One thing Joe has failed to mention is that all of these type devices, except those installed inside the tire using a band clamp around the rim, screw onto your valve stem.

In and of itself screwing something on the valve stem to read pressure is not a big deal except you have lost 50% of your valve stem leakage protection. On an ordinary valve stem the Schrader valve is the primary seal, and the cap is the secondary seal. With the pressure sensors installed the valve is held open, so if the pressure sensor leaks, so does the air in your tire. Usually the leaks occur because the pressure sensors need to be removed to check the actual pressure and to adjust the tire pressure. The more they are removed the more likely you will have a leak. At least the sensors will tell you the tire has leaked.

But Joe is pretty clever. Actually he is very clever. He installed a second set of valve stems 180 degrees opposed to the ones on the wheel. The pressure sensors, once installed and tested for leaks are then never removed. No removal virtually eliminates the risk of a leak at the sensor. The tire pressure can be checked and adjusted at the normal valve stem which has the Schrader valve and the sealing valve cap. Take a bow Joe.