Some members already have a TPMS or are familiar with them, so this post is to report that I took a risk buying one not advertised for trailer use... and it does work very well. I emailed Orange's US office and received a reply from a company official that he was using one of their units on a trailer at a distance of 31 ft. I needed about 33 ft. to get from the rear axle hub to the dash of my crew cab so I thought the risk wasn't too big. I bought the automotive model P409S made by http://www.orange-electronic.com/
and sold by http://www.tirerack.com/accessories/detail.jsp?ID=55
The Orange unit has some big advantages and also disadvantages compared to the much more common, and expensive, Doran. For me the big advantages are the OEM style sensor mounted inside the wheel and the 7 yr. lithium battery. The main disadvantages are that it only monitors 4 sensors and they don't have the pressure range of Doran's either. This is not an issue with 50 psi trailer tires. Adjustments on the receiver allow it to monitor up to 50 psi. The high pressure alert is user adjustable, but maxes at 60 psi. The bright LED display will display psi above 50. The spec sheet says it will monitor up to 76 psi, but if the audible alarm is screaming at 60 I'm not going to wait to see if it will go higher. They do make a bus model, not sold by Tirerack, that monitors 6 tires and presumably much higher pressures. Point is, this thing is good enough for 50 psi tires, but not higher.
The low pressure alert is also user adjustable, but maxes at 35 psi. I would have liked that to be more like 40 psi, but even at 35 psi a ST205/75/R15 can still carry a load of 1480 lb which means it won't be overloaded even if my trailer was at max. GVW because its partner is still carrying 1820 lb---1000 lb. on the tongue, 6000 on the tires. In any case, the pressure is displayed on the receiver and I would hope to catch a slow leak long before the alarm sounds.
Temperature alerts can be similarly set and the receiver can be set to display either psi or temp or both alternately. Temp and psi are monitored continuously and an alert will sound regardless of which measurement is on display.
The tire of course needs to be dismounted to install the sensor and valve. The sensor adds 35 g and I did have the tires balanced then too. I took one tire in first to make sure I'd get a signal in my truck. In a show of good faith I even installed the lug nuts before walking over to the truck to see that I really did have a signal. The last 2 tires I put the receiver in my car and when I got home with the tires in the trunk it almost immediately picked up the two already on the trailer even though the full length of my truck and the front of the car was adding to the distance. The sensors only send a signal every 30 seconds, but most of the time all 4 tires show up very quickly whether sitting in the driveway or out on the hwy. On my truck the cig lighter socket is live all the time so I pull the plug on the receiver when parked. Either way the receiver clears the display when I start the truck so I know I'm always getting a fresh signal and not holding onto a stale one. The sensors cannot be turned off and transmit even when the receiver is unplugged. There are instructions for the electrically inclined to hard wire the receiver. I haven't found a good spot yet to velcro the receiver, but it is an attractive unit for an add on device, about the size of a credit card.
The valve stems are Al and would look really good on an Al wheel. To keep the shiny valve caps off of bicycles and other vehicles I carry a 3/8 socket and thumb wheel ratchet which works very slick for this kind of task.
The installation was very straight forward with no problems and no leaks on the rubber seals of the valve stem. The sensors read 1 psi lower than my digital gauge so I think that is acceptable too. All in all a nice product and after more than 7000 mi and almost daily use over 5 weeks I'm very satisfied. Given tt tire reputations, driving down the road knowing what the tt tires are doing is an amazingly reassuring experience. I'd hate to ever be without it again.
Along with all the various ymmv qualifiers, here is a FAQ from Doran's web page:
"My sensor was blown off (blowout), why was no alert triggered?
Instant tire failure is rare in comparison to the more common failure caused by gradual tire deflation. It is possible, during an instantaneous blowout, to have the sensor blown off the tire before it has had a chance to send a signal to the monitor. The Doran 360™ tire pressure monitoring system is designed to monitor tire pressures, not to prevent this type of catastrophic tire failure."
I have no idea whether the Orange sensor screwed to the valve stem bolted to the wheel would do any better and I hope not to find out.