Magnetic field measurements near RF induction heaters.
Conover-DL; Murray-WE; Lary-JM; Johnson-PH
Bioelectromagnetics 1986 Jan-Mar; 7(1):83-90
A magnetic field strength monitor developed for NIOSH by the National Bureau of Standards was used for field surveys of ten radiofrequency induction heaters used for fatigue testing of metal alloys and six heaters used to melt a variety of metallic or crystalline materials. The first group of heaters had a nominal power output of 2.5 kilowatts (kW) at 450 kilohertz (kHz), and the second had a frequency range of 303 to 488kHz and a nominal power output of 10 to 50kW. The monitor consisted of three mutually orthogonal loop antennas with diode detectors connected to a signal processing/metering. It monitor displayed peak or mean squared magnetic field strengths from 0.01 to 10,000 amperes squared per square meter (A2/m2) with an accuracy of +/- 1.0 decibel at 13 frequencies from 300kHz to 100 megahertz. Measurements were taken in areas normally occupied by operators, in front of heaters and cables, and in areas not normally occupied close to applicator coils and cables. The axis of all coil applicators was vertical, and preliminary polarization measurements indicated that other magnetic field components were insignificant compared to this component. Measurements indicated operator exposure ranges varying from less than 0.01 to 0.5A2/m2 up to 3 to 300A2/m2. Duty factors were not determined, but operators' estimates established an approximate range of 0.083 to 0.333. Assuming a duty factor of 0.083, 25 percent of sources had operator exposures exceeding 2.5A2/m2, the threshold limit established by the American Conference of Governmental Industrial Hygienists in 1983. Assuming a duty factor of 0.333, 75 percent of sources had operator exposures exceeding 2.5A2/m2. Analysis of magnetic field strength as a function of distance from the applicator coil indicated that magnetic field strength varies inversely with roughly the fifth power of separation distance. The authors conclude that accurate operator position data are required to obtain meaningful occupational exposure estimates and to develop effective control technology.
BIOEDI; NIOSH-Author; Radio-waves; Magnetic-fields; Monitors; Measurement-equipment; Heating-equipment; Occupational-exposure; Industrial-equipment;
Author Keywords: RF induction heaters; magnetic field strength monitor; occupational RF exposures
Dr. David L. Conover (C-27). NIOSH, DBBS, PAEB, 4676 Columbia Parkway, Cincinnati, OH 45226