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I just purchased a tire floor pump with a gauge for my bike. The gauge seems to start off at 0 PSI when I connect the pump to the tire. So, do I need to use a separate tire gauge to first measure the current pressure of my tires prior to using the pump so as not to over-inflate them?

Additional info: I purchased the Serfas TCPG Bicycle Floor Pump and the gauge does not move at all when the hose is attached until I start pumping. Not sure if that's correct or if I'm doing something wrong.

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If you're using presta valves, you have to pump first so that the pressure can be read. Schrader valves push the pin when you attach the chuck, so this shouldnt be necessary. –  Batman Apr 25 at 3:20
    
Not sure what kind of pump you have. With the floor pump that I have, I don't need to "pump first so that the pressure can be read". For presta valves you should unlock the valve first, then attach the pump hose, then flip the top lever. –  sessyargc.jp Apr 25 at 8:57
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A pump or two to get the gauge up to the current pressure in he tires won't overinflate them so much that you will damage them. If you're at all worried, let a little air out with your fingers before you attach the pump, or using the release button after you have the pump on. –  Kibbee Apr 25 at 18:18
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I pump until I hear the click of the valve opening and then read the pressure gauge. –  Remy Apr 25 at 18:53
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Can't you use your Doctor's screwdriver thingy to determine the pressure? –  andy256 Apr 25 at 22:58
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3 Answers 3

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The pressure dial doesn't measure the pressure in the tube - it gives the pressure in the entire system (tube + hose). When you connect your pump, you need to pressurize the hose to the same level as the tube before it will read anything. I usually find this takes two or three full pumps, and the resistance jumps up massively once the tube and tire have the same pressure. At that point, the dial will show you the pressure in your tire.

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Thanks for confirming that the two or three pumps are needed before one can start reading the pressure on the gauge. I was previously assuming that it would read the pressure as soon as I connected the hose to the valve, but apparently that isn't the case. –  THE DOCTOR Apr 28 at 15:24
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I still don't understand why people are so anxious to get the pressure right to the last millibar. It will vary with load, temperature and time anyway, even on a relatively short ride of only a couple of km. As long as it's more or less correct, that's good enough.

For road bikes (~8 bar/116 psi typical tire pressure), a 0.2 bar/3 psi difference is absolutely acceptable IMHO. On an MTB, which runs far lower pressures, less is better, but as the tires are far bigger, a pump stroke will also not put that much more pressure on them. In the end, I try to end up within ~3% of the target, which has served me well the last 20 years.

So, what @Remy said in his comment is the way to go. Prestas will click open when the pressure in the hose surpasses that in the tube. So don't lean on your pump with full force until that happens and you're good to go. Also, most floor pumps designed for road bikes don't have the volume to make your tires burst with just one stroke.

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+1 I would be curious if someone could provide typical accuracy measures of some mid-range pumps (for example the one from the question). –  Vorac Apr 28 at 10:18
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For those not measuring in "bar", 0.2 bar is 2.9 psi. This is probably getting pretty close to how accurate the pump is anyway. I just checked, and my pump is rated by the manufacturer as being accurate to +/- 3%. Which is 3psi of variation at 100psi. I don't think that it's important to be that accurate with tire pressures. If you're making an effort to check and correct the pressure once a week, you'll be well ahead of most cyclists. –  Kibbee Apr 28 at 12:52
    
@Kibbee Thanks for pointing out that psi are still widely used. Updated the answer. –  arne Apr 28 at 14:02
    
PSI is all that's used in the (ever-regressive) US. –  Daniel R Hicks Apr 28 at 14:26
    
Canada has adopted the metric system, but everyone I know uses PSI. The correct metric unit of measurements for air pressure is Pascals. I just looked it up, and a "bar" is equal to 100,000 Pascals, or 100 KPa. So I guess that "bar" just allows people to use Pascals without resorting to large or small numbers. Although I think that using KPa would work better, as people wouldn't have to think about decimal places. –  Kibbee Apr 28 at 15:22
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Squeezing the tire or putting your weight on the handlebars or seat will give you a ballpark idea of how much air you will need to add, if any. Experience is still a good teacher. Expensive sometimes but reliable. Batman is right about the Presta valves: don't forget to unlock the valve (back the lock "nut" off the valve cylinder) before attaching the pump. If a Schrader is showing zero pressure after attaching the pump, the pump center-pin isn't pressing the valve stem in. I've found using a gauge often causes more air loss than the reading was worth. Pump-mounted gauges are pretty good, but checking accuracy against a trusted gauge is probably a good idea as well.

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Note that from about 2 Bar up tire pressure can't be measured by squeezing the tire with hand. –  Vorac Apr 28 at 10:15
    
Yeah, depending on the tire size and the squeezer's strength, squeezing will only tell you that the tire is over/under about 40-50psi, which is way too low for many road tires. –  Daniel R Hicks Apr 28 at 14:27
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