Magnetometer variations

Hello All

If you record data from the magnetometer continuously and you enter a
building, do you expect to see a sudden change in the magnitude of the
data?

I tried recording the data this way but I do not see a much change in the
magnetic field.

Any pointers will be helpful. 

Best Regards

Sia
---------------------------------------
Posted through http://www.DSPRelated.com

On Tue, 20 Oct 2015 09:54:28 -0500, "sia4uin" <105263@DSPRelated>
wrote:

>Hello All
>
>If you record data from the magnetometer continuously and you enter a
>building, do you expect to see a sudden change in the magnitude of the
>data?
>
>I tried recording the data this way but I do not see a much change in the
>magnetic field.
>
>Any pointers will be helpful. 
>
>Best Regards
>
>Sia

Shielding a DC magnetic field is pretty tough to do.   Unless
something in the building is generating it's own field, which is
possible, or distorting the natural field, one wouldn't expect much
change.


Eric Jacobsen
Anchor Hill Communications
http://www.anchorhill.com

>Shielding a DC magnetic field is pretty tough to do.   Unless
>something in the building is generating it's own field, which is
>possible, or distorting the natural field, one wouldn't expect much
>change.
>
>
>Eric Jacobsen
>Anchor Hill Communications
>http://www.anchorhill.com

Thank you Eric. So lets say, if I walk into a superstore, will it register
any change? I still have to try this experiment but what are your
thoughts?

Best Regards

Sia


---------------------------------------
Posted through http://www.DSPRelated.com

On Tue, 20 Oct 2015 10:35:16 -0500, "sia4uin" <105263@DSPRelated>
wrote:

>
>>Shielding a DC magnetic field is pretty tough to do.   Unless
>>something in the building is generating it's own field, which is
>>possible, or distorting the natural field, one wouldn't expect much
>>change.
>>
>>
>>Eric Jacobsen
>>Anchor Hill Communications
>>http://www.anchorhill.com
>
>Thank you Eric. So lets say, if I walk into a superstore, will it register
>any change? I still have to try this experiment but what are your
>thoughts?

It might, because there are obviously things that can change it that
might be in the store.   There is nothing inherent in "buildings"
though, that will always cause a change.


Eric Jacobsen
Anchor Hill Communications
http://www.anchorhill.com

>>Thank you Eric. So lets say, if I walk into a superstore, will it
register
>>any change? I still have to try this experiment but what are your
>>thoughts?
>
>It might, because there are obviously things that can change it that
>might be in the store.   There is nothing inherent in "buildings"
>though, that will always cause a change.
>
>
>Eric Jacobsen
>Anchor Hill Communications
>http://www.anchorhill.com


OK, I will do this experiment tomorrow and see what happens. 

Thanks Eric.

Best Regards

sia

---------------------------------------
Posted through http://www.DSPRelated.com

Eric Jacobsen <eric.jacobsen@ieee.org> wrote:
> On Tue, 20 Oct 2015 09:54:28 -0500, "sia4uin" <105263@DSPRelated>

(snip)
>>If you record data from the magnetometer continuously and you enter a
>>building, do you expect to see a sudden change in the magnitude of the
>>data?

>>I tried recording the data this way but I do not see a much change in the
>>magnetic field.

(snip)

> Shielding a DC magnetic field is pretty tough to do.   Unless
> something in the building is generating it's own field, which is
> possible, or distorting the natural field, one wouldn't expect much
> change.

Pretty tough, but it can sort of be done.

Color CRT displays normally have a mu-metal shield to keep the
effects of outside magnetic fields out. Mu-metal keeps magnetic
fields out about the same way that conductors keep electric fields
out, but it can also get magnetized.  With appropriate degaussing,
magnetizion can be reduced enough that it mostly keeps the earth's
field out.

I never knew before that degaussing originated in keeping ships
from triggering magnetic mines in WW II. 

https://en.wikipedia.org/wiki/Degaussing

But it isn't so obvious what this has to do with buildings.

If you go into a building with a metal detector magnetometer,
though, I suspect you will see an increase in the AC magnetic
field. 

-- glen

On 20.10.15 16.54, sia4uin wrote:
> If you record data from the magnetometer continuously and you enter a
> building, do you expect to see a sudden change in the magnitude of the
> data?

The magnetic field has always some local variations due to iron parts 
close to you. But it is mainly the direction that changes rather than 
the amplitude.

> I tried recording the data this way but I do not see a much change in the
> magnetic field.

Not that unexpected.

We had 3 orthogonal Helmholtz coils with several meters diameter to get 
some compensation of the earth magnetic field in our lab. Quite much effort.


Marcel

On 21.10.15 06.46, glen herrmannsfeldt wrote:
>> Shielding a DC magnetic field is pretty tough to do.
>
> Pretty tough, but it can sort of be done.
>
> Color CRT displays normally have a mu-metal shield to keep the
> effects of outside magnetic fields out.

Nope. The magnetic field will not be shielded by a CRT tube.

The (mandatory) degauss coil in fact causes the shadow mask to be 
/magnetized/ in the same way than the surrounding field. So it 
integrates seamlessly without local distortion of the magnetic flux lines.

The direct effect of the earth magnetic field is quite low and 
compensated by the static convergence setting which in fact slightly 
moves the beam source. AFAIK this is the main reason for the high 
accelerating voltage. The phosphor would be more efficient at lower 
energy electrons.

In fact it would have been much easier to use a paramagnetic or 
diamagnetic material for the shadow mask. But it need to have the same 
coefficient of thermal expansion than the lead glass of the front screen 
and it need to be a conductor to deflect the beam current. And glass has 
a much lower coefficient of expansion than any metal. So you need the 
magnet-volume effect to compensate for the thermal expansion of the 
metal. The shadow mask is made of Invar.

> If you go into a building with a metal detector magnetometer,
> though, I suspect you will see an increase in the AC magnetic
> field.

Yes, usually the line frequency is visible.


Marcel

Marcel Mueller <news.5.maazl@spamgourmet.org> wrote:
> On 21.10.15 06.46, glen herrmannsfeldt wrote:
>>> Shielding a DC magnetic field is pretty tough to do.

>> Pretty tough, but it can sort of be done.

>> Color CRT displays normally have a mu-metal shield to keep the
>> effects of outside magnetic fields out.
 
> Nope. The magnetic field will not be shielded by a CRT tube.
 
> The (mandatory) degauss coil in fact causes the shadow mask to be 
> /magnetized/ in the same way than the surrounding field. So it 
> integrates seamlessly without local distortion of the magnetic flux lines.

As far as I know, you need both. The smaller field, over the longer
distance from gun to screen needs the mu metal shield.

https://en.wikipedia.org/wiki/Mu-metal

> The direct effect of the earth magnetic field is quite low and 
> compensated by the static convergence setting which in fact slightly 
> moves the beam source. 

But you don't want to have to readjust every time you move.

> AFAIK this is the main reason for the high accelerating voltage. 
> The phosphor would be more efficient at lower energy electrons.

But lower voltage would require higher current, which isn't so easy,
especially to make a small spot. 
 
> In fact it would have been much easier to use a paramagnetic or 
> diamagnetic material for the shadow mask. But it need to have the same 
> coefficient of thermal expansion than the lead glass of the front screen 
> and it need to be a conductor to deflect the beam current. And glass has 
> a much lower coefficient of expansion than any metal. So you need the 
> magnet-volume effect to compensate for the thermal expansion of the 
> metal. The shadow mask is made of Invar.

It seesm that invar is used only for the shadow mask frame, where it
joins to glass.  As I just learned from:

https://en.wikipedia.org/wiki/Shadow_mask

The rest of the shadow mask is attached to the frame while warm,
and then as it cools, contracts and so is under tension. 

This tension keeps the shadow mask from deforming as it warms
up from the power of absorbed electrons.

-- glen

Marcel Mueller <news.5.maazl@spamgourmet.org> wrote:
> On 21.10.15 06.46, glen herrmannsfeldt wrote:
>>> Shielding a DC magnetic field is pretty tough to do.

>> Pretty tough, but it can sort of be done.

>> Color CRT displays normally have a mu-metal shield to keep the
>> effects of outside magnetic fields out.
 
> Nope. The magnetic field will not be shielded by a CRT tube.
 
> The (mandatory) degauss coil in fact causes the shadow mask to be 
> /magnetized/ in the same way than the surrounding field. So it 
> integrates seamlessly without local distortion of the magnetic flux lines.

As far as I know, you need both. The smaller field, over the longer
distance from gun to screen needs the mu metal shield.

https://en.wikipedia.org/wiki/Mu-metal

> The direct effect of the earth magnetic field is quite low and 
> compensated by the static convergence setting which in fact slightly 
> moves the beam source. 

But you don't want to have to readjust every time you move.

> AFAIK this is the main reason for the high accelerating voltage. 
> The phosphor would be more efficient at lower energy electrons.

But lower voltage would require higher current, which isn't so easy,
especially to make a small spot. 
 
> In fact it would have been much easier to use a paramagnetic or 
> diamagnetic material for the shadow mask. But it need to have the same 
> coefficient of thermal expansion than the lead glass of the front screen 
> and it need to be a conductor to deflect the beam current. And glass has 
> a much lower coefficient of expansion than any metal. So you need the 
> magnet-volume effect to compensate for the thermal expansion of the 
> metal. The shadow mask is made of Invar.

It seesm that invar is used only for the shadow mask frame, where it
joins to glass.  As I just learned from:

https://en.wikipedia.org/wiki/Shadow_mask

The rest of the shadow mask is attached to the frame while warm,
and then as it cools, contracts and so is under tension. 

This tension keeps the shadow mask from deforming as it warms
up from the power of absorbed electrons.

-- glen