[3dem] high EMF

[Svetomir B Tzokov]

Dear Alok,

We are also using field cancellation system for our CM200 for over 10 years and, more recently, on a newer T12. It works by sensing the magnetic fields outside of the microscope close to the specimen and actively compensating it through a "cage" of wires in x, y, and z direction.

i.e. such a system consists of a sensor placed outside of the objective lens (as close as possible to where the sample is sitting in the microscope); set of wires inside the walls of the microscope room (or attached to the walls), and a control box.

Such system is good for compensating fields from AC sources (like the 50/60 Herz mains currents) - we also have those compensated down to about 10% of the initial intensity; but not so good for fields from DC sources (like trams, for example) and other low-frequency fields (lifts/elevators, etc.).

You should stick with the suggestions from the engineers. In any case, if you can't choose the place for your microscopes, it is always better to have such field cancellation system. Just make sure the low-frequency fields are within (or just at) the acceptable limit.

We don't have GIF installed here, but as far as I know it is very sensitive to any external interference, and even movement of a metal chair close to it can cause image movement. Again, I would imagine that it would be better for the GIF if you had a field cancellation system.

Hope this helps.

Kind regards,
Svet






On 20 Jul 2011, at 09:03, Marin van Heel wrote:


Dear Alok,

We have had this type of dynamic field compensation in operation for our CM300 FEG and CM200 FEG instruments for more than a decade and to our full satisfaction. If I remember correctly the residual field (50Hz/60Hz) is reduced to ~10% of its original value after correction.  This type of compensation is only ment to work for magnetic strayfields generated by the 50/60 Hz mains.

Hope this helps,

Marin

On 20-Jul-11 7:55 AM, Alok Mitra wrote:
> Dear colleagues:
> 
>   The EMF measurements for the microscope rooms where our old T12 and the new TF20 (with GIF) are to be housed revealed values beyond the acceptable level. One of the remedial solution suggested is to utilize an active field cancellation device, which samples the field strength in real time and generates an opposing field to cance (placed near the objective lens)l. Does anybody have experience as to how well such a device performs? I will appreciate your input
> 
>    regards
> 
>    Alok
> 
> 
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-- 
Dr Svetomir B Tzokov 
Electron Microscopy Facility Manager
Krebs Institute for Biomolecular Research
Dept Molecular Biology and Biotechnology 
The University of Sheffield 
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