Magnetic Field Reduction

The magnet power supplies pull their power from the wall main. Quite a bit of energy is stored in the magnetic field, and that energy has to be supplied by the power supply. A 9V battery doesn't contain much energy, although it does have enough voltage. That said, the higher the supply voltage, the faster you can ramp up a magnet. Essentially, what you have is dv/dt = di/dt * L, where v is the voltage, i the current, and L the inductance of the magnet. The faster you change the current, the higher the voltage. There are also funny things with superconducting materials, where they can be driven out of the superconducting range and start making heat, so you also have to be careful to stay out of those modes.

Tom

 

Let's see........two hydrogen ions with one oxygen ion. Sounds lethal!

 

Please don't suggest that. Someone out there might actually try it!

 

DH is going to FLIP OUT when i show him this post! i can't wait to see his reaction. his biomed classes are this fall and he is so excited to learn how all this stuff works.

thanks for sharing.

 

Yeah Tom, thanks for sharing, that's fun stuff. It makes me wonder though... isn't surgical steel a ferrous material? Being steel and all? I have steel rods in my spine and I've been in an MRI machine without having bits of metal come flying out of me. Or is the field very different on the inside?

 

It's now quite clear my Calculus would have actually been used on your engineering team. Some days it's a wonder I can even add or subtract ...

 

Surgical steel is, through a miracle of metallurgy I don't fully understand, non-magnetic.

 

Surgical Steel is just Stainless Steel. Stainless Steel is non-magnetic.

 

Well, I find that mild, well-mannered ridicule is a good way to get someone's attention when they are so wildly off base. In any case, I am completely sincere in this:

Ellie: If you're trying to find ways to manage risk in your life, then you are starting on totally the wrong end of the risk spectrum. To the extent that you pay attention to things like EM fields to the detriment of spending time on substantive, proven risks, you will be increasing your overall risk, not decreasing it.

 

Some stainless is, some isn't. I could write pages on metallurgy, but trying to keep it brief, it all comes down to the alloy. The 300 series stainless is called austenitic, which is non-ferrous. Other stainless steels go through this stage when cooling, but end up being non-austenitic. The two main groups of non-austenitic stainless are ferritic and martensitic. The austenitic stainless steels are more corrosion resistant, but also softer and more expensive. Surgical steel is obviously a high grade 300 series, so there isn't a problem with magnetic attraction. While there isn't any problem with the magnetic field, the metal screws up the RF and distorts the image. Any sort of metal can make it hard to get a good scan near the metal.

Tom

 

This thead as been going on for quite some time now and I see that you are still all "ALIVE" (and well I suppose?). So what is going on? Is the field not affecting you? I know I am going to live to be a 100 years old because I am not stressing about the gas bill anymore.
Otherwise, interresting theard.

 

Hi dngrsone,

Well, shooting electrons across the vacumm of a CRT is not RF. Its electric current kinda like in a flash light. In order to get RF you need to make the electron beam go in a circle (like in a Magnetron in a Microwave oven), and the CRT is doing just the opposite. The CRT is designed to get the electron beam as straight and in the right direction as possible.

The bigger problem is misallignment of the CRT beams that then might hit shaddow mask. The shaddow mask is a perforated metal sheet behind the screen. It absorbs electrons that were to bounce off the inside of the screen, and then come back to land on other pixels than the intended pixels. If you look inside an X-Ray tube you see its just a big electron gun, like in the back of a CRT, and shoots at a metal target. The electron beam voltage in a CRT is sufficient to make X-Rays when the beams hit the shaddow mask. Which is why the CRT glass is lead-glass. Which was the second use of Lead in industry (to car batteries) prior to the LCD revolution. The lead in the glass absorbs X-rays.

So, if you want to reduce radiation effects, replacing all the monitors and TV's with LCD screens would be more effective than Prius shielding. As X-Rays are ionising radiation, and much more dangerous than RF or the static and low frequency magnetic fields in the Prius at levels less than the magnetic field of the earth.

As for copper mesh suits, they are used. But usually when one needs to climb up the mast of active TV stations antennas, to work on an antenna higher up. Like on top of the Sears Tower. In such enviorments with live multiple 50 KW emissions just yards away, this is very prudent.

 

This post made me smile. I'm the president of my own electronics company now, but in my former life as an industry guy, I worked in several medical related companies. I worked in R&D and later production at a pharmaceutical company (we were the ones that brought you cortisone, Motrin, and Xanax) . At another company we did one of the first self-reading EKG machines, and a lot of EP cathlab equipment. I already mentioned the MRI scanners. We can skip the stuff for the Navy and Edwards, plus all the vibrational analysis equipment. I've had a pretty good run.

Your comment about DH's classes made me think of another story back when I was the head of the MRI group: We had a group of instructors that trained MRI technicians to use the equipment, and I decided to go through the training with one of the classes, just to see how the technicians worked in the real world. The whole class was female, except for me. The first day in the suite with the high-field magnet, I walked in and here were two of the students standing with their backs to the magnet, twisting and turning slowly, a rapt look on their faces; kind of like dancing on qualudes. I couldn't imagine what they were doing, so I had to ask. Both were wearing underwired brassieres, and they had discovered that the field pulled on the underwire. They were experimenting with different positions.

I have another story that DH would like, but I will spare you all the details. Just tell him that at one point we were having trouble with our gradient amps, and they were failing somewhat catastrophically. Two of our repair technicians were working on-site at a scanning center when one of the rack mounted amps blew. Flames were coming out of the back of it. They pulled it out and ran through the waiting room, flames and all, and tossed it out the door into the parking lot. I don't imagine that impressed the waiting patients.

Have DH post questions if he wants to chat. I'm always happy to pontificate about technology.

Tom

 

he loves your stories! like i said, he doesn't get into biomed instrumentation, imaging tech and troubleshooting until the fall. but he is really looking forward to it. right now he's doing a lot of electronics theory- he's got the practice down but never had a formal theory class.

 

That just isn't fair. Some guys have ALL the luck! I suspect while you were in the room with the gyrating women, I was at a petrochemical facility changing ORP or conductivity sensors

 

<smirk>

 

Ah, this would explain why the nice people at the MRI clinic have to ask so many personal and embarrassing questions about body piercings.

 

Yep. Even some of the tattoo inks are ferromagnetic and cause problems.

Tom

 

Well, I see this thread has been going on for some time, but let me add my "two cents" worth.

I have the same problem with the AM radio in two Toyota vehicles. The AM radio worked perfect in both the 96 4Runner and 04 Prius when purchased new. At different times, I have begun to experience the same problem on both vehicles. The AM radio in the 4Runner started having static problems about five years ago. The FM radio still works fine. The static occurs when I drive under overpasses, high voltage lines or other structures with a lot of steal in them.

After two years, I started to experience the same problem with the Prius. When I drive across country where nothing but corn surrounds me, the problem does not manifest itself. The AM radio operates like new on both vehicles.

From years ago when I helped work on ham radios, I recall something about RF interference with radios and that we used to install a condenser or a filter to eliminate that type of interference.

G Man:fencing:

 

The RF interference you are experiencing in your two examples are as follows: When passing under a bridge/overpass, the structure gets between your AM radio antenna and the source of the transmission (the radio tower). This results in a weakening of the signal. Also, there may be some reflections of the signal causing interference because of multiple, out-of-phase signals being received.

The interference you experience when passing under high-voltage power lines involves large magnetic fields at 60Hz (plus, to a much diminished extent, harmonics of 60Hz) which will distort, reflect and deflect your incoming signal, and possibly interfere with the IF amplifiers in your AM receiver (depending on adequacy of shielding).

These problems happen with all radios-- it's in the nature of the beast. Some will work better than others because they are designed to more discriminating or noise-resistant, or have better signal to noise ratios.