Illuminating test - Light-up tire pressure monitors

Want to maximize your gas mileage, reduce tire wear, and improve safety? Well those are the sort of claims made by the makers of the Air Alert Valve Cap System. The caps they're selling are the simple screw-on type with valve indicators that blink once the tire's air pressure falls about 4 pounds per square inch (psi) or more from the initial cold inflation pressure of a tire. We bought a set of four for $24.95 direct from Aviation Upgrade Technologies to try them out for this blog-exclusive look at tire pressure monitoring systems.

Valve cap sensors aren't new; we tested a mechanical color-coded sensor some years ago under the Accu Pressure name. They worked OK, but we had problems trying to read the color-coded display, which dimmed over time due to moisture fogging. They also required stooping down close to the sensor to read the display. Plus, the sensor was susceptible to road curb damage, and we even had a set stolen on a parked car during our test.

We found that these new, blinking Air Alert stems more accurately sensed pressure changes. Think digital pressure gauge to mechanical stick pressure gauge in comparing it to previous mechanical systems.

The instructions are straightforward and the caps are simple to install, but it's necessary to follow the instructions precisely to initially calibrate the caps to your car tires' recommended inflation pressure. 

• Set your tire's pressure to the recommended inflation pressure and then install the cap. It screws on like a regular valve cap.
• The cap will automatically measure and retain the pressure setting for the life of the cap.
• Should the pressure drop 4 psi or more from the initial pressure, the top of the cap will blink red until the pressure is restored.

We bench-tested each of the four valve caps and found them to accurately flash when the pressure dropped 4 psi +/- 1 psi as claimed by the manufacturer.  In fact, against our pressure calibration gauge, the caps worked within about one half of a psi (+/- 0.5 psi), notably better than claimed. (Aviation Upgrade Chief Executive Officer Torbjorn Lundqvist says that caps are checked at the factory to be within specification of +/- 0.5 psi, despite the advertised claimed tolerance of +/- 1 psi.) We then installed the caps on a Chevrolet Silverado pickup and compared them to the truck's own tire pressure monitor system, which displays individual tire pressures. Each time the tire pressure fell 4 psi or more, the Air Alert flashed.

Pros:

• The caps are relatively small, just about 1" x ½" in size, so they should be fairly discreet, and they only weigh about 0.35 oz. each.
• We didn't experience any tire imbalance, but it's probably a good idea to have the tires rebalanced to compensate for the weight of the valve cap. Mr. Lundqvist says a more miniaturized version of the cap is in the works.
• Accuracy may be satisfactory, but durability is still an open question. They do have a long-life battery -- good for about two years of service or just three weeks if a flashing light condition persists.
• The caps are suitable for a wide range of cars, trucks, trailers with tire pressures from 10 to 110 psi.

Cons:

• Mess-up the initial calibration of the valve cap and you end up with a non-functional system.
• Like all valve cap sensors, you have to walk around the car to observe each tire. We found the blinking light noticeable in most light conditions and it was far easier to detect than some mechanical color-coded pressure loss systems. But we agree with the manufacturer's advice: the system is not meant as a substitute for monthly tire pressure checks.
• Four psi is a narrow window of air loss particularly with a tire that operates at high air pressure. The natural variance of pressure resulting from the changes in air temperature might routinely trigger the caps to flash. As a comparison, the government-mandated tire pressure monitoring system (which will be mandatory on all new cars starting in 2008 model year) requires activation at margin of 25 percent loss from the vehicle's recommended air pressure setting. For a vehicle like the Chevrolet Silverado, that would be about 8.8 psi less, resulting in fewer false alarms than the Air Alert System. But, of course, the Air Alert System is providing a more stringent and better early warning system.
• Like all twist-on pressure cap systems, it's critical to properly hand-tighten the cap. Leave the cap a bit loose and it could actually bleed air out of the tire. And because the caps do stick out a bit more than regular valve caps, they get damaged from curbs.
• Finally, some states restrict use of blinking lights on the wheels; some devices are clearly cosmetic with brilliant illumination. It's not clear if these add-ons are affected by the same restrictions since the light is not normally on when the tire is properly inflated and the illumination is subdued. But it's probably a good idea to contact your state department of motor vehicles for advice first.   
• The batteries are not replaceable.

If your car doesn't have tire-pressure monitors and you're looking for an economical and simple way of ensuring that your tires are not under-inflated, consider the Air Alert System.

--Gene Petersen