical technicians, and in airports for baggage inspection equipment maintenance. This Ion Chamber was manufactured by Victoreen which is now Fluke Biomedical.

451P MicroR Ion Chamber, display

451P Ion Chamber Specifications

Radiation Detected: Gamma and X-rays above 25 keV

Operating Ranges

0-500 µR/h or 0-5 µSv/h
0-5 mR/h or 0-50 µSv/h
0-50 mR/h or 0-500 µSv/h
0-500 mR/h or 0-5 mSv/h
0-5 R/h or 0-50 mSv/h

Accuracy: Within 10% of reading between 10% and 100% of full scale indication on any range, exclusive of energy response. Calibration source is Cs-137

Detector Chamber: 300 cc volume pressurized air ionization chamber to 6 atmospheres

Controls: ON/OFF and MODE

Automatic Features: Auto-zeroing, auto-ranging, and auto-backlight

451P Response Time: Analog response time from 10% to 90% of reading for a full scale step increase is dependent on operating range. Response time for a step increase in radiation exposure rate from background:

Step Increase Time
400 µR/h 4.8 seconds
4 mR/hr 3.3 seconds
10 mR/hr 4.3 seconds
40 mR/hr 4.5 seconds
100 mR/hr 2.7 seconds
1 R/hr 2 seconds
4 R/hr 2.7 seconds

The following table shows time measured from 10% to 90% of final value for a step increase or decrease in exposure rate such that a range change does not occur. These values are the response times for the various ranges:

Range 10%-90% Time
0-500 µR/h 5 seconds
0-5 mR/h 2 seconds
0-50 mR/hr 1.8 seconds
0-500 mR/hr 1.8 seconds
0-5 R/hr 1.8 seconds

Power: Two 9-volt alkaline, 200 hours operation

Display: LCD analog/digital with backlight

Warm-Up Time: Less than two minutes for initial operation when the instrument is in equilibrium with ambient temperature


Temperature Range: -20°C to + 50°C

Relative Humidity: 0 to 100%

Geotropism: Negligible

Typical Energy Dependence: Nitrogen gamma rays are 110% to 120% of indicated readings as determined at the University of Lowell

Dimensions: 4 in (w) x 8 in (d) x 6 in (h) (10 x 20 x 15 cm)

451P energy response curve

Weight: 2 lb 6 oz (1.07 kg)

Modes: Integrate mode operates continuously 30 seconds after the instrument has been turned on. Integration is performed even if the instrument is displaying in mR/h or R/h

Freeze mode will place a tick mark on the bar graph display to hold on the peak displayed value. The unit will continue to read and display current radiation values.

Model # Part #
DS 451P Ion Chamber 451P-RYR
451 XL Software/Cable 451EXL