Gauss Meters- created 10-04-2007

Searcy, Clint - 10/04/2007.05:37:29
MIMForum Staff, Nashville

Well, the time has come for me to get a gauss meter of some sort. I have been wanting to study gauss and it's effects on pickups for a while now. I also have a little green to go shopping with. So I'm wondering if I can make do with a simple pocket magnetometer like the model 25 from RB Annis. Or if my money would be better spent with something a little more high end like this DC Gaussmeter Model 1 from AlphaLab, Inc.

Most of the low end stuff seems to be rather Ghost Busters-ish. But if they work I can deal with that.

I've heard that the RB Annis magnetometer is not very useful for pickups. Seymour Duncan uses one though.

The guys at the pickup makers forum seem to swear by the Carlsen Melton GM-200A gauss meter.

I'm not sure how it compares to the AlphaLab.

I agree, a gaussmeter is better. You can position the small probe everywhere, and take specific localized readings. You can also make some production fixtures to hold the probe in a specific spot, and check magnets and pickups for consistency

he guys at the pickup makers forum seem to swear by the Carlsen Melton GM-200A gauss meter.

I looked at that one. I don't like it because the probe is built into the end of the case. I have heard that if you use a gauss meter a lot the probe will ware out and need to be replaced. I'm guessing that life would be even shorter for guys like me that work with neodymium a lot. Also like Charlie mentioned being able to move the probe from string to string is something I would like to try.

I leaning towards the Alpha now. I like the design more and it's cheaper.

<i>I'm guessing that life would be even shorter for guys like me that work with neodymium a lot</i>

It's mostly from handling, not from what you use it on. The ones I used were basically a thin metal strip on the end of a thin cord. That's good, because you can fit it into tight places, but it's bad because you have to treat it like it's a thin piece of metal on a thin wire. It's easy to damage it if you're not careful.

Ahhhh I see. So I need to treat it like 45ga magnet wire and I should be OK

It's not quite that delicate, but if you use that type of handing approach you won't have any issues with it

I'm leaning towards the Model

There are some do-it-yourself articles on the web that reveal how simple it is to make a gauss meter. I use a Melexis linear hall effect sensor, which costs very little compared to any of the pre-fab meters and is way simple to use: it takes a DC supply voltage and produces an output that is linear with gauss by a scaling factor of your choice. With appropriate scaling and offsetting, you can read gauss directly on your volt meter. The chip is about 3.5mm x 5mm; package it as you see fit. Make it replaceable if you like; I stuffed the leads of mine into a piece of pin header strip.

I got a couple of different versions of the chip, one from Melexis directly and one from Digi-Key.

I guess what I'm saying is that it would be really easy to start tinkering with measurements for a surprisingly small amount of money. No digital wahoo, but I think digital wahoo would only get helpful if it started offering a way to plot curves versus position and that sort of thing, anyway.

How exactly do you calibrate it, so you know it's accurate

It's pretty much taken care of for you inherently by the design of the chip. The output normally sits at half the supply voltage, and is driven higher or lower by a north or south field, respectively. So there is no "absolute" reference required, other than the power supply itself, which is easy to manage.

Then, accuracy is a function of linearity, which is 0.5% for the 90242 chip.

The other mandatory piece is an accurate voltmeter, which should be pretty easy.

I use an op-amp to zero the offset to make the meter easier to read, instead of having to subtract 250 from everything, so I have a chance to mess up the accuracy there if I want. Even 1% seems more than accurate enough for the kinds of things we do, and I could use 0.1% resistors if I cared.

One of the nice things about taking this approach is you can swap in a different chip to get a different sensitivity and range, rather than just scaling the voltage of the chip you've got, thereby losing accuracy in the lower ranges. There is a version of the 90242 that reads +/-500 gauss, and another that reads +/-1300 gauss. The scaling is different.

I found a really nice Walker Scientific MG-3D on Ebay but it's missing the Hall Probe. I wrote to the company and they will sell me one but it costs more than my car. I was thinking maybe I could make one cause I'm sort of handy. But if it's not going to be 100% accurate there's little point in owning such a fine tool.

Here's what the manual has to say about the probe.

3.2 The Hall Probe

(Fig. 3.1).

Geometrically, the sensor is essentially rectangular as would be expected from the orthogonal nature of the current-field-voltage-relationship.

Figure 3.1 - Hall Sensor

Hall Voltage = K0 x Excitation Current

where K0 is a function of geometry, current, field and temperature, and even at a fixed control current and sensor temperature, the field-voltage relationship is nonlinear. This is reflected in the linearity specifications for each Hall probe.

3.3 Functional Description (See Section 5 – Block Diagram)

The Hall probe control excitation is supplied by a constant current 4800 Hz oscillator-amplifier circuit. At the probe output terminals, there is a dominant voltage due to the field modulation of the probe current and a minor component due to the probe's zero field residual imbalance. A voltage derived from the control current is subtracted from the probe voltage in the balanced summing network before the AC

I don't know enough about hall effect sensors to know what might work on this.

You need to find a hall sensor that can measure the maximum range that the meter can. If the meter can do +/- (x)Gauss, the sensor needs to match this range. The linear hall sensors I tested needed only a DC supply voltage, anything else was done internally, and it gave me a linear voltage proportional to the strength of the field. I'm not sure what would work with this meter

Yea.... the more I ponder the more I like the Model 1. The only thing I don't like about it is that the probe is soldered in and would be a little more work to replace when it goes bad

I don't know exactly what you have in mind, but I wonder if you're over-complicating things a bit. One of the things I discovered when I started playing with the "simple and inexpensive yet surprisingly accurate" chip was that more accuracy wouldn't necessarily do me much good. A reasonable general ballpark in other areas of electronics is 10%, and when I started moving the probe around my little pickup prototypes, I discovered what we already know: magnetic fields are quite variable throughout, i.e. -- the strength you measure is based on the

I was able to learn interesting and useful things about the various configurations of cores and magnets that I was playing with, and to compare approximate field strengths between my various designs and existing pickups.

My general conclusion was that, in order to achieve the "next level" of analysis, I'd need to have accurate 3D positioning and rotation control over the probe, and plot the numbers in the 3D domain, probably searching for maximums on the rotational axes to pin down the vectors. I don't see a cost-effective way to do that "nicely," though I suppose it could be done in a limited way -- the limits being imposed by the specific questions I was trying to find answers to. Constraining the measurements to a plane would simplify it; 2D motion and one axis of rotation, and you could identify the outline of the field.

I'm curious... If I may be so bold, and if you feel at liberty to share, what exactly is it that you want to measure? I.e., what are the questions?

I'm curious... If I may be so bold, and if you feel at liberty to share, what exactly is it that you want to measure? I.e., what are the questions?

Sure you can ask.

1. I use a lot of neodymium magnets in my original designs. I would like to be able to measure the amount of gauss at the string with these pickups compared to an Alnico magnet.

2. A neodymium magnet that is 1" x 1/4" x 1/16" is noticeably lower in output than one that is 1" x 1/4" x 1/4". So which thickness is optimum and why?

3. One of my designs uses no core in the coil at all. How much penetration of the coils can I get just by changing magnets?

4. Will stacking two .25" magnets on top of each other produce the same gauss as a .50" magnet?

5. When folks send me vintage pickup to recharge have the actually lost any charge to begin with?

6. Does the heat from my potting process weaken my Neodymium magnets?

7. Are vintage magnets less powerfully than modern ones?

These are the sort of questions I hope to start unwrapping. I have read theories about some of this in books and on the web but I have learned that a lot of what we "know" about pickups is speculation and BS. I'm still having people quote the famous Seymour Duncan from 25 years ago where he said "Neodymium is useless in pickups making and would weld the string to the pickup". He was speculating. I would like to see what I'm talking about.

I think to get the the bottom of some of these questions I need a pretty accurate meter or I might as well not bother with one at all. I don't think most of what I need to know would require 3D imaging. But it would be fun to check that out someday too.

Baby steps... <g>

Thanks for indulging me, Clint! :)

I won't hold any unrealistic delusions of influencing you, but were it me... I think I could answer all of those questions with more than reasonable accuracy (about 1%) with this simple chip, a few parts stuffed into my breadboard, and a voltmeter. A cheap voltage regulator chip, a cheap op-amp, a few resistors and capacitors, and a wall-wart to make it sing. Someday I'll make a PC board and put it in a nice little box -- because I need another project. :)

Then again, I could spend that time on something else, and just buy a meter... maybe a used one, to keep the cost down. And then get on with life!

(When I had time to be a photographer, I bought several used densitometers, which I suppose is the allegorical meter. Some worked better than others, took different kinds of readings, had missing parts. It was sort of a hobby, sifting through them and find the best one for transmission, the best one for reflection, and finding a place to store the rest...)

I won't hold any unrealistic delusions of influencing you,

Alright big mouth, you've changed my mind. <g> Actually I had to buy a new saw this week end and now I'm not so eager to buy stuff. So if you are saying that this chip hooked to a multi meter will give a reading that is accurate to within 1% and I would have to do a page of equations to get the number I need than I might be willing to try it out. Think you can talk me through it?

New project... the MIMF Gauss Multi Meter conversion.

Let's do it.

Yeah... a reasonably good power supply, a chip, and a voltmeter.

Google "Build your own gauss meter". The dead-simple version requires that you do a little subtracting and dividing to get the actual gauss; a slightly more expensive version has an op-amp circuit to take care of that for you.

Their circuit uses a whole separate op-amp to provide the offset voltage; I think a simple trimmer sitting between the + and - supplies, feeding the + input of the "upper" op-amp, would do just as well. They also have a trimmer pot for the gain adjustment; I'd use precision fixed resistors if I knew the precise scale of the chip. With a 1mv/gauss chip, the scaling is 1:1, so the input and feedback resistors could both be 1k or 10k or whatever. If I can find a few minutes, I'll see if I can sketch out the circuit I use -- including the power supply, which I tend to take for granted. I just stuff it all into a breadboard when I need it.

I have NO idea whether the Allegro and/or Radio Shark parts are still available, not what their accuracy is. I use Melexis. The MLX90215 has a (one-time) programmable sensitivity, but you can get the pre-programmed MLX90215LVA-BC03 at 1mV/gauss, with a range of +/- 2000 gauss or so. If you want more sensitivity, you can get the -LA03 at 10mV/gauss, but that limits the range to about +/- 200. (Noting that one mT = 0.001 tesla = 10 gauss.) They have other chips that might work well.

Melexis' webstore shipping has doubled since I got mine... gack! Get two or three to make it worthwhile. Or DigiKey, but they charge way more for the pre-programmed chips.

Even though I'm already swamped with "ToDo's" most of the time, I'd be interesting in helping make this little project "real"; I think others might benefit from it, too.

Hi everybody..!

well i think it works well.... the A1302EUA device gets saturated at +/-1900 Gauss aprox. and the UGN3503 at +/-1300 Gauss aprox.

ciao.!

Clint,