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Customer Review

11 of 16 people found the following review helpful
4.0 out of 5 stars Don't blindly trust the gauge or auto shutoff, February 14, 2012
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This review is from: Slime 40022 12-Volt Digital Tire Inflator (Automotive)
This thing is more solidly constructed than the cheapo last one that I bought at an auto parts store, and seems like it could tolerate being banged around a bit in your trunk. The auto shutoff feature seems to basically work and is useful, but after a few tire inflations it seems that its pressure gauge is probably too inaccurate to just blindly trust the auto shutoff feature. I'm not really surprised since most non-dedicated gauges seem to be a little deficient, and I wasn't planning to trust this one anyway.

Basically what happened to me is that I set it to inflate all four of my tires back to 32psi using the auto-shutoff thing, and according to my TPMS (which is generally within +/- 1psi of my dedicated tire gauge, so not too bad) the tire pressures were initially OK, but after driving 10 min, parking, waiting at least 4 hours, and walking back to the car, the TPMS was reading something like 33,33,34,35. I got out my Accutire MS-4350B Tire Gauge and sure enough the tires were within +/- 1psi of what the TPMS said. So I just let air out of them until they all read 32psi. (It's possible that the temperature went up 10F while I was doing stuff, resulting in an increase of 1psi per tire, but that doesn't explain the the 35.)

With the one that was at 35, I was already suspicious because the thing sat there pumping for 10-20 seconds AFTER it was already reading 32 (as if the gauge stuck at 32 then finally hit whatever fraction of a psi above 32 that triggers the shutoff). I actually shut the thing off on the first tire manually, then went back and tried it again after the shutoff worked on other tires. So I went back to the first tire to see what it would do and it continued to run at 32 for a bit and finally shut off, resulting in the 35psi pressure. Odd.

Now, it was only 10-20F outside when I inflated the tires and it's possible that the tires had deformed or something due to the cold, then hardened such that they had slightly different internal volumes, possibly explaining why the pressures seemed OK originally but then read too high after a 10 minute drive and a 4+ hour wait. The one that was overinflated was also the one that was very low compared to the others before I inflated them, so it wouldn't be surprising if cooled and hardened into a different shape than the others. (I inflated them all when I got up in the morning and the car was dead cold.)

So anyway, I'm still reasonably happy with this as it seems to be durable, it does have the auto shutoff feature which is still kind of useful, the power cord is long enough for my small car (as long as you run it out of the door on the same side as the tires you're inflating), and I didn't really plan to trust the pump's pressure gauge anyway.

My advice is that if you use this, use the auto shutoff but then get out a real tire gauge (like the previously mentioned Accutire), check the pressure, and then let some air out until the proper pressure is reached. If the auto shutoff is routinely underinflating, just set the shutoff threshold a little higher so you can then let air out with your real gauge to get to the proper pressure. Watch the number on the pump's display the whole time it's running, and if the pressure gauge on the thing seems to stick at the same value while the pump runs, play it safe, shut the thing off manually, and take a reading with a real gauge. I doubt the thing is so inaccurate as to explode a typical car tire before shutting off, but certainly it's work needs to be verified with a known accurate gauge.
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Showing 1-2 of 2 posts in this discussion
Initial post: Mar 23, 2012, 12:53:42 PM PDT
J. Braun says:
This is meant to educate. The reason the car makers have instructions that tell you the "cold" tire pressure to inflate to, is that tire pressure increases in tires when driving and when sitting in the sun. Always inflate tires to recommended pressure when cold. Don't worry about pressure after driving or heated as this will always be higher. Serious problems happen to tires that are under inflated

In reply to an earlier post on Apr 1, 2012, 2:53:36 PM PDT
Omega Man says:
True. I wasn't talking about the cold pressure vs the pressure after driving, but the cold pressure at one point (after the car was sitting overnight) and the cold pressure later in the day (when the car had been sitting around a few hours since last driven).

Also, the problem is that the "cold pressure" depends on the weather conditions that day. Around here the weather has been varying between 30-80F at times, so one morning I have the tires inflated to a cold pressure of 32psi when it's 70F outside, then the next morning the temperature has dropped to 30F and the cold pressure is now 28psi which is too low.

As a result it's rather difficult to keep the tires at exactly one pressure all the time with varying ambient "cold pressure" temperatures. For my car the pressure is supposed to be 32-36psi depending on how loaded the vehicle is, so I tend to adjust the pressure that I inflate them to based on how much I expect the temperature to vary. For example, if it's 60F outside and I expect that it may swing between 40-80F then I inflate the tires to 34psi. At 40F they end up at 32psi and at 80F they end up at 36psi which are both ok.
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