Guy Prevost
Well Known Member
I've been asked this question several times, so I thought I would write up a quick FAQ.
I really wanted to put my Vertical Card Compass in the upper left corner of my RV-8A. However, when I placed my compass there, it swung approximately 60?. Some experimentation with a handheld hiking compass indicated that my rollbar had a definite magnetic field. I tried a few tricks to locate the exact location of the problem. As it turns out it doesn't matter--we're going to degauss the whole rollbar here.
Here's a couple of photos of my rollbar prior to degaussing.
First photo--The plane is facing ~North.
Here's what the compass does when near the roll bar. Notice the needle always points to the rollbar:
All right. I definitely need to degauss the roll bar if I want to put a compass here. The little Television screen degaussing tools you can buy, or even a soldering gun don't have nearly the oomph to degauss something as massive as this weldment. We need power here. After a bit of research, I discovered the growler.
Basically a growler is a big coil of wire with a stack of iron plates going through it. The alternating current (regular household current) going through the coil creates a magnetic field. The iron plates are there to direct the magnetic field. The plates end in a V-shaped notch which is where the field is strongest. That?s the part you move back and forth over the item you want to demagnetize. In the old days, you would have put the armature from your car or tractor there to test it. The magnetic field generated by the growler would induce a current in the armature which was usually connected to a light bulb. If the light bulb came on?the armature was good. If not, there was a broken wire in the armature somewhere. You could then use the growler and a probe to find the break.
Here?s how it works for demagnetization. If your rollbar (or other item) is magnetized, it means that many of the iron particles in it are aligned with each other. This usually happens during welding. The metal gets melted, allowing the particles to align with the earth?s magnetic field. Once the weld pool cools, they?re pretty much stuck that way. Over time they can move, but it?s a slow process. The magnetic field the growler produces switches directions 60 times a second, because that?s how often your household current switches directions. By passing this field from the growler over a magnetized piece of steel, you make the orientation of the magnetic particles random again. They're trying to align with a field that's changing directions so fast it's basically random.
I padded the jaws of the growler with duct tape to protect the powder coating of the rollbar. I removed all sensitive items from my panel, especially the compass.
Take your watch off, and if you have any medical devices that might be affected by a strong magnetic field--Don't do this!
When the growler is powered up, it will emit an ominous 60Hz hum. It draws 4 amps, so it's turning ~480 Watts of household current into a magnetic field (OK, some of that is going to heat...). Pass the growler slowly and continuously over the entire structure you want to demagnetize. Be patient--it takes time. Eventually the growler will become too warm to comfortably handle. Just shut if off, let it cool, and come back after a bit. I want to emphasize that this takes a bit of time. The more powerful your growler is, the less time it will take. It took me about 3- 10 minute sessions to do my rollbar.
Here's a photo of the handheld compass after the process:
It's now pointing North instead of toward the rollbar. My vertical card compass, located in the left 2.25" hole, went from pointing 60? off of heading to pointing within 2? of the proper heading. I can take that out with its built in adjustment.
Cheers,
Guy
I really wanted to put my Vertical Card Compass in the upper left corner of my RV-8A. However, when I placed my compass there, it swung approximately 60?. Some experimentation with a handheld hiking compass indicated that my rollbar had a definite magnetic field. I tried a few tricks to locate the exact location of the problem. As it turns out it doesn't matter--we're going to degauss the whole rollbar here.
Here's a couple of photos of my rollbar prior to degaussing.
First photo--The plane is facing ~North.
Here's what the compass does when near the roll bar. Notice the needle always points to the rollbar:
All right. I definitely need to degauss the roll bar if I want to put a compass here. The little Television screen degaussing tools you can buy, or even a soldering gun don't have nearly the oomph to degauss something as massive as this weldment. We need power here. After a bit of research, I discovered the growler.
Basically a growler is a big coil of wire with a stack of iron plates going through it. The alternating current (regular household current) going through the coil creates a magnetic field. The iron plates are there to direct the magnetic field. The plates end in a V-shaped notch which is where the field is strongest. That?s the part you move back and forth over the item you want to demagnetize. In the old days, you would have put the armature from your car or tractor there to test it. The magnetic field generated by the growler would induce a current in the armature which was usually connected to a light bulb. If the light bulb came on?the armature was good. If not, there was a broken wire in the armature somewhere. You could then use the growler and a probe to find the break.
Here?s how it works for demagnetization. If your rollbar (or other item) is magnetized, it means that many of the iron particles in it are aligned with each other. This usually happens during welding. The metal gets melted, allowing the particles to align with the earth?s magnetic field. Once the weld pool cools, they?re pretty much stuck that way. Over time they can move, but it?s a slow process. The magnetic field the growler produces switches directions 60 times a second, because that?s how often your household current switches directions. By passing this field from the growler over a magnetized piece of steel, you make the orientation of the magnetic particles random again. They're trying to align with a field that's changing directions so fast it's basically random.
I padded the jaws of the growler with duct tape to protect the powder coating of the rollbar. I removed all sensitive items from my panel, especially the compass.
Take your watch off, and if you have any medical devices that might be affected by a strong magnetic field--Don't do this!
When the growler is powered up, it will emit an ominous 60Hz hum. It draws 4 amps, so it's turning ~480 Watts of household current into a magnetic field (OK, some of that is going to heat...). Pass the growler slowly and continuously over the entire structure you want to demagnetize. Be patient--it takes time. Eventually the growler will become too warm to comfortably handle. Just shut if off, let it cool, and come back after a bit. I want to emphasize that this takes a bit of time. The more powerful your growler is, the less time it will take. It took me about 3- 10 minute sessions to do my rollbar.
Here's a photo of the handheld compass after the process:
It's now pointing North instead of toward the rollbar. My vertical card compass, located in the left 2.25" hole, went from pointing 60? off of heading to pointing within 2? of the proper heading. I can take that out with its built in adjustment.
Cheers,
Guy