Loss of pressure as you disconnect pump...

[spire]

Now this may sound pedantic, but bear with me.

Your tyre is recommended to be inflated to 120psi, so you do this.

When you disconnect from the valve there is a brief rush of air.

If you were really keen, should you then immediately check the pressure again to see how much it had dropped in order that you could over-inflate by the same amount next time?

 

[nickb]

The rush of air will be the overpressure in the pump.

 

[mickle]

What nickb said, it's the 120psi air leaving the hose.

 

[Tynan]

120psi should close the valve on the tyre pdq

 

[briank]

What's all this about "next time"?
If you were really keen, surely you'd re-inflate right away!
Any idea how many psi are lost in that sudden rush? Or does it depend greatly on type of pump, degree of cackhandedness etc?
(I defer to no one when it comes to pedantry.)

 

[Crackle]

Yep. Air leaving pump. If you reconnect it what does the gauge say (I'm assuming track pump here)?

 

[spire]

It is a track pump, and yes there will be a rush of air out of the pump, but there will be a momentary escape of air from the inner tube before the valve closes. It is to this small loss that I am referring.

 

[tyred]

It is a track pump, and yes there will be a rush of air out of the pump, but there will be a momentary escape of air from the inner tube before the valve closes. It is to this small loss that I am referring.

I would consider it to be so small as to be insignificant. If you're really worried about, pop in an extra PSI or two for luck before disconnecting the pump.

 

[02GF74]

yeah but if you then connect a pressure gauge, the gauge itself has a small volume of air so you will not get the true pressure reading in the tyre itself.

 

[BentMikey]

It is a track pump, and yes there will be a rush of air out of the pump, but there will be a momentary escape of air from the inner tube before the valve closes. It is to this small loss that I am referring.

How are you measuring it? By reconnecting the pump? Then you'll be using some of the air in the tube to re-pressurise the pump and gauge, so it won't be an accurate reading anyway.

I'd bet the real loss of air from the tube on disconnect is so small that it's not worth worrying about.

 

[spire]

This is like Quantum Mechanics where the act of observing something affects what is being observed!

Measuring air pressure affects the air pressure. Aarrgghh!

 

[yello]

On a related note, I can't check air pressures with my track pump (a TopPeak Joe Blow) because I have to let the tyre down a bit first. I don't know why exactly but I guess it somehow releases the valve in the tyre. Similarly, I can't pump it up.

So, for me, any pumping up of the tyre first starts by letting some air out!

 

[jimboalee]

I've got a Joe Blow.

Before you connect the pump head to the tyre valve (Presta), tap the presta with the pump head to loosen its seat.

This works better.

BTW, the loss of pressure when you disconect the pump head is a Gnat's fart. Not worth worrying about.
You've just got to whip the pump head off quick!

 

[Stick on a Giant]

Just disconnecting once will release such a small amount of air/pressure that you probably won't see the needle move on the dial at all.
What you need to do, is to disconnect and reconnect the pump 10 times. On the tenth go, see how much the pressure has dropped. Then divided that by 10 to see how much pressure is lost each time. Then figure out how much pressure is added per pump stroke, figure out how that relates to the loss of pressure, and overpump by that amount before removing the pump head.
Is it worth it?







I don't think so!

 

[skwerl]

as you say, you're into quanta here. ie tiny, tiny amounts

try worrying about something else - like negative equity or who's going to win Strictly or whether you'll really get your money back from the phone vote