I have one of the cheaper systems that I got on eBay where the pressure sensor screws on to the valve stem. It does not show the actual pressure - it just gives an alarm if the pressure goes below 28psi. I test the sensor each time that I check the tire pressure, and it takes maybe five seconds to go into alarm ....
Bill and Kate
Stone Harbor, NJ
2005 Jayco Jay Feather 29N (Salty Dog House)
2005 Ford/Quigley 4x4 E350 Van 6.0 TD (Moby)
Reese Pro Series SC Hitch