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Author Topic: Meyer-Mace Isotropic NMR Generator  (Read 24760 times)

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The flux density measurement would be much more accurate if you'd placed the sensor like on the diagram below in the green zone.

Optionally, you could plug the hole in the yoke with a second piece of that iron rod, if you have any left, so the hole is not in the way and the long rod does not have to be skewed like that.

Oo oops, the last piece might be on holiday in N Wales!! :)

As an afterthought, do you have any Emery tape Peterae? You could remove a few thou by that means.


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Hi Grumage
it is meant to be a interference fit

   

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Hi Peterae.

Indeed. I was suggesting what I would term a " Squeaky " fit. A fine Emery tape will just knock off those high spots so you can rotate the rod into the hole, without lube it would squeak.   ;)

Cheers Grum.


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@Smudge

What do you think of linearly interpolating this  μr vs. f curve, all the way down to DC in order to obtain the permeability, that Peterae can use for his DC bias coil calculations ?

Can you think of a better method ?
Something seriously wrong with those measurements.  This is showing enormous inductance at low frequencies.  The inductance  L=V/(w*i) is part of the hidden math used in those calcs and it is giving nonsensical values.  What is the DCR of the 150 turn coil used?  If that is any near to the 10 ohms then it could perhaps explain things.

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Something seriously wrong with those measurements.  This is showing enormous inductance at low frequencies.  The inductance  L=V/(w*i) is part of the hidden math used in those calcs and it is giving nonsensical values.  What is the DCR of the 150 turn coil used?  If that is any near to the 10 ohms then it could perhaps explain things.
Strange, i dont think the coil could be 10Ohms, i will check in the morning, could there be a mistake in my maths in the spread sheet, or are you saying that my measured voltages are incorrect.
I posted the spreadsheet and data in that post

Oh hang on the CSR was 10 Ohm that was used to measure current
   
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On thinking about it the formula you used to calculate mu is only accurate where the reactance is much greater than the coil resistance.  Clearly at frequencies down to 1 Hz there is a problem.  The internet site you got the formula from should have a health warning about this, but it doesn't.  The problem is the voltage that is claimed to be across the inductor isn't, it is across the series coil R plus inductance.  So when there is current involved it is not the inductor voltage that you are seeing.  I'll put up a correction in a moment, but first I have to have breakfast.

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OK here we go.  If you have available the current i into an inductor and the voltage V across it then from V=omega*L*i you can get the inductance as L=V/(omega*i).  Then from L=mu*N^2*Area/length you can derive a formula for absolute mu as mu=V*length/(N^2*Area*omega*i) which is the formula you found on the web.  But if the coil has series R then what you measure as V is actually across the impedance Z=sqrt((omega*L)^2+R^2)=V/i.  So the formula for mu has to be modified to mu=sqrt((V/i)^2-R^2)*length/(N^2*Area*omega).

I thought I could modify your spread sheet with different values for R to get something sensible but that didn't work out too well.  What I can say is that there is no material on earth that has the mu values plotted against frequency that you found.  You mention that your core has an air gap, and it is a fact that in that situation the reluctance of the magnetic circuit is not dependent on the core mu, it gets very close to the reluctance of the air gap itself.  So there can't be that variation of effective mu with frequency that you found.  A drop-off at high frequencies can be explained by eddy currents, but the steady increase at ever lower frequencies is just nonsense.

If I read things correctly you wish to establish the DC bias current necessary to get 0.5 Tesla in your core.  I would be inclined to forget about calculating the mu and concentrate on getting a measure of the flux.  Have two windings, one to carry the bias current and the other as a sense winding.  Use a low frequency as close to DC as you can get and still have sensible results.  Drive known current into the bias coil then measure the open circuit voltage on the sense coil.  That voltage is not affected by coil resistance since no significant current is flowing there.  But the voltage is related to the flux in the core. If you have pure sine waves  then you can easily obtain the flux from V=N*omega*flux.  If not sine waves then you really need to integrate the voltage over a complete cycle, but the answer you get is flux.  Remember flux is B*Area.  Now you have flux against current so you can easily determine the current you need for your 0.5 Tesla.  Use that value for your DC bias.

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Thanks for your time Smudge

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If I read things correctly you wish to establish the DC bias current necessary to get 0.5 Tesla in your core.
Yes thats right, i will work on your proposed method, Thank you  O0
   

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Have two windings, one to carry the bias current and the other as a sense winding.  Use a low frequency as close to DC as you can get and still have sensible results.  Drive known current into the bias coil then measure the open circuit voltage on the sense coil. 
Why wouldn't the open circuit voltage on the sense coil be simply defined by the transformer's turns ratio?
   
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Why wouldn't the open circuit voltage on the sense coil be simply defined by the transformer's turns ratio?
If there is significant voltage drop across the primary coil resistance then the open circuit secondary voltage is not defined by the turns ratio.  And if the core is lossy the primary current is high even with the secondary open circuit.  At 45 MHz the core will be quite lossy because of eddy currents if nothing else.  The secondary voltage will give an accurate representation of the flux whereas the primary voltage won't.

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Dont forget the whole idea was to use a low frequency as close to DC maybe 1Hz or 10Hz to get a value for my Bias
Coil, so 45MHz does not come into it for this test  O0

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If there is significant voltage drop across the primary coil resistance then the open circuit secondary voltage is not defined by the turns ratio.
So from this it looks like i need very thin wire and lots of turns for each coil?
   
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Dont forget the whole idea was to use a low frequency as close to DC maybe 1Hz or 10Hz to get a value for my Bias
Coil, so 45MHz does not come into it for this test  O0
So from this it looks like i need very thin wire and lots of turns for each coil?
Maybe lots of turns for the sense winding.  But the drive coil may need fewer turns so that you are driving it with current and not voltage.  Don't really know.

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Could i drive with a square wave, that way i would easily know what current flows when in the on state
   

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So what happened?
   

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Time & Money have been short, too many other things been going on, i will pick it up again soon though, i desperately need to get hold of a Geiger Counter next, i need to be able to tell the difference between X-rays & Beta and preferably cheapish, any ideas are well welcome, i have tried building an ION chamber but have been unable to make it sensitive enough so far.
   

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I desperately need to get hold of a Geiger Counter next, i need to be able to tell the difference between X-rays & Beta and preferably cheapish, any ideas are well welcome,
Did you try this source ?
They have kits, too.
   

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Thanks verpies that's a great link.


I just found this on ebay it would plug into my tablet.
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It detect low levels of hard beta and gamma particles. Running the test for longer than two minutes will yield more accurate results

http://www.ebay.co.uk/itm/Smart-Geiger-Pro-SGP-001-Nuclear-Radiation-Detector-Counter-For-Smartphone-iOS-/172143967386?hash=item2814947c9a:g:iIwAAOSwu1VW7sHf

The biggest problem i will have is RF induced readings and whether they are real readings.

I wonder if this is using pin diodes as the detector, maybe i better try and stick with a tube, data logging would be real handy but i guess it will always come down to cost.
   

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The biggest problem i will have is RF induced readings and whether they are real readings.
Yes, these things are really susceptible to RF EMI.
   

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Is there no way to shield the circuit under test to
prevent the undesired RF from reaching the high
energy radiation detector?  Perhaps it could be
enclosed in a fairly small metal cage or box?

In studying the link Peterae provided to the Smart
Geiger Pro
it appears that the app is capable of
recording a "History" to memory.  Does anyone know
if it is capable of recording to memory a replica of
the sampling event?  Something that could be "played
back" in order to re-create the sampling?

If so, perhaps the small unit itself could be enclosed in
a shielded box to monitor the circuit under test; recording
the test to memory free from RF interference.

It is no wonder the "Smartphone" has become a nearly
indispensable tool for this modern generation. But, I'm not
ready for one yet.  Probably never will be. ;)

I do marvel at how people are almost constantly fiddling
with their cellphones though. C.C
« Last Edit: 2016-05-03, 04:44:20 by muDped »


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