If anyone is shopping for a gaussmeter, this one seems to be good.
I had to replace my gaussmeter last month and decided to try out one of these..
Really happy with it so far and the 0-1v output is great for data logging.
http://www.trifield.com/GM2.htm (http://www.trifield.com/GM2.htm)
Price is $490
I can build ten for that...if not more...
I quit buying meters. :)
I learned a long time ago, if you build all of your tools - you never get any work done ;D
Its good to know how though!!!
For low Gauss measurements, yeah the $3 Hall sensors available are great, but if you want a serious DIY meter that can measure up to 20,000 Gauss, you are out of luck I'm afraid. They are out there, but you will pay about $600 just for the Hall sensor.
I searched extensively a few years ago. The only one I found that didn't cost an arm and a leg, and was able to come close to being "useful", was a part by Analog Devices: AD22151 @~ $4.00
The unit DS bought is a good unit, and I've asked the designer a few years ago what Hall sensor he used. The are not available any more, so he must have bought a few thousand from China when he had the chance. His unit sells for $300 I believe.
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I'm considering using a DVM and a Hall sensor:
http://www.lakeshore.com/mag/hs/hsmm.html#uhg
http://www.lakeshore.com/mag/hs/hsoi.html
Using the DVM is fine but those hall sensors are pricey and basically raw in that there is no built in amplifier. The output level will be rather low. The HGA 2010 puts out about 20mV/kG, and requires a precision current source.
If you are on a budget, I would go with something like these: http://www.allegromicro.com/en/Products/Categories/Sensors/linear.asp Just a couple of bucks vs. $120
These are about 1000X more sensitive because of the built in amplifier, typically 2-5 mV/G (vs kG), plus they can be excited with a very simple DC supply, no precision required.
Bottom of the page you will find a pdf app note for linear hall sensors.
Also try Micronas linear hall effect sensors $1 to $2 range
If any of these are too sensitive for your application, you can provide a front to back magnetic shield / shunt.
I had fun with some hall sensors years ago, i wired 2 up facing back to back, had 1 feed the inverting and the other feed the non inverting input of an op amp, i then fed the output to a V to F and added an amp and little speaker, it was easily sensitive enough to act as an audible compass
Thanks Gents.