Tyre pressure

My favourite translation is in the instruction manual for the Bafang e-bike hub motor I purchased (Bafang is a well known Chinese hub motor manufacturer). The manual describes the power connector to the motor as a "standard beauty mouth". Makes sense actually. It is a standard connector. It is secure and watertight (beauty). One end plugs into the socket of the other (socket = opening = mouth).

It’s probably a translation error… I be seen it all the time troubleshooting foreign built equipment.

Your tire pressure gauge is measuring the difference in pressure between the inside of the tire and the outside of the tire, the gauge pressure.

P_abs = P_atm + P_guage

The absolute pressure inside the tire stays constant, assuming constant temperature. The absolute pressure is determined by the volume of the tire, the amount of air inside, and the temperature.

PV=nRT: (P=absolute pressure inside tire, V=volume of tire, T=absolute temperature, and nR= amount of air inside tire.)

Since the atmospheric pressure is less at altitude, you need to increase gauge pressure to have the correct absolute pressure in the tire.

I don’t think that’s right. Almost certainly the airline pressure indicator reads gauge pressure i.e. relative to the actual ambient atmospheric pressure. In that case, for a given tyre absolute pressure (which is constant, as you say) a gauge reads higher at altitude (lower pressure on the atmosphere side) and reducing it to the specified sea-level figure gives the correct (by my reasoning) pressure difference between tyre and atmosphere.

To take a specific example – assume the correct sea-level pressure is 30 psig, and the change due to elevation is 3 psi. If the pressure at sea-level is reduced to 27 psig, at altitude the gauge reads 30 psig. This gives the correct 30 psi difference between tyre and atmosphere. If nothing is done till at the altitude, the gauge there reads 33 psig. Reducing that to 30 psig gives the correct 30 psi difference.

I'm thinking we're saying the same thing.

My interpretation is that they are referring to driving at a higher altitude than your home. If you are on a mountain where the atmospheric pressure is 3 psi less than your home and you measure your tire pressure as 25 psi, you should inflate to 33 psi so that it will be 30 psi when at home. If you inflate to 30 psi on the mountain, it will be 27 psi at home.

It is the weight of the vehicle and absolute pressure that will determine the contact patch between the tire and the pavement. Atmospheric pressure is insignificant.

To insure absolute pressure is correct, guage pressure should be higher at altitude.

Huh?

Please disregard my previous post. After thinking it through I was obviously incorrect.

The performance of the tyre is dependent on its shape, which is a function of gauge pressure, not absolute pressure.

While this may be insignificant in England, I definatly notice the difference elevation makes when I was living in Mexico. 8,000 ft elevation where I lived vs sea leval when I drove to the coast, a one day trip.

G'day Gringogreg. What was the difference? Did the tyres feel softer?

Jim

...".... if tire inflation were set with a tire pressure gauge at sea level (where the atmospheric pressure of 14.7 pounds per square inch is used as ambient atmospheric pressure by the gauge), the same tire pressure gauge would indicate the pressure has increased at higher elevations where the ambient atmospheric pressure is lower. Those measured at the 5,000-foot level (where an atmospheric pressure of only 12.2 pounds per square inch is the ambient pressure) would indicate about 2-3 psi higher than at sea level. On the other hand, traveling from a high altitude location to sea level would result in an apparent loss of pressure of about 2-3 psi.

However, the differences indicated above assume that the tire pressures are measured at the same ambient temperatures. Since tire pressures change about 1 psi for every 10° Fahrenheit change in ambient temperature, the tire pressure measured in the relatively moderate climate typically experienced at sea level will change when exposed to the colder temperatures associated with higher elevations.

This means that in many cases differences in ambient temperature may come close to offsetting the differences due to the change in altitude. Depending on the length of their stay at different altitudes, drivers may want to simply set their cold tire pressures the morning after arriving at their destination, as well as reset them the morning after they return home."...

You must also take into account the difference in temperature...the higher you go the colder it gets...this is the opposite effect of altitude increase...not really worth worrying about unless you plan on staying for a while...

https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=167

Well, the gauge since it does measure relative to local atmospheric pressure, will actually show an increase with altitude, even if you do nothing. The rate won't be the exact of the following diagram, it will be a bit less and not all that predictable, since not all tires are created equal and a softer skeleton tire will show less increase.

The manufacturer possibly instructed by tire vendor just wants to avoid liability, since another thing that goes with altitude is that the ability of the thinner air to absorb heat (including that from tires), drops more dramatically than pressure. And there is no thermostat or radiator fan on tires. You will find the same logic on power electronics, temp sensitive instruments etc. S.M.

It is all pretty academic, show me any number of tyre pressure gauges that agree with each other when compared on a set pressure and anyway the gauges used are likely 5% or 10% tolerance.

I spent a lot of time calibrating gauges and instruments in industry and without doing the correct adjustments against a standard gauge or a dead weight tester there is no way of knowing how accurate the gauge you are using is likely to be.

Seems like Oriental lawyer naval gazing to me.

5% is good, but 10% is too much. Also - how can you be sure that the gauge you use to show you set pressure is correct?

Easy. Read its calibration certificate!

I agree.The type of pressure gauge also will affect the accuracy.A simple shirt pocket ink pen type relies on a spring ,so atmospheric pressure does not matter.The Bourdon tube type does respond to atmospheric pressure, and are available in PSIG or PSIA pressure,and the digital type and depend on how they are calibrated,either at the factory or in the field.Before digital and electronics any critical pressure measurement instrumentation was shop calibrated,using,as you say, a dead weigh tester or traceable standard reference.I cut my eye teeth on analog instruments and still prefer them in certain situations,such as explosive and hazardous atmospheres.My knowledge evolved to encompass digital,including several programming languages, but I am still amazed at the accuracy and durability available with properly calibrated analog instruments.

The original Panama controls were analog and functioned properly until the recent upgrade when widening the canal.

I cant think of any digital instruments that can function that long reliably.

"A simple shirt pocket ink pen type relies on a spring ,so atmospheric pressure does not matter."

Huh?

Well,perhaps a little bit,but not enough to matter,for practical purposes.

If you want to get precise,none of the OTC tire gauges are adequate.

They would have to include tire temperature,and atmospheric pressure,and require special handling and storage in a special container.

"Well, perhaps a little bit, but not enough to matter, for practical purposes"

The OP is only talking about changes in atmospheric pressure, and, the simple gauge will adjust its reading by exactly the same amount as the change in atmospheric pressure.

Several people have said that pressure guages measure pressure RELATIVE to ambient pressure. Is this so? What sort of guage is affected by the ambient pressure? The pocket type of tyre pressure guage is a sliding rod pulling against a spring. I am sure the external ( ambient ) pressure has no effect on that.

Weather balloons are inflated with very low pressure, then they inflate as they ascend. The pressure doesn't increase until the balloon starts to constrain the expanding gas. Extrapolating from that i would expect the tyres to expand as you went up a mountain, if they could. BUT does that attempt to expand mean that the pressure in the tyres is going up? Only the relative difference to the outside pressure increases. If it is important to decrease the expansion then the tyres should have air released to keep "rubber on the ground".

There’s gauge pressure, Absolute Pressure, atmospheric Pressure and Differential pressure.

What your referencing is gauge pressure.

Gauge pressure is zero-referenced against ambient air (or atmospheric) pressure, so gauge pressure readings include the pressure from the weight of the atmosphere.

Thank you. If a gauge has the ability to read negative ( compared to S.T.P. ) then as it ascends it will go into the negative, lets say -50KPa. But once applied to a rigid pressure vessel that was pumped up to 500 KPa at sea level i expect it will read 500KPa giving a differential pressure of 550 KPa. Extrapolating from this i expect a tire pressure gauge to still read the same pressure as it did at the bottom of the hill. Given that the tyre is a non-rigid pressure vessel i expect it will expand a little giving less tread in contact with the, icy?, ground. Thereby a need to REDUCE the pressure is required.

I think my reference to scuba diving (post #21) remains an aid to understanding. A bathysphere can descend to any watery depth and ascend again and the occupants do not need to decompress, because, as you point out, the pressure inside a rigid pressure vessel remains constant regardless of the outside pressure. But in an underwater research station that enables divers to enter and exit through a bottom opening (essentially a non-rigid pressure vessel), the air pressure inside is much greater than at the surface and therefore the occupants must go through a long decompression chamber session (pressure reduction) when they return to sea-level. Similarly, and in agreement with you, a car tire inside a descending and ascending bathysphere would not need any pressure adjustment, and also in agreement, a car tire inside the described underwater research station and properly inflated there, if transported to the surface would require pressure to be released to prevent it from exploding.

"There’s gauge pressure, Absolute Pressure, atmospheric Pressure and Differential pressure."

Surely, gauge pressure and differential pressure are the same thing?

Jimrat: why do you believe that:-

"The pocket type of tyre pressure guage is a sliding rod pulling against a spring. I am sure the external ( ambient ) pressure has no effect on that."

The forces acting on the rod are the spring, the tyre pressure at one end and atmospheric pressure at the other end?

"smack" Me hitting forehead with palm. It doesn't have a way to calibrate it so, i didn't think any harder about it.

Lower ambient pressure = further extension of the rod = higher reading = need to pump tyres up to a higher reading.

So i am now confused. The tyres are inflating more due to lower ambient pressure, lower temps are lowering the inflation, the gauge is reading higher, i don't have any nearby mountains to drive up to see for myself what actually happens.

Woe is me, i shall go to the sea. ;-)