Noise when entering Faraday cage

[amaranta]

Hi,

I am a biologist and I do a lot of experiments on cellular electrophysiology. I keep my preamplifier and the biological subject inside a Faraday cage, and it works well. In some experiments I need to deliver a drug to the cell so I have to enter the cage briefly, with my hand only, and the delivering instrument. This introduces a low amplitude noise, visible in the recording, that lasts for as long as I am inside the cage. Why does this happen? How do you call this noise?

I will really appreciate your answers!

Amaranta.

 

[Bobbywhy]

amaranta, Welcome to Physics Forums!

It is impossible to know for sure, but my first guess is you are intrducing a 50 Hz or 60 Hz signal into the cage where it enters the preamplifier. This signal is from your laboatory electrical system, for example, the lighting.

Do you have access to an ocilloscope? Experiment: hold the 'scope probe in your fingers and increase the sensitivity (volts/centimeter) control. You should see an AC signal being picked up by your body from the surrounding electrical power system. It is this signal that causes the low amplitude noise visible in the recording. That's why you keep your experiment shielded in a Faraday cage! To eliminate this noise.

 

[CWatters]

What frequency is the noise? That would be a good clue to it's origin.

Wrist watch? Mobile phone? 50/60Hz from the mains?

Any hole in the faraday cage can potentially allow noise in so one solution might be to devise a way to introduce the drugs remotely? eg use a cannula extension?

 

[amaranta]

Hi, and thanks for replying!

Yes, a fast count at the number of "peaks" in 1 second looks like it is 50-60 Hz. In normal conditions there is no noise, so it is not a problem of the Faraday cage. It is just that in some of my data there is this low amplitude 50-60 Hz noise that cannot be avoided, only when I deliver the treatment (for logistic reasons, it is not possible to use a long-distance device) and I need to at least describe it properly (it is for a paper I am writing). Any help?!

Thanks again!

 

[CWatters]

Basically your body is picking up a signal from the mains and injecting it into the cage. Try holding the cage with one hand while using the other to inject the drugs.

If that's not possible try grounding your body to the cage with a length of copper wire.

I suppose another option might be to power your equipent using a computer UPS. Then turn off power to the room/building when you need to access the experiment?

Caution: You should seek local electrical safety advice before you experiment with this sort of thing. If you are operating electrical equipment and that equipment turns out to be faulty then grounding yourself to Earth or the faraday cage might make any shock you get worse. In addition if there was a fault outside the building (or nearby lightning strike) then high voltages might be conducted into the building via the mains wiring (including the earth!) and onto anything that is meant to be earthed. Normally measures are taken to prevent this happening (such as grounding Earth cables where they come into a building) but it needs to be checked. In short, earthing yourself is like taking off your rubber soled shoes and going for a walk in a storm. It's going to make any shock you get worse.

 

[the_emi_guy]

Do you need 60Hz bandwidth in your results? If not, you can post process your data through a low pass filter to eliminate the 60Hz.

 

[amaranta]

Thank you very much for your replies. They are very interesting, I didn't know that the body could pick up electromagnetic signals. I will try and ground myself to the cage or grab it, see if that works. Otherwise, the suggestion of getting a 50 Hz filter may be good, but I don't know so much about electronics.

Thank you again!

Amaranta

 

[sophiecentaur]

Mains Hum is a common problem and it can sometimes be reduced just by suitable routing of Earthing / Grounding wires or even by disconnecting them. The interference is very likely to be because of currents flowing, causing magnetic fields and not due to electric fields.
In audio circuits, hum is often tackled by using just a single ground connection - 'daisy chained', with the whole thing connected to mains Earth just at one end. 'Earth Loops' should be avoided. Sorry if that is all gobledegook but hum is a devil to get rid of and it can be a bit of a black art.
It would probably help to make all the equipment in the cage run off batteries and, possibly, to send the data via an optical coupler (readily available). That way, sticking your hand into the cage would have much less effect.

 

[the_emi_guy]

Otherwise, the suggestion of getting a 50 Hz filter may be good, but I don't know so much about electronics.

Thank you again!

Amaranta

I was not proposing an electronic filter. If you data sampling rate is fairly high, and your bandwidth of interest is much lower than 60Hz, then you can process your data through a low pass filter numerically (essentially average it).

 

[marcusl]

TYou can also make a noise-tight penetration tbrough your shield to run a plastic tube for remote fluid delivery. Solder a copper tube that is 10 diameters long into the wall (ie. 3/8" tubing that is 4" long or longer).This is called a waveguide beyond cutoff. Slip your plastic tubing through. Stay away from metal, plastic tubing only.