Dumb question

I have a dumb question about tubes with presta valves. When you're
inflating them -- or at least, when I'm inflating mine -- the psi
reading on my floor pump (it's a Blackburn) goes up and down and up and
down, up as I push to pump air in, down as I pull the pump handle back up.

Which is the correct pressure reading? The high reading or the low reading?

I've been assuming the high reading is correct, so my tires are either
inflated to the correct pressure, or they're underinflated.

Dumb, I know.

-km

--
Only cowards fight kids -- unidentified Moscow protester

http://community.webshots.com/user/blackrosequilts
proud to be owned by a yorkie

On Tue, 19 Oct 2004 14:08:55 GMT, the black rose
wrote:

I have a dumb question about tubes with presta valves. When you're
inflating them -- or at least, when I'm inflating mine -- the psi
reading on my floor pump (it's a Blackburn) goes up and down and up and
down, up as I push to pump air in, down as I pull the pump handle back up.

Which is the correct pressure reading? The high reading or the low reading?

I've been assuming the high reading is correct, so my tires are either
inflated to the correct pressure, or they're underinflated.

Dumb, I know.

-km

Howdy,

In my understanding, the high pressure reading is caused by the
hesitation of the valve to open.

The low is measuring the pressure in the tire...

HTH,

--
Kenneth

If you email... Please remove the "SPAMLESS."

the black rose wrote:
I have a dumb question about tubes with presta valves. When you're
inflating them -- or at least, when I'm inflating mine -- the psi
reading on my floor pump (it's a Blackburn) goes up and down and up and
down, up as I push to pump air in, down as I pull the pump handle back up.

Which is the correct pressure reading? The high reading or the low
reading?

I've been assuming the high reading is correct, so my tires are either
inflated to the correct pressure, or they're underinflated.

Dumb, I know.

-km


Presta or Schraeder, doesn't matter. The gauge on your pump is reading
the pressure in your pump, not in your tube; if the tube valve is open
they should be the same. Most likely you have some leakage in your pump
or around the seal so when you push on the handle the valve opens and
the pressure reading should be accurate. When you pull up the valve
shuts and some of the air in the pump leaks out so the gauge reading
starts to drop.

--
My bike blog:
http://diabloscott.blogspot.com/

On Tue, 19 Oct 2004 10:39:54 -0400, Kenneth wrote:

on my floor pump (it's a Blackburn) goes up and down and up and down, up
as I push to pump air in, down as I pull the pump handle back up.

Which is the correct pressure reading? The high reading or the low
reading?

I've been assuming the high reading is correct, so my tires are either
inflated to the correct pressure, or they're underinflated.

In my understanding, the high pressure reading is caused by the hesitation
of the valve to open.

The low is measuring the pressure in the tire...

No, that is not correct. When the valve sticks (Hint: to minimize this,
press the valve to let out some air before putting the pump head on), the
pressure reading will rise until it forces the valve open, then it will
drop. This happens before you pull the handle back up, if it will happen
at all.

But the pressure reading should stay constant as you pull the lever up. I
agree with the earlier poster, you have either a problem with the pump, or
the seal of the head around the valve stem.

--

David L. Johnson

__o | "A foolish consistency is the hobgoblin of little minds, adored
_`\(,_ | by little statesmen and philosophers and divines." --Ralph Waldo
(_)/ (_) | Emerson

"Diablo Scott" wrote: Presta or Schraeder, doesn't matter. The gauge on
your pump is reading the pressure in your pump, not in your tube; if the
tube valve is open they should be the same. (clip)
^^^^^^^^^^^^^^^
If you push the pump down quickly, the pressure in the barrel will rise
faster than the flow through the hose can equalize it, so it will be higher
than the tire pressure. It is normal to see the gauge rise and fall in
rhythm with the pumping cycles. If you pause at the bottom of a stroke, you
should see the pressure come down to the tire pressure. As long as the
check valves and stem/hose seal are good, the pressure should stay constant
on the up-stroke. You should NOT feel any pressure against your hand from
the pump handle on the up-stroke. In a perfect system, the gauge should
stay constant when you are not pumping, but a very slight leak at the stem
seal is not uncommon. Pump faster to keep ahead of it.

the black rose wrote:

I have a dumb question about tubes with presta valves. When you're
inflating them -- or at least, when I'm inflating mine -- the psi
reading on my floor pump (it's a Blackburn) goes up and down and up and
down, up as I push to pump air in, down as I pull the pump handle back up.

Which is the correct pressure reading? The high reading or the low
reading?

I've been assuming the high reading is correct, so my tires are either
inflated to the correct pressure, or they're underinflated.

i have seen in some cases when i do not open the presta valve fully
(meaning the valve nut is not fully open), this kind of thing happens.
my guess is if i open it fully, the pump can push down on the valve -
there by forcing it open completely. When it is not fully open (i mean
the lock nut is not at the top most position), then the pump cannot push
the valve open - so the the rising/falling.

my 2 cents,
ravi

Dumb, I know.

-km

On Tue, 19 Oct 2004 09:51:43 -0700, Ravi wrote:

the black rose wrote:

I have a dumb question about tubes with presta valves. When you're
inflating them -- or at least, when I'm inflating mine -- the psi
reading on my floor pump (it's a Blackburn) goes up and down and up and
down, up as I push to pump air in, down as I pull the pump handle back up.

Which is the correct pressure reading? The high reading or the low
reading?

I've been assuming the high reading is correct, so my tires are either
inflated to the correct pressure, or they're underinflated.

i have seen in some cases when i do not open the presta valve fully
(meaning the valve nut is not fully open), this kind of thing happens.
my guess is if i open it fully, the pump can push down on the valve -
there by forcing it open completely. When it is not fully open (i mean
the lock nut is not at the top most position), then the pump cannot push
the valve open - so the the rising/falling.

my 2 cents,
ravi

Beware of having the valve open too far. IME, you only need to unscrew it
about half-way and this prevents bending the delicate post, but still
allows easy filling. I had been having trouble getting good sealing and
read a poster's comment on this and now seems to be working ok.

Logging in on the psi readings, the first reading is due to pressure inside
the pump tubing, and the place the needle bobs back to on upstroke is the
actual reading inside the tire, but I've not done any definitive studies
nor read this spelled out anywhere. It seems like a commonsense interp.,
and we all know how wrong that can be... ;-p

-B

someone writes:

When inflating tires the psi reading on my floor pump (it's a
Blackburn) goes up and down and up and down, while pumping.

Which is the correct pressure reading? The high or the low reading?

The high reading is the pressure required to free the conical Presta
valve from its seat. The low reading, if the pump head grommet does
not leak, is the pressure in the inner tube and the pressure at which
the valve settled back into its seat.

I've been assuming the high reading is correct, so my tires are
either inflated to the correct pressure, or they're underinflated.

The tires are underinflated, the amount depending on how sticky the
valve is. If the valve is not popped open manually before pumping,
pressure can exceed 200psi (damaging the gauge), before the valve will
open.

Jobst Brandt

A Presta valve has no spring to close it unlike a Schraeder valve. THe
pressure differential should be the same but there is a bit of
"stiction" in the seal. This is the reason for the momentary pressure
rise.

If all is working correctly and no leaks, the LOWEST pressure reading
on the gauge is the tire pressure. This should also be stable without
pumping. If there is the slightest leak, this pressure will not be
stable.

The higher pressures are just dynamic, caused by the volume of air
flowing over the restriction of the valves and hose.


--
Weisse Luft

On Tue, 19 Oct 2004 18:25:08 GMT, wrote:

someone writes:

When inflating tires the psi reading on my floor pump (it's a
Blackburn) goes up and down and up and down, while pumping.

Which is the correct pressure reading? The high or the low reading?

The high reading is the pressure required to free the conical Presta
valve from its seat. The low reading, if the pump head grommet does
not leak, is the pressure in the inner tube and the pressure at which
the valve settled back into its seat.

I've been assuming the high reading is correct, so my tires are
either inflated to the correct pressure, or they're underinflated.

The tires are underinflated, the amount depending on how sticky the
valve is. If the valve is not popped open manually before pumping,
pressure can exceed 200psi (damaging the gauge), before the valve will
open.

Jobst Brandt


Isn't the gauge in the pump at least as reliable as putting a hand-held
tire pressure gauge on it?

A sure test, but one which will probably make you want to add more air
again, would be to measure it with one and you'll surely find that the
existing pressure shares one of the readings, high or low. (most likely
low).

I routinely give mine about 5-10 lbs more air than it calls for anyway.
Since you're not a heavy rider, km, why not over inflate a bit? Of course
I'm not running skinny tires, so YMMV.

-B