Use of magnetic field dosemeters for occupational exposure assessment

Philip Chadwick, EMF Dosimetry Handbook Project Co-ordinator

 

 This summary should be read in conjunction with John Swanson's chapter on Power frequency EMF measurements

 

In 1997 the UK Health and Safety Executive published Contract Research Report CRR 1997:128 on the suitability and use of an EMDEX II magnetic field dosemeter for the assessment of the magnetic field exposure of induction heating workers. 

 

This is a report of good general usefulness as the induction heating environment can give rise to some of the highest occupational exposures to magnetic fields at a range of frequencies, from les than 10 Hz to greater than 10 kHz.  The waveforms of the magnetic fields can be complex, with a high harmonic content, and exposures can be transitory and very dependent on the details of working practices.

 

This brief summary is provided for the EMF Dosimetry Handbook by permission of HSE


 

 

 

The HSE report contains some very useful general conclusions about the use of personal dosemeters in industry:

 

·               Dosemeters will give better estimates of true operator exposure than can be derived from spot measurements and observations of working practices

 

·               The use of dosemeters on multiple workers or for prolonged periods will minimise the perturbation of working practices

 

·               Exposures which occur infrequently and which would not be predicted from routine observation may be captured by a dosemeter

 

·               Variations in emitted magnetic flux density during a long production cycle will be accounted for

 

 

Several aspects of magnetic field dosimetry in industrial environments were emphasised:

 

·               The exposure environment must be characterised, to allow comparison with exposure standards and to ensure that the magnetic flux densities encountered are within the frequency and dynamic ranges of the dosemeter

 

·               The dosemeter data should be examined carefully for evidence of bad data and the range of recorded exposures compared with those predicted from spot measurements. Discrepancies should be investigated further

 

·               The functionality of the dosemeter should be checked immediately before it is issued and after it has been returned

 

·               It is a feature of many induction heating environments that the magnetic flux density varies across the dimension of the human body. It must be recognised that the position at which the dosemeter is worn may affect the maximum magnetic flux density that it records.

 


Particular conclusions related to practical aspects of the use of dosemeters included:

 

·               Most of the operators wore dosemeters for one or two days only, and it is conceivable that their working practices may have been affected, consciously or otherwise.

 

·               Some of the dosemeter data are not consistent with the spot measurements of magnetic flux density made at each site and it is likely that the magnetic field emissions vary markedly between different stages in the production process

 

·               Reliable personal dosimetry would require the wearing of a dosemeter for many days.

 

·               The exposure of induction heater operators is characterised by transient exposure maxima as they approach the coils and the dosemeter should be set to the highest sample rate possible to avoid problems of aliasing. Where aliasing does occur, the amplitude of the maxima is likely to be underestimated but warning of this is given by the presence of the bad data flag in the key byte of the EMDEX data file. Workers' exposure records should be checked for the presence of this flag and if it is present, the data treated with caution.

 

·               Sample rate is often limited by the available memory of the dosemeter, and memory constraints also preclude routine measurement of the separate vector components of magnetic flux density.

 

·               It is vital that the battery not be allowed to run down before the data file is transferred to a computer or the exposure information will be lost.

 

·               If the dosemeter is dropped, one or more sensing coils may be damaged. This may not be obvious to the user but could seriously affect the validity of the data gathered. It is important that the integrity of each data channel is checked with a test source before and after each exposure assessment is made.

 

 

 


 

 

Dosemeter record from operator of induction furnace