Alathea

Ive used three pumps in the last two weeks-1 WM foot pump, one WM Schwinn, one Pedros, and my current one today a Blackburn AirTower1.

I returned the Pedro's because the gauge would settle in at 40 right when hooking up, and then barely move-testing the tire with a hand gauge would then net 65-75+ lbs. The LBS staff verified this today on three of their bikes at the shop-then gave me a Blackburn at a discount for my trouble. The footpump, a 9.00 walmart thing, is the ONLY one of all these that registers my 60lb tires right after hooking it up.

The WM Schwinn wouldn't pump past 50. The foot pump wont pump past 60.

The Blackburn settles in at around 40ish right when hooking up and then after a pump goes up to around 60 or whereever the tires beginning pressure is. I test the tire with my hand gauge before I hook up the pump and it reads about 60. The LBS guys used my hand gauge today too-so its about as accurate as one can expect.

What do I have to do to get a pump that reads what the pressure is BEFORE I start pumping air with it?

rdtompki

Do presta pumps every read the pressure correctly before you start pumping? Doesn't seem like this would be the case since the pump doesn't depress the stem and equalize the pressure until you start pumping. Once you start pumping the higher pressure in the pump forces air past the valve during which time the pressures on either side (tire and pump) are equal. Anyway, makes sense to me.

Alathea

my tires are schrader, but maybe im just paranoid.

Mr. Beanz

Not sure why it is so important. You know you topped off at 60 last time you filled the tire. Tires lose pressure overnight so you can't possible overfill in one stroke. So if I need a starting point, I'd place an airgauge on the stem before pumping.

Alathea

I didnt do it over night, though. The issue is, shouldn't the starting pressure on the guage on the pump read what is already is? Im leaning towards no since three of the 4 pumps all read differently. Granted im only spending 25-30 per pump, but Id like to know what the pressure is before I pump.

If I have a pump with a guage I shouldn't have to check the pressure with ANOTHER gauge, right? I checked with a hand gauge and they were at 60, but hooking up yet another floor pump shows the starting pressure indicator says 40ish.

Mr. Beanz

Ok!

Wogster

With a Schrader they should read correctly before you start, now I don't know if it matters that much, because you have a separate gauge, your not needing to attach the pump to determine starting pressure. So if your gauge shows you don't need air, the pump does not matter. If your pumping then the fact it reads low at first does not matter, it's what it reads when your finished pumping that matters.

Gauges on pumps are not known for being super accurate anyway, it's a convenience item to get you in the right pressure range. My tires are intended for 85PSI, I pump them to 90 according to my pump, pull the pump off, and they should be somewhere between 83 and 87, which is close enough. I know there are some riders who need to be within .0000001 PSI and spend 2 hours getting the pressure exactly right before every 3 minute ride -- me, I would rather spend 3 minutes getting the pressure close, and the 2 hours riding

Alathea

Thanks. Its frustrating to have gone through 4 pumps and a gauge to do something that seems so simple in application-testing pressure. I like to be able to trust my instruments-must come from being in a tech field.

lambo_vt

A floor pump isn't an instrument and, like any other pressure gauge that probably cost about a buck, shouldn't be trusted.

Like Wogster said, don't overthink the pressure thing. Inflate, and ride.

d4c4c8

I have NEVER in 25+ years of riding or the 6 years I was part owner of a shop had a pump that immediately registered the current pressure in a tire. I always CHECK with a guage, then add air if needed.

Like they all said, DON' WORRY ABOUT IT

cyccommute 

A pump will never give an accurate reading of a tire initially. Neither presta nor schrader. For a schrader valve stem to work, you have to depress the valve and open it so that air can enter. Depressing the valve will fill the hose and, sometimes, the pump chamber. This will, of course, decrease the pressure in the tire. You can't get around it. It's the way the system works. Once you've finished pumping, you have to release the valve and you can lose a little air while doing that.

A presta valve, on the other hand, requires the pressure in the pump line to open the valve. You have to pressurized the hose and pump chamber to at least the pressure in the tire...often over the pressure in the tire...to get air into the tube. Once you remove the chuck on a presta valve, the valve closes and keeps pressure in the tire. The release of air you hear with a presta on removing the hose is the line depressurizing and the tire isn't losing anything.

If you want the pressure in the tire before pumping, the only way to get that is with a pressure gauge. However Mr. Heisenberg has thrown a bit of a monkey wrench into the reading. By using the gauge, you can know what the pressure in the tire was when you put the gauge on but you can't know what the pressure is after you take the gauge off. You lose some pressure each time you use the gauge.

LandKurt

Actually that's the Observer Effect. Heisenberg's uncertainty principle say you can't know both the exact speed and position of a particle. It's not about changes due to observation, rather that you can only measure one or the other precisely. Very counter intuitive.

Kind of like how you can't get a bike that's inexpensive, quality built, and has good service. You can't have it all, so choose what's most important to you. Life is made of trade-offs, after all.