Good EMF meter?

[TohKah]

Thought I would post here instead of trying to buy one from amazon with reviews about how many ghosts they've seen or how poisonous the microwave or their cars heated seats are...

Anyways at work we have an NMRI machine (Nuclear Magnetic Resonance Imaging - literally a scientific version of your standard hospital MRI machine but instead of people being the "samples" different liquids are inserted and observed - same technology) and we keep getting little errors that pop up and are borderline affecting the readings. The company wants to know what kind of electronic noise may be present and how much of it. If we have to we may build a Faraday cage around it or try some different things.

The buildings main transformer is right outside the outer wall of the building (14KV trans...) and were betting that is the culprit. Originally the equipment and the transformer were supposed to be at opposite ends of the building when it was being built but at the last minute someone flipped one to the other end so now they are near each other...

So what I need is a good meter that can detect mainly your standard 60hz AC power but can also detect a wide range of other frequencies relatively accurately. Would be nice for one that can measure and display magnetic field strength(amps per meter) and magnetic flux density(teslas/gauss).

Anyone have any good suggestions? Price range $200 and under would be nice.(yes I know just like any piece of test equipment you get what you pay for but there has gotta be a decent one for under $200).

Thank you guys/gals in advance!

[MosherIV]

Hi

Forget anything that is sold to the general public, they are just a useless for what you want to do.

In an ideal world, what you want is the equipment used to measure radiated EM for EMC compliance testing.
The equipment is very expensive.
One option is to rent the equipment.

Something else more affordable is a spectrum analyzer.
Do a search on DIY test for emc and there are examples of how to use an oscilloscope and spectrum analyzers.

(In Europe, CE marking requires companies to do self certification for electronic equipment. This includes tests for conducted and radiated EM, hence there is a market for this kind of thing)

[jjoonathan]

Even cheap spectrum analyzers are still going to be well out of that price range. You *could* buy one of those $20SDR dongles and turn it into a spectrum analyzer, but the sweep rate will be miserably slow which is not good for catching transients never mind direction finding. The reality is that anything resembling professional equipment for your task will cost $1k (old, used, limited) to $10k (entry level lab/industrial stuff). That doesn't mean you should give up, just realize that you'll either need to schmooze someone who does have the equipment or hack together something that would give most folks over in the Metrology subforum a severe case of the "tut, tut"s punctuated by fainting spells.

Assuming you want to do the latter, my suggestion would be to stick in the time domain for your first pass at hunting down the noise (we're taking advantage of the fact that you suspect it's intermittent here). Buy a cheese-grade ($50-$150) portable oscilloscope; first make sure it supports "Normal" in addition to "Auto" sweeps, then maximize bandwidth subject to price constraints. To detect E field noise, stick a wire in the socket and let the other end dangle. To detect B field noise, stick a wire in the socket and connect the other end to ground (not earth ground, the pocket scope's ground). You will want to research the types of sockets on these scopes to either pick one that facilitates these wire attachments or to buy an adapter. Read up on dipole and loop antennas to understand the limitations of this approach, but if the interference is bad enough you'll see the wiggles it leaves behind. If you don't, it might be conducted emissions. Grab a 10X probe and look for spikes on the mains. Preferably a fixed 10X probe because cheese-grade "scopes" don't support mains voltage at 1X and even accidental momentary exposure to mains at 1X could fry them and you. Hell, I'd probably wear gloves while handling the thing. But between those 3 techniques I'd give you slightly better than even odds of finding the problem.

The less-fun but more likely to work approach is to schmooze someone with the proper gear. It's easier than you think if you find someone who isn't busy at the moment (which is the real trick). People with expensive equipment and hard-won expertise like to use it. Just approach them when they are 1. not busy, and 2. hungry (you *did* remember to bring a food bribe, yes?) and it'll work 80% of the time. Honestly I'd suggest starting with this approach but since you're in grad school I would guess that you want to take a swing at it yourself first :-)

EDIT: careful about focusing too much on the transformer;  I've got a cheap wall-wart that I bet could give your NMR machine the fits from twice as far away.

[uncle_bob]

Hi

This is the classic single use application. Don't buy anything. Rent the gear you need, get the job done, move on. That's even more true since the gear you need won't even rent for a week and fit in your budget ....

Bob

[InterestedTom]

Given the low frequency of interest you could use a gauss/tesla meter with a hall-effect probe or possibly a hall-effect sensor.

What kind of magnetic flux density do you want to measure, uT, mT or T?

If it's only a few hundred mT or less then it should be within the range of sensors costing less than £5. The good thing about hall effect devices is they will not be damaged by being exposed to too high a flux density. These sensors tend to output a voltage which is proportional to the applied magnetic flux density to within a few %. The output of this sensor could then be measured with a standard multimeter (which you may already have) and powered with a voltage reference (for a few pounds at the most) and a fairly cheap power supply or just use a quality power supply with more stable and accurate output. Depending on what you already have there is no reason why this couldn't be done for less than £200 if you don't need GHz of bandwidth or better than 2% accuracy. The other thing to consider is renting a ready made gaussmeter or teslameter (same thing, different name and units, modern ones are normally capable of both units despite the name gaussmeter), there are single axis and multi-axis variants available at various accuracy levels from 5% all the way down to 0.5% (at DC).

The difficulty you may have is separating out the disturbances from the transformer from the fields of the NMRI machine. You may have to experiment with a few different things but as I said, an initial homebrew system needn't cost more £200.