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Interpreting some Paint Gauge Thickness meter readings.

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  • #1

    Interpreting some Paint Gauge Thickness meter readings.

    I have an electronic Paint Gauge Thickness meter, which can measure both ferrous and non-ferrous metal substrates. Model number is CM-8825NF.

    Overall it seems to be working, however sometimes I get two different readings, depending on whether I put the meter into ferrous (F) or non-ferrous (NF) mode:

    My car, which is very new, has a metallic black finish, and it looks like it has a clearcoat. In most areas of the car, I get readings around 140um in F mode, and around 80um in NF mode. On some areas (the doors, for example), however, the meter automatically switches to F mode, even if I had first put it in NF mode, and then gives the same F readings as before - around 140um. So, it appears that some panels aren't strongly enough ferrous or non-ferrous for the meter to automatically sense the "correct" type, so it doesn't override the user's mode selection.

    Now, I also have a test panel (a bonnet from a completely different make/model of car), and this finish appears to be non-metallic black, and also appears to have a clearcoat. (certainly, when I polished it, I got no black on my polishing pads at all). On this panel, I get around 140um in F mode, and a very large 400um reading in NF mode.

    Has anyone encountered this kind of ambiguity before? I'm assuming that in both cases the F readings are the correct ones.

    Is metallic paint really metallic? I.e - are the flakes actually metallic?

    Greg.
    Tags: None
  • #2
    Re: Interpreting some Paint Gauge Thickness meter readings.

    You need the other PC-

    Hey Paaaaaaaaaaul !!!!!!!!!!!!!!!!!!!
    Freedom prospers when Christianity is vibrant and the rule of law under God is acknowledged

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    • #3
      Re: Interpreting some Paint Gauge Thickness meter readings.

      If the panel is steel the ferrous readings will be correct.

      Warning – nerdy content:

      Eddy current sensors used to measure distance to non-ferrous metals will still output a signal when presented with a ferromagnetic metal at the same distance, but it will be have a completely different value.

      For a given sensor, the calibration is different between the two types of materials. So if the meter gets confused and thinks it’s the wrong type it will interpret the return signal incorrectly and produce an erroneous reading.

      The difference comes from how the sensor reacts to the magnetic (permeability) and energy dissipating (resistivity) characteristics of the target material. Here’s a cool article on the subject.


      PC.



      ps- Thanks for asking, Joe. The research was fun. Here’s a cookie:

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      • #4
        Re: Interpreting some Paint Gauge Thickness meter readings.

        gives me the warm fuzzzzzzzzzzzies!!

        Freedom prospers when Christianity is vibrant and the rule of law under God is acknowledged

        Comment

        • #5
          Re: Interpreting some Paint Gauge Thickness meter readings.

          PC,
          Thanks for the article, and the summary of the problem - much appreciated. Note that I have not yet fully read the article.

          Note that when I use this meter on a stainless steel kitchen knife, the readings are appreciably wrong - it doesn't measure zero on the bare metal, and delta thickness readings from the calibration foils are also wrong. The meter does automatically go to F mode though - it will not produce an NF reading. I am however able to recalibrate it (needs a LOT of adjusting) for this knife. Anyway, I then went back to my test panel, and found that for both F and NF measurements, the delta thickness readings were at least correct. I.e, placing a calibration foil on the panel produced an accurate change in reading, even though there is a big difference in the absolute thicknesses between F and NF measurements. I haven't tried this test on my car yet.

          By the way, the instructions recommend that if possible, the meter first be zeroed on the bare metal. (it appears that it should also be calibrated on the bare metal too, using the calibration foils - not just zeroed!)

          Greg.

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          • #6
            Re: Interpreting some Paint Gauge Thickness meter readings.

            I've now taken some measurements with a test foil placed on my vehicle. In F mode, the delta is accurate - the thickness reading increases by the foil thickness. In NF mode, the thickness reading actually decreases with the foil in place. Bizarre! (and this reduction is about the same as the thickness of the foil - I've only tested it with one foil thickness so far though - it may well just be a coincidence)

            Anyway, I think it's reasonably safe for me to assume that my F readings are kosher. (and the NF readings are completely invalid)

            On the test panel, however, I wonder whether perhaps there are two layers, of different material, and the gauge is able to measure them individually. (I say this because the delta readings are accurate in both F and NF mode)

            Greg.

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            • #7
              Re: Interpreting some Paint Gauge Thickness meter readings.

              I tried using the PTG on a friend's '03 Subaru Liberty. This car doesn't give any F readings for the bonnet, and switching to NF mode produces sensible readings. Later on my friend did a bit of Googling, and found some evidence that this car actually does have an aluminium bonnet.

              Greg.

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              • #8
                Re: Interpreting some Paint Gauge Thickness meter readings.

                Not to take away from the technical side of this discussion, but after all the readings sooner or later the decision has to be made...

                Go
                No go


                If the decision is Go, then once you bring your hand or the polisher down on the paint there's no turning back. If the decision is No go, then you kindly tell the customer thanks, but no thanks and here are the reasons why,

                1. Reason
                2. Reason
                3. Reason


                Personally, I've never used a Paint Thickness Gauge when I detailed full time, instead I would,

                Evaluate the customer
                Evaluate the car
                Do a test spot

                Make a decision and move forward

                I think they're a useful tool, but it is possible to make decisions on Go or No go without them. A little luck and experience helps a lot too....

                Mike Phillips
                760-515-0444
                showcargarage@gmail.com
                "Find something you like and use it often"

                Comment

                • #9
                  Re: Interpreting some Paint Gauge Thickness meter readings.

                  Thanks Mike. Yes, I had a feeling before I bought it that I could probably do without it. However, considering the low price, and also my inexperience and my situation, I decided that it would be of some benefit.

                  Greg.
                  p.s I like gadgets, too.

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