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How much air goes from the tire to the pump when you plug it on the valve?

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How much air goes from the tire to the pump when you plug it on the valve?

Old 04-18-21, 03:07 PM
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eduskator
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How much air goes from the tire to the pump when you plug it on the valve?

I am trying to figure out how much air my tires lose between rides by using my floor pump's pressure gauge. I was always told it could be between 0 to 20PSI, but I have never verified.

The thing is that there is a loss of pressure inside the tire as soon as I connect the head on the valve (physics ) so the reading is not accurate. I am trying to figure out how much pressure is lost during the connection. Google wasn't helpful. Just in case, I know I can buy a gauge to read the pressure directly at the valve.

Has anyone ever wondered / found the answer or am I the only bored cyclist in the world who has time to think about these things? I know it will vary based on the length of the pump's air hose and surely a few other factors, but there should be a known average, right?
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Old 04-18-21, 03:16 PM
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A $15 Meiser air pressure gauge lets just a smidgeon of air out (or maybe a micro-smidgeon) when you use it and gives decent accuracy.
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Old 04-18-21, 03:39 PM
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Originally Posted by tyrion View Post
A $15 Meiser air pressure gauge lets just a smidgeon of air out (or maybe a micro-smidgeon) when you use it and gives decent accuracy.
As I said in my initial post, I know I can buy a gauge to read directly at the valve. That does not answer my question though.
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Old 04-18-21, 03:42 PM
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Too many variables I suspect.
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Old 04-18-21, 04:13 PM
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If you don't check the pressure, but just check by squeezing the tire, does the tire lose pressure over time?

I would just check the pressure loss over a longer time. For example, how much pressure does your tire lose over 2 months? Then divide it by the number of days.
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Old 04-18-21, 04:35 PM
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No one cares. It can be measured, based on the tire’s known volume and pressure using the common gas law and the size (length, internal diameter) of the hose, plus a variable for how sloppily the OP applied the pump head to the valve and botched that process, leading to excessive and unnecessary losses, plus a fudge factor of 3*(z-W/kg) as related to hydraulic vs mechanical disc brakes and sock height. This time chain lube has no effect whatsoever. Just this once. But, you *MUST* know the EXACT starting pressure inside the tire BEFORE beginning this measurement or the whole thing is off.

Last edited by AdkMtnMonster; 04-18-21 at 04:37 PM. Reason: Equation error correction
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Old 04-18-21, 04:39 PM
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I lose 0 psi from the tube when I connect my pump. 1st stroke of the pump shows the cracking pressure when the valve to the tube opens. With ultra thin butyl tubes I consistently lose about 10 psi overnight.
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Old 04-18-21, 07:24 PM
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Originally Posted by eduskator View Post
As I said in my initial post, I know I can buy a gauge to read directly at the valve. That does not answer my question though.
An accurate and inexpensive air gauge will answer all you questions, if you know the proper questions to ask.
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Old 04-18-21, 07:38 PM
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It's probably different every time you put the pump head on the valve. I'll bet you never do it exactly the same twice. Does it matter? Not a bit. Are you overthinking this? Definitely.
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Old 04-18-21, 11:25 PM
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Why don't you just measure the pressure 10 times in a row and see how much the readings drop?
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Old 04-18-21, 11:37 PM
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Originally Posted by tyrion View Post
An accurate and inexpensive air gauge will answer all you questions, if you know the proper questions to ask.
I bought an air gauge, at your advice. I asked it what kinds of questions I'm supposed to be asking, no reply. This is worse than when you sold me those magic beans. 😡
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Old 04-19-21, 12:05 AM
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Relatively easy calculation using ideal gas law. P1*V1 = P2*V2, where P1 is your tire pressure and V1 is your tire volume, V2 is your combined volume of tire plus gage, and solve for P2. Don't worry about the units as long as you are consistent on both sides of the equation.
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Old 04-19-21, 02:55 AM
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Originally Posted by eduskator View Post
I am trying to figure out how much air my tires lose between rides by using my floor pump's pressure gauge. I was always told it could be between 0 to 20PSI, but I have never verified.

The thing is that there is a loss of pressure inside the tire as soon as I connect the head on the valve (physics ) so the reading is not accurate. I am trying to figure out how much pressure is lost during the connection. Google wasn't helpful. Just in case, I know I can buy a gauge to read the pressure directly at the valve.

Has anyone ever wondered / found the answer or am I the only bored cyclist in the world who has time to think about these things? I know it will vary based on the length of the pump's air hose and surely a few other factors, but there should be a known average, right?
Assuming you air up to the correct pressure before each new ride why do you care?
It’s not an interesting topic.
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Old 04-19-21, 07:32 AM
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Originally Posted by Seattle Forrest View Post
I bought an air gauge, at your advice. I asked it what kinds of questions I'm supposed to be asking, no reply. This is worse than when you sold me those magic beans. 😡
I was just wondering since when people started to give their two cents instead of trying to answer the damn question.
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Old 04-19-21, 07:46 AM
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If a presta valve is used, the answer is none. When a tire is inflated, the presta valve seals as soon as you stop pumping. The only way to lower the pressure in the tire is to disconnect the pump head and push on the top nut with your finger. The pump head should never contact the top nut with rubber seal at the end. I've forgotten to tighten the nut at the top and ridden 50 miles with no pressure loss. You'll also find that after the nut is loosened, the seal needs to be broken by pushing on the nut to let a little air out. If that's not done, it can take a huge pressure to break the seal.
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Old 04-19-21, 08:26 AM
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Originally Posted by DaveSSS View Post
If a presta valve is used, the answer is none. When a tire is inflated, the presta valve seals as soon as you stop pumping. The only way to lower the pressure in the tire is to disconnect the pump head and push on the top nut with your finger. The pump head should never contact the top nut with rubber seal at the end. I've forgotten to tighten the nut at the top and ridden 50 miles with no pressure loss. You'll also find that after the nut is loosened, the seal needs to be broken by pushing on the nut to let a little air out. If that's not done, it can take a huge pressure to break the seal.
I am not sure to fully get what you are saying. As soon as you plug the pump's head on the valve, there is a loss of pressure inside the tire (air leaving the tire and going into the pump's hose), it's physics. When you connect the pump on the valve, it opens the ''circuit'' and the valve stays open until it's disconnected.

If you are referring to the air loss when disconnecting the pump's head after inflating the tire, then yes, I'd say there is no loss of pressure (or barely) since the valve closes quickly due to the internal pressure being high.
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Old 04-19-21, 08:33 AM
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Originally Posted by DaveSSS View Post
The pump head should never contact the top nut with rubber seal at the end.
Plenty of them do and, if the OP is getting a pressure reading by simply attaching his pump head, his pump is one that certainly does.
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Old 04-19-21, 08:33 AM
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Originally Posted by DaveSSS View Post
If a presta valve is used, the answer is none. When a tire is inflated, the presta valve seals as soon as you stop pumping. The only way to lower the pressure in the tire is to disconnect the pump head and push on the top nut with your finger. The pump head should never contact the top nut with rubber seal at the end. I've forgotten to tighten the nut at the top and ridden 50 miles with no pressure loss. You'll also find that after the nut is loosened, the seal needs to be broken by pushing on the nut to let a little air out. If that's not done, it can take a huge pressure to break the seal.
Originally Posted by eduskator View Post
I am not sure to fully get what you are saying. As soon as you plug the pump's head on the valve, there is a loss of pressure inside the tire (air leaving the tire and going into the pump's hose), it's physics. When you connect the pump on the valve, it opens the ''circuit'' and the valve stays open until it's disconnected.

If you are referring to the air loss when disconnecting the pump's head after inflating the tire, then yes, I'd say there is no loss of pressure (or barely) since the valve closes quickly due to the internal pressure being high.
Some pump heads do indeed depress the presta valve and cause some air loss when you put on the chuck. Some do not. I prefer the ones that do not.

Obviously the OP has the first type.

How much air is lost in this operation will depend on the pump's air chamber and hose volume and will depend not only on the pump but also on the size/volume of the tire.

It's not worth worrying about the details.
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Old 04-19-21, 08:39 AM
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So I woke up this morning with my head a little more rested than yesterday (gotta love Satur-Wine-Days!) and I figured out that I could simply pump the tire up to X pressure, disconnect it and subtract the new reading to get an approximate air loss.

At 70psi, I reconnected it and got 55psi. Air loss is more or less 15psi.

Last edited by eduskator; 04-19-21 at 08:46 AM.
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Old 04-19-21, 08:44 AM
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Assuming the pump is at the bottom of it's stroke, then you are just filling the hose, so look at the inside diameter of the hose and the length and figure out the volume. Once you figured out the total volume, you compare that to the volume of the tire/tube and figure out how much the pressure would change based on the adding the additional volume. I suspect it is minimal.
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Old 04-19-21, 08:48 AM
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Originally Posted by DaveSSS View Post
If a presta valve is used, the answer is none. When a tire is inflated, the presta valve seals as soon as you stop pumping.
If that were true then the gauge wouldn't read tire pressure, it would only read the pressure in the pump.
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Old 04-19-21, 09:35 AM
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Originally Posted by big john View Post
If that were true then the gauge wouldn't read tire pressure, it would only read the pressure in the pump.
What makes you think it doesn't? And how big a difference do you think there is?
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Old 04-19-21, 09:54 AM
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Originally Posted by asgelle View Post
What makes you think it doesn't? And how big a difference do you think there is?
It doesn't what, read the pressure in the tire? He said the valve closed as soon as you stopped pumping, therefore you couldn't get a reading until you applied pressure to open the valve.
As to your other point, I don't know how much difference there would be, maybe very little but I would have to assume at least a little higher in the tire/tube to force the valve closed. This is assuming you subscribe to the theory that the pump head does not depress the valve. If you've ever heard a lot of air escaping as you have a tough time removing the pump head then you know the valve is being depressed, at least in that instance.

As with the original question, all of this is actually moot.
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Old 04-19-21, 10:43 AM
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Originally Posted by big john View Post
It doesn't what, read the pressure in the tire? He said the valve closed as soon as you stopped pumping, therefore you couldn't get a reading until you applied pressure to open the valve.
As to your other point, I don't know how much difference there would be, maybe very little but I would have to assume at least a little higher in the tire/tube to force the valve closed. This is assuming you subscribe to the theory that the pump head does not depress the valve. If you've ever heard a lot of air escaping as you have a tough time removing the pump head then you know the valve is being depressed, at least in that instance.

As with the original question, all of this is actually moot.
Wrong. As someone else ( DaveSSS ) posted earlier as soon as you stop pumping the pressure in the tube forces the valve closed. You don't lose any pressure from a presta valve when removing the pump head. The air you hear is coming from the pump, not the tube. It's no 'theory'...the pump head doesn't depress the valve, air pressure from the pump does. You continue to get a pressure reading when you stop pumping because there is still positive pressure in the pump/hose. Oddly enough it's the same as what the pressure is in the tube.

Last edited by cxwrench; 04-19-21 at 10:46 AM.
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Old 04-19-21, 10:56 AM
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Originally Posted by cxwrench View Post
Wrong. As someone else ( DaveSSS ) posted earlier as soon as you stop pumping the pressure in the tube forces the valve closed. You don't lose any pressure from a presta valve when removing the pump head. The air you hear is coming from the pump, not the tube. It's no 'theory'...the pump head doesn't depress the valve, air pressure from the pump does. You continue to get a pressure reading when you stop pumping because there is still positive pressure in the pump/hose. Oddly enough it's the same as what the pressure is in the tube.
So you're saying you get no reading on the gauge when you first install the pump head? I have two floor pumps with different heads and they both show pressure before pumping.
Sometimes when trying to remove the pump head after inflating the tire if it's in an awkward position and I twist it around air will escape from the tire. Sure, if it is removed straight off the only air released is from the hose but if you fumble around with it the tire will lose air.
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