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Using a multimeter as a gaussmeter

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  • #16
    Originally posted by J M Fahey View Post
    I need to measure loudspeakers, typical values between 10k and 15k Gauss (way above saturation point for any Hall sensor I found) and inside down to 0.8mm gaps in loudspeakers, where to boot path is curved and voice coils can be as small as 12mm diameter.
    I always wondered how JBL etc were able to provide field strength specs for the gaps in their HF drivers for instance, claiming in some cases 2 Tesla ( = 20,000 Gauss ). How they do that, indeed.

    I'm having some fun translating LtKojak's query. There must be some idiomatic meaning as the direct translation leaves me puzzled.

    On one occasion I did witness a brand new JBL diaphragm get flung across the room when the bozo installing it "click checked" it with a 9V battery before it was bolted into place. Boy was that fun! Sort of a low yield rail gun...
    Enjoy. Every. Sandwich.

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    • #17
      AlphaLab Inc. (https://www.alphalabinc.com/product-...y/gaussmeters/) has meters that measure up to 3T and Hall probes as thin as 0.8mm. There may be other makers also.
      When there is a market demand there will be a solution. But as production volumes are low, prices will be high.

      But the ballistic induction + voltage integrator method mentioned by JMF surely also works. I think it means completely immersing a voice coil in the magnet assy through the airgap and then removing it completely while integrating the induced voltage. The result is total (change of) flux times number of turns. Needs to be divided by airgap area and number of turns to get airgap induction (flux density). The results might be somewhat high, as they tend to include some internal leakage and outside fringe flux. So calibration may be necessary.
      Last edited by Helmholtz; 08-03-2019, 07:18 PM.
      - Own Opinions Only -

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      • #18
        Originally posted by J M Fahey View Post
        Thanks a lot, but most posted gaussmeters are useless to me.
        ...
        I need to measure loudspeakers, typical values between 10k and 15k Gauss (way above saturation point for any Hall sensor I found) and inside down to 0.8mm gaps in loudspeakers, where to boot path is curved and voice coils can be as small as 12mm diameter.

        So the main problem is the sensor, both in sensitivity and size.
        There may be a way.

        Old school is to mechanically move a conductive coil in a magnetic field and measure the coil output.

        Not-so-old is to make a small skinny coil and shake it fast, e.g., put a single layer coil on some thick tape (0.005") and shake the Dickens out of it with a piezo speaker.

        Thickness of a single build #42 coil between a folded piece of 5 mil kapton tape should be no more than 0.016" or 0.4mm. Thin film inductors on flexible plastic are already available from printed circuit board houses.

        Cheap piezo speakers driven at 10V typically have a 0.005mm (~0.0002") displacement. It may be enough to generate a useful AC voltage.

        Latest greatest tech is the transverse magnetoresistive sensor (TMR) but I don't see the thin ones yet.

        Please keep us posted on how you solve this. It sounds interesting.

        -drh
        He who moderates least moderates best.

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        • #19
          Not-so-old is to make a small skinny coil and shake it fast, e.g., put a single layer coil on some thick tape (0.005") and shake the Dickens out of it with a piezo speaker.
          Simply shaking a coil in a homogenious magnet field won't produce/induce EMF. The number of field lines penetrating the coil needs to be changing (preferably between zero and max).
          That's why electromechanical field meters have rotating coils.
          Last edited by Helmholtz; 08-03-2019, 09:45 PM.
          - Own Opinions Only -

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          • #20
            Originally posted by LtKojak View Post
            Juan Manuel, no rompas màs las pelotas, querés?

            No translation for the above statement will be provided.

            Por supuesto, estoy bromeando, hermano.

            Que las pases bien!

            Tu compatriota,

            Pepe
            Me acabo de comer una picada de chorizo seco tandilero, queso de campo, seguida de un buen asado de tira y regada con Don Valentín lacrado, tinto por supuesto, así que en este estado de paz mental decime cualquiera que no me importa
            Google translate crashed trying so unfortunately no translation is possible
            But it involves coal cooked meat, smoked country cheese,1 year cured homemade salami and 4 year old red wine
            Plus the ethereal bliss caused by its consumption
            Juan Manuel Fahey

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            • #21
              Originally posted by J M Fahey View Post
              Me acabo de comer una picada de chorizo seco tandilero, queso de campo, seguida de un buen asado de tira y regada con Don Valentín lacrado
              Como te odio...

              J/K

              Buen provecho!

              PS: to the ones interested in knowing what we're talking about, it's all secret messages written in code directed to the Alien Overlord class living in the innards of the Antarctic continent. No biggie!
              Pepe aka Lt. Kojak
              Milano, Italy

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              • #22
                English please!
                Last edited by big_teee; 08-07-2019, 03:22 PM.


                "If Wishes Were Horses, Beggars Would Ride!" Scottish Proverb 1600s
                Terry

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                • #23
                  Originally posted by LtKojak View Post
                  Como te odio...

                  J/K

                  Buen provecho!

                  PS: to the ones interested in knowing what we're talking about, it's all secret messages written in code directed to the Alien Overlord class living in the innards of the Antarctic continent. No biggie!
                  Apparently so. You write Juan asking him why he's not kicking the ball, and that you're just joshing him, then he replies with a recipe. I figured there's some idiomatic/regional dialect going on, or you guys like to send non sequiturae back n forth. All good fun anyway, I get to exercise rarely used translation brain cells. No I haven't resorted to Google translate or anything of the sort, as Juan said, they would just make hash of your writings anyway. Carry on!
                  Enjoy. Every. Sandwich.

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                  • #24
                    Originally posted by LtKojak View Post
                    PS: to the ones interested in knowing what we're talking about, it's all secret messages written in code directed to the Alien Overlord class living in the innards of the Antarctic continent.
                    What have you been smoking with your cubensis mushrooms?

                    Everybody knows they hang at South Georgia Island.
                    Any cursory meeting with the Grytviken city locals there will show you
                    how ...um... otherworldly they are.

                    -hizself
                    He who moderates least moderates best.

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                    • #25
                      Originally posted by J M Fahey View Post
                      Partly because typical values run from 10000 to 15000 Gauss (1 to 1.5 Tesla) which is well beyond usual Hall sensors, but even more because speaker gaps go from 0.9 to 1.3 mm wide and usual meters simply do not fit.
                      Okey, fine. I'll try again.

                      There are hand-roll sensors called B-dots used for NMR bores, synchrotrons, and plasma containment research. Be prepared to design and build from first principles.

                      Commercial magnetometers in the 0-2T range are either expensive or OhMyGodMakeItStop "institutionally priced."
                      GMW Associates has a cute USB dongle attached to a 1mm thick sensor on a stick priced at a mere $3310.

                      While there are similarly ranged versions priced from $100 to $700, their probes are too thick.
                      He who moderates least moderates best.

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                      • #26
                        Originally posted by DrStrangelove View Post
                        Okey, fine. I'll try again.

                        There are hand-roll sensors called B-dots used for NMR bores, synchrotrons, and plasma containment research. Be prepared to design and build from first principles.

                        Commercial magnetometers in the 0-2T range are either expensive or OhMyGodMakeItStop "institutionally priced."
                        GMW Associates has a cute USB dongle attached to a 1mm thick sensor on a stick priced at a mere $3310.

                        While there are similarly ranged versions priced from $100 to $700, their probes are too thick.
                        Thanks for suggestions.

                        Just googled something on B Dot probes, apparently their main feature is measuring relatively weak magnetic fields immersed in a strong electrostatic field so they use some kind of differential capacitance to null it out.
                        And to boot, measuring a high frequency varying one.
                        Maybe that explains the high price.

                        My objective is way more modest: to measure a quite stable magnetic field (loudspeaker magnets) , and no electrostatic fields nearby, so my task is not that complex.

                        As mentioned before, when the modern whizbang stuff is unavailable, I roll back to "basic principles", straight from , say, a 1925/1940 Physics Labs book which are buildable with copper, iron, and not much else.

                        In my case I sweep a coil across the voice coil gap and integrate its output to get a reading.

                        In the old days the integrator was mechanical, a heavy disk with much inbertia attached to a galvanometer needle and with a mechanical inertia/time constant much longer (say 5 or 10 seconds or more) than the time used to swep the coil, which even if pulled by hand can be achieved in fractions of a second.

                        In that case the disk starts moving and continues doing so and the needle way overshoots (on purpose) reaching a peak displacement proportional to the integral of current created by the coil sweep, so it shows the total flux density along that path, in Maxwells.
                        Then divide that by gap length (plate thickness) to get Intensity (not Density) in Gauss/Tesla.

                        Those clever guys even much amplified the reading (before the invention of Electronics) by gluing a small mirror to the needle and shining a ray of light on it, them projecting that on a whitewashed wall.
                        A mm or two needle movement could mean many meters on a wall



                        My only modernization is to feed the voice coil current into the inverting input of an Op Amp (which is nominal ground) , where the NFB element is a capacitor.

                        Said capacitor will charge with and integrate the current created by the swept voice coil and the peak value is a voltage directly proportional to total flux in Maxwells.

                        I can calculate what voltage corresponds to what Maxwell level by applying basic , universal Physics concepts and simple Math ... the way Pioneers did it over 150 years ago.

                        I also kludge similar basic devices to measure almost anything, often surprising guys who don´t understand how I manage to measure things WITHOUT the "specific" detector, transducer , IC , meter, whatever.

                        Not surprising to Forum members here who, say, can measure a bipolar transistor, diode, FET , etc. using a plain Multimeter (and a pen, paper and a calculator) without using, say, a Peak/Atlas meter , measure ESR without an "ESR meter", inductance or capacitance without a bridge, bias without a bias probe, and even less a dedicated microprocessed thingie and so on.

                        At least within 10% or even 20% which is close enough and then some for everyday Tech problems.

                        I find it sort of funny when some armchair designer publishes a circuit in DIY Audio (somebody who never ever grabbed a soldering iron) and defiantly states "my design is better than yours because it achieves 0.000127448% distortion" ... just because Simulator just spit out that number.

                        REALLY? ... 0.000127448% ? ... not 0.000127447% or 0.000127449%?

                        Amazing !!!!!

                        PS: I am mubling building my own Hall Effect sensor out of ... flexible PCB material ... which is thin and flexible.

                        So far and unless I´m mistaken, I just need to etch a small rectangle on it, say 8mm long (gap depth) by, say, 2 mm wide, plus tracks leading to the center of each side, one pair feeding current top to bottom, other measuring voltage developed side to side.

                        Single face material is 0.22 mm thick and if I add a Mylar sheet to insulate exposed copper I might keep it 0.3 or 0.35 mm thick ... and still flexible.

                        Modern Hall probes are all made out of semiconductor material (so silicon so like a thin slab of glass so inflexible) but that´s so they are very sensitive able to measure Earth magnetic field.

                        Mine will be very basic and raw, essentially same as Hall used in the 1860´s, but who cares?, I am measuring the highest magnetic fields found in everyday use so not worried about "lack of sensitivity" at all.
                        And it will be linear and "unsaturable" by definition.

                        That it´s somewhat "deaf" is not a problem if I am measuring nearby pistol shots.

                        I guess a plain TL071 Op Amp will be more than enough to process its signal and to boot it has adjustable input offset, so .....
                        Juan Manuel Fahey

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                        • #27
                          And the ballistic galvanometer idea comes from an even earlier clever idea:

                          Guess how did Leonardo da Vinci *measure* the velocity of a bullet he couldn´t even *see*?

                          No Electronics, photocells, nothing.

                          He shot at a known lead weight, which took the bullet and swang a little, and applied a little Math:



                          O, fine, but how did he measure the peak displacement of the heavy pendulum?

                          oh, "easy" he added a silk ribbon: it would be pulled through a slot to peak length by pendulum, but it would not get back inside said slot, so you caould measure the *exact* displacement.



                          Clever chaps indeed.

                          We have wonderful Science, but some scientist said: "I can see very far ... but only because I am sitting on the shoulder of giants"!!!!
                          Juan Manuel Fahey

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                          • #28
                            Originally posted by J M Fahey View Post
                            I am mubling building my own Hall Effect sensor out of ... flexible PCB material ... which is thin and flexible.
                            If you build it, they will come - all the way from South Georgia Island. To be calibrated to the 9th decimal place.

                            Accuracy is important when navigating the cosmos! No wonder there are so many ufo crashes reported on Earth. Talk about a kool new occupation... geeze, this is IT!
                            Enjoy. Every. Sandwich.

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                            • #29
                              Originally posted by DrStrangelove View Post
                              Everybody knows they hang at South Georgia Island.
                              SHHHHHHHH!!!!

                              You're not supposed to say that...
                              Pepe aka Lt. Kojak
                              Milano, Italy

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                              • #30
                                Originally posted by J M Fahey View Post
                                We have wonderful Science, but some scientist said: "I can see very far ... but only because I am sitting on the shoulder of giants"!!!!
                                Remember where you are. As often as not, we're standing on each other's feet.

                                Snark dispensed, congrats on some outstanding first principle thinking.

                                Reminds me of content in John Strong's 1938 reference called Principles in Experimental Physics, ISBN-13: 978-0917914560. Available used for under $10, I note. I still reread sections when parts seem too expensive.
                                He who moderates least moderates best.

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