Dumb question

the black rose said:

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 said:

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/

Kenneth said:

Quoted message said:

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.

Quoted message said:

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 said:

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

Quoted message said:

Dumb, I know.

-km

Ravi said:

the black rose said:

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 said:

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

Quoted message said:

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.

Quoted message said:

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
[email hidden]

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.

Quoted message said:

someone said:

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

Quoted message said:

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.

Quoted message said:

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
[email hidden]

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

Badger South said:

Quoted message said:

Quoted message said:

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

Quoted message said:

Quoted message said:

Quoted message said:

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

Quoted message said:

Quoted message said:

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.

Quoted message said:

Quoted message said:

Quoted message said:

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

Quoted message said:

Quoted message said:

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.

Quoted message said:

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

The gauge may be but it can't engaged without letting air out, so the
pump gauge, if it is any good, gives a more accurate reading.

Quoted message said:

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).

As I said, that will let out air.

Quoted message said:

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.

That may work if you don't make any steep descents, but after you have
blown a tire off the rim, you may think differently about that. If
you don't ride in mountains then it's probably safe.

I like to descend and corner fast, and for that rock hard tire is not
ideal.

Jobst Brandt
[email hidden]

jobst.brandt said:

Quoted message said:

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.

That may work if you don't make any steep descents, but after you have
blown a tire off the rim, you may think differently about that. If you
don't ride in mountains then it's probably safe.

I like to descend and corner fast, and for that rock hard tire is not
ideal.

ditto, I recently switched to 700x28s on the roadie for that reason--I can
run the recommended max pressure, 85-90psi, and get nice
grip over the rough pavement around here. Skinnies have their place I
suppose, but I don't ride in ideal conditions and find cornering much more
comfortable on the slightly wider and softer tires. The ride is *much*
improved on the flats, and try as I might, I can't perceive any more RR
than a set of 23s.

If I lived in the desert with nice glassy roads then things might be
different...

Quoted message said:

hat may work if you don't make any steep descents, but after you have
blown a tire off the rim, you may think differently about that. If
you don't ride in mountains then it's probably safe.

Yeah, uh compared to present company, I have no conception of a mountain,
let alone frequent them. Plus, never west of the Miss. ;-D

Quoted message said:

I like to descend and corner fast, and for that rock hard tire is not
ideal.

Good to know, if I ever learn how to corner. <g>

-B

Quoted message said:

Jobst Brandt
[email hidden]