I'm no medical Guru either but I know the sort of field levels involved. They range from about .5 to 3 teslas. This is not much higher than the fields in the gaps of electrical motors and generators (up to 1.8T in normal silicon iron and up to 2.1 T in cobalt iron used in aerospace machines) The difference is the length of the effective magnetic dipole. The field in an MRI machine is almost constant across the whole bore of the machine and for whole body machines this gives a dipole length of a metre or more. You are still in the near field up to a couple of dipole lengths before the field strength starts drooping as the inverse cube of the distance. Thus it is quite possible to have fields up 1T a metre or more from the machine this corresponds to an H field of 790 kA/m and this is sufficient to lift large ferromagnetic objects. The dipole length in a normal electrical machine is about the size of the air gap, about 1mm or less. Thus there are very strong magnetic fields only out to a few centimetres. I don't know if this is the reasoning behind the field values in IEC 60601-2-24 but if it is the levels do not seem unreasonable.
Nick Rouse . ----- Original Message ----- From: "Chris Maxwell" <[email protected]> To: <[email protected]>; <[email protected]> Sent: Tuesday, April 30, 2002 4:22 PM Subject: RE: IEC 60601-2-24 / Magnetic Field > > Hi Amund, > > I read your email (below). I'm no medical device expert; but I'm > wondering if the magnetic strength limit was set so high due to MRI > (Magnetic Resnonance Imaging) devices. My understanding is that MRI > devices produce HUGE magnetic field levels. According to a recent > newspaper story that I read, these fields are strong enough to draw a > metal oxygen tank across the room to the MRI machine. > > Is there any chance that the device in question could be used around an > MRI? It would be a real bummer if someone's infusion pump suddenly > changed its run rate due to a nearby magnetic field. > > Could that be what the standard is safeguarding against? Any medical > device gurus care to comment? > > > > Chris Maxwell | Design Engineer - Optical Division > email [email protected] | dir +1 315 266 5128 | fax +1 315 797 ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
