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Magnet Strength Testing Variance

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  • Magnet Strength Testing Variance

    Like others, I have set up a weight scale to measure the strength of T-Jet and MT magnets. For those who do not know how this setup works, a metal weight is placed on a scale with a piece of plastic/glass suspended over it. The scale is then zeroed out and a magnet is placed on the suspended glass. The strength of the magnet is then measured by the amount of “pull” (as expressed by a negative weight) exerted on the weight. Basically it is how much weight is being lifted off the scale by the magnet.

    While the number gained does not correlate to any official measurement chart, what this setup gives you is the relationship among all your magnets, allowing you to match magnets based on their strength.

    In order to know my measurements are consistent from day to day, I have a few test magnets whose strength is recorded. I use these magnets when I set up the scale to determine everything is set up correctly. But on certain days I run into the issue where the readings are not the same on these test magnets as they were when the readings were first recorded. And this has me confused.

    I understand this is not a unique situation and others have had the same problem. Logically the magnets should measure the same (or very close) to the same readings every day. When the test magnets measure differently than the original recorded readings, there is no reason to test more magnets because the readings won’t be valid when compared to readings of other magnets made on other days.

    So the question becomes…why the difference? What makes the readings vary greatly from one day to the next. Is it temperature, barometric pressure, changes in the Earth’s magnetic field? Any ideas?

    Last edited by Grandcheapskate; 02-07-2019, 09:10 PM.

  • #2
    All of the above. Magnet strength is inversely proportional to the distance. It is very difficult to maintain the same distance from the plastic cover and the metal plate over time. Having built several magnet checkers using the same scale from Harbor Freight, no two are identical to the tenth of a gram. Try adding some paper shims under the magnet to see the impact on results. Is the cover bowing, installed fully? Does the scale move in one direction over time? Is the temperature the same?


    • #3
      The scale I'm using runs off electricity and not batteries, so I can eliminate battery strength as a factor. The metal plate of my scale is not magnetic, so I need to place a metal plate on the sclae and then zero it out.

      My setup consists of supports on two sides of the scale with a 1/8" glass plate laid across them suspended over the scale. I then have thick plastic "cards" I place on the scale to bring a metal plate very close to the underside of the glass. I use the same supports, glass, cards and plate every time, placed the same way.

      The glass plate does have a slight bow to it (undetectable by the naked eye) so I have it marked and always place it in the same orientation. I actually measured the magnets using the glass plate with both sides "up" and when the numbers are less than my recorded numbers (it's almost always less), they are less no matter which way I place the glass.

      When the numbers are off, like yesterday, no amount of shifting the pieces around changes the numbers more than a tenth of a gram. As an example, there is an AW XT magnet whose number is recorded as 14.6 grams. Yesterday I could not get a reading greater than 13.5.

      There is some other factor at play here. I am guessing temperature and humidity.



      • #4
        I would guess it's the scale, and NOT the magnets.

        I don't know all of the particulars, but scales can and do read differently depending on any number of things. Altitude, tides, and moon phase are said to be factors.

        I know scales read differently in Kansas City than they do in Ferndale Ca. Almost 2 tens of a gram different. And this is not just one scale, it's all of the scales we took. The St. Louise guys had the same reaction.

        When you're trying to be that accurate, small things matter.

        Your mileage may vary.


        • #5
          I considered it could be the scale but I do not think so, and here is why.

          I have weighed and recorded the weight of the "cards" and metal plate. When I set up the scale, I make sure I get the recorded readings as I add the cards and then when I add the metal plate. If this weight were drastically different than the recorded weight, then I would know there is an issue.

          The total weight of the cards and plate is recorded as 118.1 grams. Most times I get 118.0 but I consider that close enough. So the scale may, over a total weight of 118 grams, show a .1 gram difference between sessions. That is nothing.

          Therefore, a negative "pull" in the range of 6-14 grams should show even less than a .1 gram variation. Even if it is .1 grams I would consider that acceptable. But I am seeing a drop of over 1 full gram.

          Granted there is a difference between putting weight on the scale and removing weight as the magnets exert pull on the metal plate. But it seems more likely something is effecting the magnet strength.



          • #6
            Hey, Joe

            Just from interest, is the use of a measuring device such as the Magnet Marshal used much by you Pancake racers?

            I noticed that there's a new MM2 which apparently works with all scales from HO thru 1/24.


            • #7
              Originally posted by Wet Coast Racer View Post
              Hey, Joe

              Just from interest, is the use of a measuring device such as the Magnet Marshal used much by you Pancake racers?

              I noticed that there's a new MM2 which apparently works with all scales from HO thru 1/24.
              I have no idea. I am not a hardcore racer and never have been. I am now part of a growing race group but I don't put in enough time to build a winning car and compete with the best in the group, so I just go for the fun and try to stay out of the way.

              If anyone in our group does measure magnet strength they are pretty much using the scale found at Harbor Freight. As far as I know, no one has gone all out and gotten any type of expensive (or dedicated) magnet measuring device.

              By the way, just to insure the differences I was getting in the magnet readings was not due to temperature, I took the scale and magnets from the basement and moved them upstairs where it is probably 8-10 degrees warmer. I let everything sit overnight and then measured the magnets. I got the same readings as the day before in the basement.
              Last edited by Grandcheapskate; 02-08-2019, 10:27 PM.


              • #8
                Measuring magnets is a tricky business no matter how you do it. The method that you outlined should work well if all you need are relative strength readings, but you indicated that you are having a problem getting consistent readings. I have a proper magnetometer, but even with that I must be careful. For a start the magnetic fields are quite irregular, so moving the probe just slightly can make a big difference in the reading. The strength of a magnetic field will also vary with the square of the distance, so any variation in that will have a bigger impact than you would expect.
                When I use the magnetometer to measure pancake magnets I place the probe in the middle of the side that will face the armature and move the probe around until I find the highest value. I mark that value on the magnet with pencil. I have re-checked magnets many times and I am always able to reproduce them within a few gauss.
                My 1/32nd club used digital scales to measure the downforce of cars with traction magnets. We used a marked calibration magnet to get consistent readings. In that case the readings did not have to be terribly accurate, but we found the method to work well.
                Others have suggested various causes for your problem. You may want to make something to position the magnets, a piece of cardboard with a cutout for a magnet would work. The magnets should always be turned the same way. It is not unusual for a pancake magnet to be stronger at one end that at the other. I have not compared top and bottom measurements, but I expect that those can vary as well.
                It is possible to assemble a very inexpensive homemade gauss meter to get relative readings. For actual gauss readings you would have to use a more expensive calibrated Hall effect sensor.
                I have an article on building the meter as a PDF, I can send that to anyone that is interested.
                With careful attention to the variables I believe that the rig that you are using can be made to work satisfactorily.


                • #9
                  Just to clarify a few things. I am only measuring the magnets in one direction. I place the magnet face down (the side facing the armature is down) and record that value.

                  It seems that I can move the magnet around on the glass plate and the reading varies very little, so the 1+ gram difference I am seeing day to day is not from the magnet position on the glass. Nor is it from the glass bowing from day to day as the readings are lower no matter which side of the glass is facing up (I have two recorded readings for each magnet).



                  • #10
                    As I said before the method that you are using should give repeatable results and I would expect that it would not matter much if the magnet was not centered except that you noted that the glass plate was slightly bowed. My experience with electronic scales is very extensive, even the cheaper ones give reliable results. What you get when you pay more for a scale is added features and more decimal places. I have an actual 500g calibration weight, that was checked VS a GLP five decimal place balance. GLP standards require that the scale is checked several times a year by a licensed technician. The scale that I use displays 0.1 g and can be calibrated so it reads correctly.
                    Since you are not getting repeatable results something must be changing, the problem is figuring out what. I would tend to rule out the scale itself, but you can check that with a weight. That does not have to be a calibration weight, just something that is not likely to change, like a heavy metal nut from a hardware store. If the nut reads the same every time you turn on the scale you would be good to go. I do not think that your magnets are changing strength. My experience has been that modern ceramic magnets are very stable, they are hardly affected by heat and it takes a very powerful field to have any effect on them. You can count on the Earth's magnetic field to remain constant, relative to magnets it is very weak in any case. It seems to me that the only variable left is the distance between the metal plate and the top surface of the glass. Any material that changes size with changes in temperature, humidity or barometric pressure would not be a good thing to use to support the glass plate or as shim material under the metal plate.
                    You might avoid taking your apparatus apart if possible. The fact that it has to be assembled when you want to take readings makes me nervous. I have noticed that some scales are fussy about being perfectly level. The ones that I used in the lab included a bubble gauge. If you are using blocks of some sort to support the glass plate those nay not necessarily have exactly the same dimensions and every time you assemble things the spacing may change.
                    If I was really ambitious I might try to duplicate what you are doing and see what I see. I am curious to see how the gram values would compare to my gauss readings. There is a considerable difference in what the magnets might read if you use a magnetometer. The original Dash magnets that I got as loose items were good for 900 gauss, the ones that came in the 1st generation T-Dash cars all measured less. The magnets in the only 2nd generation Dash car that I got were only 650 gauss.


                    • #11
                      It seems the readings I have been getting over the past few days are consistent yet they are less than the readings I got when taking my benchmark. This becomes a problem for me because within the past 2 months I took all my runners, measured their magnets and recorded the results. That was done over a period of at least a week and I was able to calibrate the scale every day to the benchmarks even though I had to assemble it every day.

                      Just a note: I need to reassemble the scale every day because it cannot be turned on while weight is on the scale.

                      The metal plate is sitting very close to the underside of the glass...about 1-2 sheets of paper is all that will slide between the metal and glass. So it will be very difficult to move the metal even closer to get higher readings.

                      As to checking the scale, the weight of all the cards and plate I have to put on the scale is consistent day to day. The weight s always between 118.0 and 118.1 grams.

                      Another point of interest...if the bowing became more pronounced it would change the readings on both sides of the glass. In other words, if the readings went down when measured with the glass with side 1 up, then they should go up when measured with side 2 up. But both sides are lower than before.

                      Last edited by Grandcheapskate; 02-11-2019, 03:42 PM.


                      • #12
                        I added a piece of .002" shim steel which moved the metal plate closer to the underside of the glass plate. This resulted in my benchmark magnets measuring very close to the numbers I have recorded for them.

                        My only conclusion is the amount of humidity affects the plastic cards and/or the glass plate. So I will use the .002" when needed if the readings are too low without it.



                        • #13
                          What are you using to support the glass plate? I do not believe that the thickness of the plate will change with changes in humidity. If the supports are wood humidity could have an effect. Anything that changes the distance between the metal plate and the magnet would have an effect. As I stated earlier the magnetic attraction will vary with the square of the distance, so if you double the distance you will only have 25% of the downforce.
                          A small change in the procedure might work. Assemble your apparatus, it would be best if you had a few very thin shims between the supports and the glass plate. Put a magnet on the glass. Mark your reading on that magnet, which will now be your calibration standard. Now measure a few other magnets and make note of the readings. Always orient the magnets the same way, if it was me I would try to keep them centered more or less. Next take your rig apart and put it back together. Put the calibration magnet on the scale to see how it reads. Possibly the reading will be different, if so you would have to add or remove shims to get the reading the same. Adding shims will decrease the reading and subtracting shims will increase the reading. once you are reading the correct calibration value you can read your other magnets and their values should be the same as before.


                          • #14
                            i noticed that you'd get different readings when placing the magnet in different locations on the plastic tray. I marked off an area in the center to always measure from.
                            Still not 100% accurate or consistent. But a ball park figure on overall strength is good.

                            Anyone ever assemble the gauss meter that was in archives. I did. with the aid of a multimeter you'd get a reading , use a formula and get a Gauss reading.

                            I didn't make mine that sturdy, and it isn't functioning right now. I'd use it to get a general reading at both poles. I think that's where the value is in a magnet matcher. Matching the strengths at the poles helps the armature maximize revs and torque.
                            I am sure I saved a file of it before the website done gone kablewey.


                            • #15
                              I had mentioned a home built gauss meter in an earlier post. I had saved screen grabs of the original article and converted them to a PDF.
                              An inexpensive Hall effect sensor can be used if all you need is a ball park reading. If you want a more precise reading you can spend a lot more money on a calibrated Hall effect sensor.