Back of box says "FOR USE ON METAL VALVE STEMS". They selectively omitted the word "only," but it's equally clear that their intention is to redirect liability if you use flow sensors on your towed vehicles or trailers. My closed car trailer has rubber valve stems, so I don't think the weight of the flow sensors isn't a factor for TST to make such a claim clearly visible on the back of the box. Not mentioned in the description and not shown. But for that reason I think the product is excellent,