I'm not allowed in the workshop today because I won't hear the door bell and the Time Electronics 2004 is allegedly arriving today. So I thought it was high time that I calibrated my search coils (in the lab). I have a selection of coils wound on RM5 formers, starting from 40 turns, then 80, 160, and 325. All are fragile, being soldered to BNCs by their former's pins. I also have a much more robust single layer coil wound on Paxolin tube into which a BNC is glued. For calibration, I have a pair of 10" Helmholtz coils that I calibrated against the Earth's magnetic field which is stated to be 19.662uT in my back garden according to a BGS survey (you have to do this sort of experiment well away from anything ferrous). The coils were calibrated using DC from a 2400 sourcemeter whose current was adjusted to give 45 degrees of deflection from the Earth's field on a large compass, thereby indicating equal fields. This is not an easy experiment to carry out (yet it's often described as a school experiment), but I eventually produced results that I believe have an uncertainty of 1%. Satisfyingly, the sensitivity I found matched theory to 0.2%.
In use, the coils are resonated at 50Hz because mains susceptibility is what I'm interested in, and produce a flux in their central space of 5.27uT per mA of driving current. 50Hz sine wave came from an Agilent 33220A arbitrary function generator that developed 3.631VRMS across the 53.9R load, so current was 67.4mA and flux was 355uT. I started with the robust coil and found 1.033mVRMS driving 50R oscilloscope with the coil aligned with the Helmholtz coils in the central space where flux is pretty much constant. The TDS3032 oscilloscope measured cycle RMS and was averaged over 32 records. I fully expected the RM5 coils to produce much larger signals because they have more turns (although smaller area). What I found:
325 turns 1.639mVRMS
160 turns 806uVRMS
80 turns 413uVRMS
40 turns 228uVRMS
I would have thought that I had made a poorly coupled transformer and that output voltage would be proportional to the number of secondary turns, but this is clearly not the case. Can anyone tell me why?
EDIT/ANSWER: How stupid can I be? Those numbers DO scale by a factor of two at doubling of turns. Their low output compared to the robust coil is due to their smaller area.
In retrospect, it turns out that the large robust coil is the most useful, so I did the sums and determined that 1mV into 50R indicates 367uT of magnetic flux density. Putting the coil under the travelling microscope, I counted 61 turns of 0.2mm enamelled copper wire on the 13.9mm tube. Sensitivity is proportional to area and turns, so if you have some tube, you should be able to calculate your coil's sensitivity from my results.
The point of all this is to be able to quantify hum leakage from mains transformers.
Calibrating search coils
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