getting accurate tire pressures
 

Hey all, I was wondering how everyone gets accurate pressures in their tires. When I pump up my tires I go to a little over what my target is because when I disconnect the pump head there's some air that comes out of the tire. If it were a constant amount then it'd be no problem but it's not constant. Is there such thing as a presta valve gauge with a bleeder?

Usually a non-issue. Hopefully there is only a llittle bit of air leaking out when you remove the head. If not then maybe work on your technique/pump to make it easier to get the head on and off.

Be consistant. That's all that matters.


BTW - if you're checking to see how much pressure you lost by putting the pump head back on then there's your problem. There's no real need to do that.

I used to ride around with an accurate gauge using a presta adapter. Then over time I realized it was pointless. Haven't done it since the early 90s.

I don't worry about things like this. Pump it up till it's around 100psi, then I pop off and ride.

Honestly you think you could really tell the difference beetween 5 PSI, i bet in a blind study no one could.

That air is not coming from the tire. It's coming from the hose of the pump. If you understood how the valve worked, you'd know why.

QFT.

I have devised a procedure to figure it out for real though. Repeatedly remove and replace the pump head from the valve stem, taking a measurement each time. Do this at various pressures. After your data is collected, you can calculate the average pressure lost as a function of temperature and beginning pressure.

Easy.

Ah, got it.
Now I feel dumb getting confused by a freekin' valve & a pump... :rolleyes:
duhhhhh the valve only goes one way unless you press the button.
i need more coffee or some food

As a member of the computer generation, I have no idea what QFT means :o

You're not the first to ask this question, so if you're implying that many people on this forum are dumb... oh, nevermind.

I just had my thumb re-calibrated. It's accurate to +-.174 psi.

QFT = Quoted For Truthiness/Truthfulness/Truth.

Okay, I have come to accept the fact that the "lost" air is coming from the pump itself. And I'm aware of how the valve works. So.... Can somebody explain to me the purpose of the "Air Release Button" on my Topeak floor pump. It's claimed use is to allow the user to bleed small amounts of air from the tire to fine-tune the pressure.

But isn't the air being released simply coming from the pump itself and not the tire? Is the "Air Release Button" one of the more blatant bits of marketing BS out there?

http://en.wikipedia.org/wiki/QFT

I have a pump that used to work and had one of those. You could deflate a tire completely using that thing. I theorize that the pump head physically depresses the valve.

Place index and middle finger on rim. Place thumb on tire. Move thumb toward fingers. Assess force needed to compress tire.

It doesn't have to be more complicated.

That doesn't account for changes in the surface area of the thumb. That could spell disaster.

Was the button located directly on the pump head? On mine (Topeak JoeBlow Sprint) the button is located on the pump body, so I'm not sure how it could depress the presta valve. But I've never attempted to deflate the tire with it. I'll give that a try, for kicks.

The problem is that you're also measuring the amount of air lost when you replace the pump head. There's no way to know how much of your measurement is from replacement vs. removal. I think a VERY small amount is lost from removal, and almost all of what you're seeing is from replacement.

Also, with my pumps, attaching the head doesn't pressurize the pump since it's at a lower pressure than the tube. So I lose some air getting the thing attached, but I have to pump it a bit to equalize the pressure and get a reading. If you're provides a reading upon connection, then you're pressurizing the pump with air from the tube, which lets air out of the tube. Inconsistencies with this process will also greatly affect the measurements.

The real way to do this would be to set up a rig made from PVC with a pressure gauge attached to it. Then pump it up to 100psi and see what happens to the fixed gauge when you remove the pump.

I have no experience with that pump, but am curious as to how it works as well.

I was thinking that the head always depresses the valve. Really don't know.

I defer to your superior method.

I am not fully evolved and do not have opposable thumbs. What method do I use? Also the stub of my prehensile tail gets sore on longer rides, any advice there?

I don't think this will work because each time you connect the pump to the valve, the pressure from the tire will fill up the pump hose loosing a certain amount of psi from the tire. This is not the case when removing the hose after pumping since the pump hose is already full.

To answer the OPs question, IMO it's not an issue. Most of the pressure release comes from the hose, not the valve...

Yes the air escaping when you do a decent job of seating and unseating the pump head is the air that is still pressurized and in the pump hose. My implication about technique refers to whether or not the OP is good at taking the head on and off. If not then you can lose a lot of air bumping the tube valve without being sealed anymore.

Release valves on the pump body work by allowing flow of air in the opposite direction - bypassing the check valve in the pump. This allows you to overpressurize and then slowly bleed off air until you reach pressure.

When you reach pressure everything in the system is at the same pressure...that is everything after the check valve:

1. Hose
2. Pump head area
3. Tube Valve
4. Tube

When you kill the seal at the pump head the following happens:

1. Air begins to escape from the hose - leaking around the pump head.
2. Pressure drop in the hose causes the Presta tube valve to close preventing air from the tube from escaping.
3. Air continues to bleed from the head until the hose and head pressures reach stability with the general surroundings - removing the pressure differetial.

When attching the pump head sometimes - depending on your pump's head design - the presta valve is not depressed. This means that you have a 2 chamber system (hose/head vs valve/tube). The valve does not then open until the pressure on the hose/head side is greater than the pressure inside the tube. The pressure differential opens the presta valve. Example...Silca pumps. Pump, pump, pump, pressure jumps then Psss as the valve opens.

No I wasn't implying any umm.....

+1 What he said ^