TPMS-RS21 is a new improvements which include a vast reduction in size. Creating a lighter, more compact, easier to install sensor with great rim compatibility. We’ve also added relative humidity measurement and a user-replaceable battery. For a budget TPMS sensor, the ability to measure relative humidity is a real step forward, the replaceable battery also reduces unnecessary scrapping of wheel sensors after 1 or 2 race seasons. Some big improvements that set us apart from the competition.