The Gaussmeter Project

 

A while ago, a member of the ATIS Stationary Engine List, Ed Stoller and I began discussing the design of a low-cost Gauss Meter kit for engine folks.  A Gauss meter is used to tell the strength of magnets and it could be used to test magneto magnets for strength.

 

What we ended up was an inexpensive meter kit that Ed can supply.  It consists of a printed circuit board and all the components (except a 9 to 12 Volt battery) and your digital meter to make the meter work.

 

To measure a magnet, the sensor (supplied in the kit) is held away from the magnet and the voltage is noted (usually around 2.5V).  Now, put the sensor against the magnet and move it until the voltage is at itís highest (for the South pole) or at itís lowest (for the North pole).

 

Subtract the voltage you measured without the magnet from the voltage you got with the magnet and divide by 0.0013 and the result is the strength of the magnet in Gauss.

 

You can get more information by emailing Ed Stoller at:   [email protected]

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July 2003:

The Meter I Built in 2003

 

The meter isnít linear above about 800 Gauss but will read with reduced accuracy to nearly 1,400 Gauss.  This range should be fine for testing old magnets.  If the range turns out to not be high enough, a friend can design a little Mu Metal shield to go over the sensor to reduce the sensitivity by a known amount.  Weíll check into that later if we have to.

 

Hereís a picture of the fancy meter I made using the kit Ed supplies plus some parts I had in my junkpile.  My meter measures South pole magnets only and I made a scale for the meter so it reads directly in Gauss.  Accuracy isnít traceable to the National Bureau of Standards but it will be fine for most applications. 

 

The actual Hall Effect sensor is the little square black thingy at the end of the cable. 

 

The little adjustment to the right of the switch is to set the electrical zero of the meter (ď0 GaussĒ).

 

To test the battery, flip the switch down and the meter will read 700 Gauss or more for a good battery. 

 

To test a magnet, hold the probe next to the magnet, flip the switch up and find the strongest reading.

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August 19, 2003

Ed Stoller writes:

 

I have been playing with my Magnet Meter and have some observations.

 

 

The sensor in the Magnet Meter is specified to be linear over the range from - 800 Gauss ( North ) to + 800 Gauss ( South), that is from .5 to 4.5 Volts output. The sensor output is in saturation from 0 to .030 Volts (North) and from 4.97 to 5 Volts(South) . The question is what happens above 800 Gauss until it really saturates ?

 

 

To answer the question, I set up an Electro magnet with a variable DC power supply to see if the Electro magnet was linear with respect to current and to map the magnetic field to both extremes. The data was collected at 2 milli amp increments form 15 to 60 milli amps in both directions and put into a spread sheet (Excel). The spread sheet made it easy to convert the output Volts to Gauss and make a graph.

 

The graph clearly indicates the Electro magnet was linear to 800 Gauss and well beyond, to 960 Gauss. So what does this mean?  

 

Keep in mind that once the sensor is in saturation, you really can't tell it by varying the magnetic field, By looking at the graph and picking the last point that was still in the linear region,  I conclude that the measurements are valid any where from .2 Volts to 4.9 Volts.  Your Magnet Meter should work fine up to 960 Gauss vice the advertised 800.

 

 

Have fun,

ED

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Fall 2003:

  

Lyle Nelson has made his version of the gauss meter.  Here are a couple of pictures of his creation.  He used the housing from a felt pen and a telephone cord.

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February 11 2010:

 

In response to renewed interest in the Gaussmeter, I've further documented the meter I made in 2003.  Note that with the following images, you can click on them and get a full resolution screen.

 

  

Construction Details

The case is made from copper clad printed circuit board material that I have soldered together.  The meter itself is one that came from my junk box, for which I made a new scale using my CAD program.

 

   

Details of The Sensor.

 

Since I wanted to get the most expanded scale I could get, I decided that I only needed to measure South Gauss so I modified the circuit so I could offset the meter reading so the far left-hand end of the scale was zero Gauss and the far right-hand end of the scale was 1,400 Gauss.  Remember that the meter is not very accurate over about 800 Gauss (but is useable).

 

The sensor physically is like a sort of flattened TO92 transistor.  I epoxy glued this one on a piece of 1/16" thick printed circuit board material that was about as wide as the sensor and about an inch long.  This was done in order to give it some mechanical robustness.  The sensor was oriented so the correct side exposed surface would be closest to the magnet being measured.

 

Then, I soldered the leads to the cable and glued the cable to the PC board.  After the glue had set, I shrank a piece of shrink tubing over the  whole works, just leaving the sensor exposed.

Schematic of The Meter.

This is the schematic of the meter above.  In order to make sure that the meter readings are stable over the life of the "transistor" battery, a 78L05 voltage regulator is used which supplies a steady 5.0 volt power source to the Hall-Effect sensor.

 

The meter happened to have a 100 microamp DC movement so I selected resistors that would cause the meter to have 100 microamps driving it at full scale (1,400 Gauss).  The "Calibrate" pot sets the meter to read accurately.  The "Zero Set" pot offsets the negative terminal of the meter to make the meter read exactly zero when the sensor detects no magnetic field. 

 

The Gaussmeter is calibrated follows:

- With the meter off, adjust the mechanical zero on the meter so it reads exactly zero.  You might want to bump the meter a little and check to make sure it returns to zero.

- With a known good battery, flip the switch to the "Battery Test" position and note that the meter goes past half scale or 700 Gauss.  This indicates that the battery is good.  *:(NOTE that you can change where the "good battery" reading is by changing the value of the 6.8K resistor.  Do this AFTER you've calibrated the meter).

- Now, with no magnetic field around the sensor, flip the switch to the "Measure" position.  Adjust the "Zero Set" pot until the meter reads zero.  Bump it a little to make sure the meter returns to zero.

- There are a couple of ways to complete the final calibration step:  

A: If your Hall Effect sensor didn't come with a calibration chart, you can calibrate your meter using a magnet with accurately known strength.  Adjust the "Calibrate" pot until the meter shows this strength.  (Remember that the "Calibrate" adjustment and the electrical "Zero Set" adjustments interact so you will have to do both of them a couple or three times so zero is zero and the strength reading matches the strength of the known magnet.

B: If your Hall Effect sensor came with a calibration chart, you can use a digital voltmeter to calibrate your Gaussmeter.  After electrically zeroing the meter (and with with no magnet close to the sensor), connect the digital voltmeter between ground and the output of the Hall sensor.  Note the zero Gauss voltage.  It should be somewhere around 2.5 volts and note it.  Now, slowly slide the sensor and a strong magnet toward each other.  When the voltage -difference- between the zero Gauss reading and the magnet reading reaches near where it should be for 800 Gauss, hold them in position and set the electrical "Calibrate" pot to 800 Gauss.  Again, you will have to tweak the "Zero Set" and "Calibrate" settings several times to make them track. 

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If enough guys make meters for themselves, I will publish a database of magnet strengths for various magnetos here so other guys can use their meter to see if their magnets are up to par.

 

Comments are appreciated.  You can send them to either:

Ed Stoller:  [email protected]

or me: [email protected]

Have fun!

Elden

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