Question:

Hi Glass Detective,

We’re a startup developing a glass weight scale which can measure one’s blood pressure, EKG and other cardiac parameters in addition to weight and body fat %. Like other scales, we’re designing around a large piece of glass, but unlike most, we’ve got 4 small holes to pass electrodes through the glass, and we would like these to be within countersunk holes.

We’re confused about the best glass for this purpose; something that is safe for a 400lb person to stand on without weighing 20 lbs and being 3/4″ thick. Most scales are built from a 6mm thick panel, presumably tempered soda-lime in most cases. Our prototypes have been made from machined and annealed borofloat, but we’re thinking we’ll need to switch to tempered glass of some stripe. Can we temper an 8mm plate of glass with 4 x 3.5mm diameter holes (located well away from any edges)?

Thanks,
Mike

Answer:

Mike,

Let me begin by wishing you well with the project you are undertaking! I have been involved with a few start-ups over the years and while they can be exciting and energizing, they can also be challenging and frustrating in that nothing ever seems to happen quite quickly enough.

As for the matter at hand regarding the panel portion of your glass weight scale: 1. I checked at a local retail store selling glass scales and the thickness of the glass. In both of the models I measured, it was 8mm (5/16″). Both of the observed models were simple point supported applications with no holes. 2. I think you will be better served going with 10mm (3/8″) thickness because of the holes and the fact that you want to countersink them. The countersinks may cause some trouble. Hopefully you won’t have to go any thicker. 3. I believe you will be best served with fully tempered (soda-lime as you refer to it) float glass supplied by a reputable tempering firm. 4. I have concerns about the 400 lb. load requirement. The concern is that the 400 lbs. will not be uniformly loaded (evenly distributed) and the corner areas are going to be the most vulnerable.

Conclusion/Recommendation: Have a dozen or so prototype glass panels produced and test them in-house. This glass is not expensive and your in-house testing and experimenting will confirm if you are on the right path. I suggest the 3/8″ (10mm) approach as a start. Assuming you go to market, you are going to want (need) to have the assembly tested by an independent test facility to assure yourselves, your distributors and insurance company that the product as built is free from design errors that could create downstream concerns. Lastly, the comments and opinions I am herein providing are nothing more than opinions and comments and do not constitute an endorsement of any products, designs or manufacturing approach. And if you don’t mind, could you let me know how it all works out for you and your team?

-The Glass Detective