Compressor/tire pressure equalization?

Just a question for something to think on, really doesn’t matter. If
there’s any reply, or if I decide I care, I’ll check the answer (for
time, not the reason) myself.

Say there’s a compressor setup @ 130+PSI providing house air. You’re
using a drop with a reg to inflate a bike tire, with the reg set to
100PSI. If you just add air until it seems to stop transferring, say
3-5 seconds, the bike tire will actually only be @ 85-90PSI. How long
do you figure you’d have to leave the air chuck on the inner tube
valve to get the tire pressure to match the pressure coming out of the
reg? Will it ever get quite that high? Why the delay? At first
thought it seems like 100PSI pushing out should equalize @ 100PSI @
the tire pretty damn quick, especially if the reg is holding the air
to 100 when there’s well over 130 coming into the reg. Experience
tells me this isn’t the case. Any thoughts or explanations on why, or
guesses as to how long you’d have to hold the chuck on the valve to
get them equal? It’ll get the tire to ~85 in a second, two tops.
Very, very quick.

wrote:
Just a question for something to think on, really doesn’t matter. If
there’s any reply, or if I decide I care, I’ll check the answer (for
time, not the reason) myself.

Say there’s a compressor setup @ 130+PSI providing house air. You’re
using a drop with a reg to inflate a bike tire, with the reg set to
100PSI. If you just add air until it seems to stop transferring, say
3-5 seconds, the bike tire will actually only be @ 85-90PSI. How long
do you figure you’d have to leave the air chuck on the inner tube
valve to get the tire pressure to match the pressure coming out of the
reg? Will it ever get quite that high? Why the delay? At first
thought it seems like 100PSI pushing out should equalize @ 100PSI @
the tire pretty damn quick, especially if the reg is holding the air
to 100 when there’s well over 130 coming into the reg. Experience
tells me this isn’t the case. Any thoughts or explanations on why, or
guesses as to how long you’d have to hold the chuck on the valve to
get them equal? It’ll get the tire to ~85 in a second, two tops.
Very, very quick.

Use a gauge at the valve and reset your regulator as needed
to account for the line drop. That's why paint directions
always specify pressure "at the gun".

--
Andrew Muzi
www.yellowjersey.org/
Open every day since 1 April, 1971

On Aug 7, 4:29*pm, AMuzi wrote:
wrote:
Just a question for something to think on, really doesn’t matter. *If
there’s any reply, or if I decide I care, I’ll check the answer (for
time, not the reason) myself.

Say there’s a compressor setup @ 130+PSI providing house air. *You’re
using a drop with a reg to inflate a bike tire, with the reg set to
100PSI. *If you just add air until it seems to stop transferring, say
3-5 seconds, the bike tire will actually only be @ 85-90PSI. *How long
do you figure you’d have to leave the air chuck on the inner tube
valve to get the tire pressure to match the pressure coming out of the
reg? *Will it ever get quite that high? *Why the delay? *At first
thought it seems like 100PSI pushing out should equalize @ 100PSI @
the tire pretty damn quick, especially if the reg is holding the air
to 100 when there’s well over 130 coming into the reg. *Experience
tells me this isn’t the case. *Any thoughts or explanations on why, or
guesses as to how long you’d have to hold the chuck on the valve to
get them equal? *It’ll get the tire to ~85 in a second, two tops.
Very, very quick.

Use a gauge at the valve and reset your regulator as needed
to account for the line drop. That's why paint directions
always specify pressure "at the gun".

--
Andrew Muzi
* www.yellowjersey.org/
* Open every day since 1 April, 1971- Hide quoted text -

- Show quoted text -

This is the setup at work, I can't be playing w/ the regs. They just
happen to be set @ 100, which is a decent pressure for my road bike.
I was more curious about the how & why, trying to learn something.
"line drop" you say? Now I've at least got something to work with to
try to understand how/why it happens like it does. Thanks.

"DanKMTB" wrote: They just
happen to be set @ 100, which is a decent pressure for my road bike.
I was more curious about the how & why,
^^^^^^^^^^^^^^^^^^^
Okay, here's how and why: The flow rate between the supply (100 psi) and
the tire (somewhere between 0 and 100 psi) is proportional to the pressure
difference between the two. As the tire pressure comes up, the flow rate
drops. Evidently, after a few minutes, this difference has dropped to 15
psi, and the flow is so slow that you no longer hear it. If you waited
several more minutes, the difference would probably drop to 7 or 8 psi
(92--93 psi in the tire.) For it to reach a full 100 psi you would
theoretically have to wait forever. This is what is called an asymptotic
approach.

"Leo Lichtman" wrote in message
...

"DanKMTB" wrote: They just
happen to be set @ 100, which is a decent pressure for my road bike.
I was more curious about the how & why,
^^^^^^^^^^^^^^^^^^^
Okay, here's how and why: The flow rate between the supply (100 psi) and
the tire (somewhere between 0 and 100 psi) is proportional to the pressure
difference between the two. As the tire pressure comes up, the flow rate
drops. Evidently, after a few minutes, this difference has dropped to 15
psi, and the flow is so slow that you no longer hear it. If you waited
several more minutes, the difference would probably drop to 7 or 8 psi
(92--93 psi in the tire.) For it to reach a full 100 psi you would
theoretically have to wait forever. This is what is called an asymptotic
approach.

I don't buy it. Resistance to flow in this setup should be low. For
practical purposes, flow and pressure difference between tires and
compressor tank should immeasureably close to zero after just a few seconds.
In my home compressor / bike tire (with either Schraeder or Presta valves)
this seems to be 5 or 10 seconds.

BobT

Leo Lichtman wrote:
"DanKMTB" wrote: They just
happen to be set @ 100, which is a decent pressure for my road bike.
I was more curious about the how & why,
^^^^^^^^^^^^^^^^^^^
Okay, here's how and why: The flow rate between the supply (100 psi) and
the tire (somewhere between 0 and 100 psi) is proportional to the pressure
difference between the two. As the tire pressure comes up, the flow rate
drops. Evidently, after a few minutes, this difference has dropped to 15
psi, and the flow is so slow that you no longer hear it. If you waited
several more minutes, the difference would probably drop to 7 or 8 psi
(92--93 psi in the tire.) For it to reach a full 100 psi you would
theoretically have to wait forever. This is what is called an asymptotic
approach.


That is exactly the case you need to have the compressor at about 130psi
and then inflate the tire and check with gage. The set it a 100 psi and
wait will take a long time and they are two different materials, a tire,
and the compressor. I would not use a compressor would be better with a
floor pump, things can happen too fast with a compressor.

--
Deacon Mark Cleary
Epiphany Roman Catholic Church

wrote:
Just a question for something to think on, really doesn’t matter. If
there’s any reply, or if I decide I care, I’ll check the answer (for
time, not the reason) myself.

Say there’s a compressor setup @ 130+PSI providing house air. You’re
using a drop with a reg to inflate a bike tire, with the reg set to
100PSI. If you just add air until it seems to stop transferring, say
3-5 seconds, the bike tire will actually only be @ 85-90PSI. How long
do you figure you’d have to leave the air chuck on the inner tube
valve to get the tire pressure to match the pressure coming out of the
reg? Will it ever get quite that high? Why the delay? At first
thought it seems like 100PSI pushing out should equalize @ 100PSI @
the tire pretty damn quick, especially if the reg is holding the air
to 100 when there’s well over 130 coming into the reg. Experience
tells me this isn’t the case. Any thoughts or explanations on why, or
guesses as to how long you’d have to hold the chuck on the valve to
get them equal? It’ll get the tire to ~85 in a second, two tops.
Very, very quick.

My guess is that regulators regulate (the pressure of) flowing air
better than static pressure, inflation to an exact pressure isn't what
they're designed to do.

On Aug 8, 1:52*am, "Leo Lichtman" wrote:
"DanKMTB" wrote: * *They just

happen to be set @ 100, which is a decent pressure for my road bike.
I was more curious about the how & why,
^^^^^^^^^^^^^^^^^^^
Okay, here's how and why: *The flow rate between the supply (100 psi) and
the tire (somewhere between 0 and 100 psi) is proportional to the pressure
difference between the two. *As the tire pressure comes up, the flow rate
drops. *Evidently, after a few minutes, this difference has dropped to 15
psi, and the flow is so slow that you no longer hear it. *If you waited
several more minutes, the difference would probably drop to 7 or 8 psi
(92--93 psi in the tire.) *For it to reach a full 100 psi you would
theoretically have to wait forever. *This is what is called an asymptotic
approach.

Okay, seems reasonable, with the regulator set at 100 and assuming a
hose of length x (held steady) for the drop to occur in. Now, not
arguing, just asking:

What, assuming Dan could reset the regulator at work, should he set it
to in order to get a reasonably close approximation to 100psi (say
plus-minus 2 per cent) after holding the nozzle on for a reasonable
time (seconds, not minutes)?

I notice Andrew didn't even go there; he just straightaway started
talking about a gauge. But where would most roadies carry a gauge?
Even the lightweight gauge that comes in the box with Schwalbe tyres,
that looks suggestively like a tampon? Any better gauge must also
weigh more; my lightweight digital BBB gauge certainly has the mass of
several of those Schwalbe giveaways.

Andre Jute
Always with the questions! -- Granny

On Aug 7, 3:35*pm, " wrote:
Just a question for something to think on, really doesn’t matter. *If
there’s any reply, or if I decide I care, I’ll check the answer (for
time, not the reason) myself.

Say there’s a compressor setup @ 130+PSI providing house air. *You’re
using a drop with a reg to inflate a bike tire, with the reg set to
100PSI. *If you just add air until it seems to stop transferring, say
3-5 seconds, the bike tire will actually only be @ 85-90PSI. *How long
do you figure you’d have to leave the air chuck on the inner tube
valve to get the tire pressure to match the pressure coming out of the
reg? *Will it ever get quite that high? *Why the delay? *At first
thought it seems like 100PSI pushing out should equalize @ 100PSI @
the tire pretty damn quick, especially if the reg is holding the air
to 100 when there’s well over 130 coming into the reg. *Experience
tells me this isn’t the case. *Any thoughts or explanations on why, or
guesses as to how long you’d have to hold the chuck on the valve to
get them equal? *It’ll get the tire to ~85 in a second, two tops.
Very, very quick.

I don't know why that's happening. Some questions, then some
thoughts:

1) Are you sure you're holding the chuck on the valve for about three
to five seconds? IME that's an extremely long time for a compressor
to fill a bike tire. Most often it happens in less than a second.

2) How are you determining the actual pressure in the tire? IOW is
your pressure gage reliable? I've tested half a dozen and found up to
10 psi errors.

3) Have you tried this at different locations in the plant? Do they
all behave the same? Could there be a problem with your pressure
regulator?

4) How long is the hose between the regulator and your tire? Is
there any chance of other restrictions in that flow path?

Thoughts:

IIRC, regulators work by using a small orifice on the low pressure
side to feed back onto a piston supported by a spring, which closes
the main flow port from the HP side to the LP side. The spring holds
the main port open, the downstream (low) pressure closes it. ISTM a
plugged downstream port or a sticking piston would give more, not
less, pressure in the hose. But perhaps something weird is affecting
the regulator. Trying a different one might give a clue.

There's always restriction between the regulator and the inside of the
inner tube. The Schrader valve is a big restriction. So you could
have 100 psi in the hose while air's flowing, but less in the tube
itself. If the hose is pinched somewhere, or if there are other flow
restrictions, you could have less in the tube. But if you leave the
chuck on the tube for an honest three seconds, flow should definitely
stop, and the tube's pressure, the hose pressure and the regulator's
set point should all be very close.

- Frank Krygowski

On Aug 7, 8:52*pm, "Leo Lichtman" wrote:
"DanKMTB" wrote: * *They just

happen to be set @ 100, which is a decent pressure for my road bike.
I was more curious about the how & why,
^^^^^^^^^^^^^^^^^^^
Okay, here's how and why: *The flow rate between the supply (100 psi) and
the tire (somewhere between 0 and 100 psi) is proportional to the pressure
difference between the two. *As the tire pressure comes up, the flow rate
drops. *Evidently, after a few minutes, this difference has dropped to 15
psi, and the flow is so slow that you no longer hear it. *If you waited
several more minutes, the difference would probably drop to 7 or 8 psi
(92--93 psi in the tire.) *For it to reach a full 100 psi you would
theoretically have to wait forever. *This is what is called an asymptotic
approach.

Interesting reply. Thanks.