EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: Mpegger on December 03, 2024, 04:38:27 pm
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Just picked up my BM786 and noticed that there are 2 cracks on the plastic case portion about 1/6 down from the top of the clear screen, which is starting to encrouch onto the clear screen itself on the left side. This meter has been pretty much babied, never dropped, and if it wasn't for my old eyes, I probably would have noticed these craks earlier. However, the crack starting to form on the clear screen itself are definetely new, as the crack causes the light going through it to have a prismatic effect and is very noticeable, even with poor eyesight, and that was not there last week when I last picked up the meter. Looks like plastic is failing because of a stress or a mounting point for a internal screw? Plastic getting old? The meter is about to be 3 years old as of purchase next month.
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I checked my BM789 and I can see four potential weak points in the plastic on each side, one of which corresponds to the cracks in your photo. Interesting issue. Even if dropped, I'd expect it to not do that. Dave could probably find a replacement top half and ship it to you or replace the DMM. This would be an easy fix if you had a donor DMM...
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Looking at the ones I have, there does not appear to be any signs of a clearance issue or cracks. The cracks you show appear to align with the first two divots. I attempted to break the meter's upper shell in my hands without the lower half attached. Should be very weak comparatively speaking.
I wonder if something went wrong when the lens was assembled into the upper shell. That our something was binding when the meter was assembled and someone went ahead and cranked down the screws.
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Bm789 - the same, it is 3 years old. The reduction in cost of materials is obvious.
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It could be the combination of the walls being a bit too thin near stress risers, with repeated heat cycling
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Just in case, I unscrewed all 6 screws a little.
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Does the crack extend down the sides? You'd have to look from inside the case.
OP's shows a horizontal offset which seems really odd, what forces are pushing that.
edit: pics of an old Fluke 27 with corner cracks. They extend partially down the side. pics taken from youtube.com/watch?v=VX-XzjO_ix8
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yep had this on fluke 83, had to redesign a new window bezel, slightly "looser" and loctite tak pak glued ....
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Does the crack extend down the sides? You'd have to look from inside the case.
OP's shows a horizontal offset which seems really odd, what forces are pushing that.
edit: pics of an old Fluke 27 with corner cracks. They extend partially down the side. pics taken from youtube.com/watch?v=VX-XzjO_ix8
Oof... the cracked screen is what I'm dreading. I can live with the cracked sides (again, I didnt even notice them cause of my eyesight), but having the screen crack just irks me. I've had some cheap plastic items get brittle over decades and eventually start to crack or break, but I wasn't expecting something that seemed quality to do so in a few short years. I still have a Radio Shack 22-163 multimeter from the early 90s that still works (though I wouldn't rely on it for accurate readings), and other then some normal wear and tear, the case is still fine.
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My wild theory is the cracking is due to different thermal expansion coefficients between the two plastics - the cast acrylic display window and the multimeter enclosure. To keep it sealed, the window is glued in place, rigid. So I would imagine high strain along the edge axis.
If this is true, then the approach is pretty much doomed. Older (water proof) multimeters use a FIP silicone gasket for the window, which has some cush. The Radio Shack one is likely a loose fitting snap-in window which can move a bit.
What base plastic is used for multimeter housings? I think polycarbonate, ABS, polyolefins are used.
Anything glass-fiber reinforced does not expand or contract nearly as much and maybe Brymen uses that? There's no plastic recycling markings that I can see.
When I worked with high volume products, we did aggressive drop testing but temperature cycling was for the electronics only.
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do the cracks in the case compromise blast containment within the case? really, the bezel should have an inside overlap with the case and should be retained in place with a silicon sealant - thereby allowing for thermal expansion of one part without affecting the integrity of the other.
cheers,
rob :-)
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The Fluke 27 is not explosion-proof, I believe it's IS and given the cracks and age... you wouldn't be using it in Haz Loc anymore.
For the Brymen multimeters, I don't see them having any ratings for water resistance, no IP rating.
Fluke 87V is IP30, Fluke 87V MAX is IP67, Fluke 17B+/187/189 no spec.
I rarely use a multimeter outdoors in snow and rain, on a boat etc.
I agree, the display window will need a flexible mount that is waterproof but this adds manufacturing cost.
Hopefully Dave is talking with them about the cracking.
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My wild theory is the cracking is due to different thermal expansion coefficients between the two plastics - the cast acrylic display window and the multimeter enclosure. To keep it sealed, the window is glued in place, rigid. So I would imagine high strain along the edge axis.
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I would have thought they used the same plastic. I did run a fair amount of thermal tests on various meters, including shock. If it is a TC, I'm surprised none of them cracked.
My guess is ozone machines.
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some plastic shrinks slowly with age.
Nintendo got hit by this on some of the early twin-screen handhelds - the plastic hinge shrank around a steel core and cracked.
interestingly it effected some colours of case more than others!
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First I've heard of that. I might have a parts unit front face moulding somewhere.
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First I've heard of that. I might have a parts unit front face moulding somewhere.
I have one of their non-working prototypes with a good case if you want it for destructive testing.
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My wild theory is the cracking is due to different thermal expansion coefficients between the two plastics - the cast acrylic display window and the multimeter enclosure. To keep it sealed, the window is glued in place, rigid. So I would imagine high strain along the edge axis.
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I would have thought they used the same plastic. I did run a fair amount of thermal tests on various meters, including shock. If it is a TC, I'm surprised none of them cracked.
My guess is ozone machines.
If it is related to thermal expension, it might be marginal enough that it requires quite a lot of cycles.
Easy enough to ask Mpegger and Antrus if they use ozone generators.
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I suspect that over-tightening the housing screws may cause unnecessary stress on the materials used. This may not be the main reason for the cracks that appear. But this can be a strong provocation.
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I suspect that over-tightening the housing screws may cause unnecessary stress on the materials used. This may not be the main reason for the cracks that appear. But this can be a strong provocation.
Good idea! You might have somemthing there. I just eased off of all eight screws on the backside of mine. Thanks!
WoD