The car Bibles product reviews: Michelin Digital Tyre Inflator.
Product Reviews
Michelin Digital Tyre Inflator
Tested July 2006 
This is also part of Michelin's new line of automotive products which came out in 2006. I tested this in conjunction with their new pressure gauge, above. At the time of writing, this inflator cost $29.99 from motoring parts stores. It comes in a medium quality cardboard box and will likely have been opened and looked over a few times before you get to it. It has a long power lead with a 12v adaptor on the end which stores neatly underneath the unit, and a good-quality rubber hose with a clip-on style valve adapter on the end of it. The hose wraps around the outside of the unit when not in use. Underneath there are recesses containing a couple of adapters for footballs and bicycle tyres. The instructions are a 4-page booklet. The instructions themselves take up about 2 line inches and are reproduced on a label on the bottom of the inflator. The rest of the booklet is consumed with (basically unnecessary) warnings and notices. The top of the unit has an on/off rocker switch at the bottom, and three buttons at the top surrounding a digital display. The top button has an 'R' on it. This is the reset button - hold it down for three seconds and it resets the device to zero. It doesn't mention this in the booklet and if you do this whilst the inflator is connected to your tyre, it will recalibrate the gauge to believe the current tyre pressure is zero. Be warned! The other two buttons are a '+' and '-'. In use, you plug the unit in to a 12v supply and the display lights up blue with black LCD figures. Press the 'R' button to cycle through the pressure units, same as the gauge reviewed above. Use the '+' and '-' buttons to set the desired pressure, starting at 30psi and working up, and once you're done, the display resets to zero. Attach the clip-on nozzle to your valve stem and the display reads your current tyre pressure. If its below the pressure you tapped in to the inflator, when you turn the inflator on, it will start pumping up the tyre until it reaches your desired pressure, then cut off. The first unit I had didn't cut off though - it must have been a dud because it just kept on going. The second unit worked just fine. Don't be alarmed if you're watching the display and it goes up to 1psi above your chosen pressure; once the pump cuts off, it will read correctly. I think the pulsing of the pump causes the reading to be too high whilst it's running. This doesn't seem to affect the cutoff feature though. The noise when the unit is inflating is pretty loud. I guess they're using an RS540 electric motor attached to a meaty one-way valve, and if you leave it on the floor, it will vibrate around and 'walk' off to the extend of the rubber hose if you let it.
Something weird happens with this unit though - both my test units had the same issue so I think it's a design 'feature'. Once it gets hot, the unit can over-read by as much as 10psi. This means that if you leave it to inflate something, it will underinflate it by up to 10psi. I noticed this first when I took the nozzle off the fourth test tyre and connected to nothing, it read 2psi. I reset the unit and it zero'd out and after a moment, went back up to 2psi. Then 5psi. I let it cool off for 5 minutes or so, and it read zero again. There is a warning in the booklet about letting it cool off for 10 minutes after any 'inflation activity' and I guess this is why. You need to be careful then - if you do all four tyres in one go, by the time you get to the last one, you could be off by as much as 10psi. For that reason I took half a star off my rating, with another half removed for the inability to deflate overpressurised tyres. You have to do that by hand then re-check the pressure. I finally took another half a star off for the initial dud unit. In this day and age, it ought to be possible to buy something like this that works first time.
I tested it against the digital pressure gauge, as well as an old stick-type pressure gauge and a quality analogue gauge which I have calibrated every 6 months. At 40psi, the stick gauge reads 39psi and Michelin's digital inflator reads 40.5psi so its well within tolerance, and it matches the pressure gauge tested above.