Wireless Tire Pressure Monitoring System (TPMS): Anyone have it?

[carusoam]

21 minutes ago, jaylw314 said:

It stays the same --  it's the outside pressure that changes  

I think I fell for my own trick question....

 

See if I can stay on target....

The colder the air in the tire gets, the more dense it becomes, lowering the pressure inside the tire...

The tire will see its highest pressure when it is at its warmest... on the ground, after use...

1atm is 14.7psi... something to consider would be the pressure differential at the ground vs, in flight...

at a super high altitude where the pressure becomes 0psi or 0atm... that would be like boosting the tire pressure an extra 14.7atm...

 

That little cap would probably feel some small extra force to hold it in place and keep it sealed...  but... if you can get it to work on the ground for a long time... it won’t be much different than its automotive cousin...

 

Let us know if you try one....

I fear the flat tire... a lot.  Never had one before...

Best regards,

-a-

[BDPetersen]

Yes, the football pressure sensor idea runs into the obstacle of the inner tube. Getting it in there is the problem. Too late for me to qualify as a boy genius, I guess.

[tigers2007]

When folks fill with nitrogen, do they vacuum out the room air from the tube in order to completely fill with nitrogen?


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[EricJ]

9 hours ago, jaylw314 said:

It stays the same --  it's the outside pressure that changes  

The tire pressure may change because the temperature changes.   Since it is getting colder, I'd expect the pressure to drop a bit as you go up.

2 hours ago, BDPetersen said:

Yes, the football pressure sensor idea runs into the obstacle of the inner tube. Getting it in there is the problem. Too late for me to qualify as a boy genius, I guess.

I'm curious about this for anything with a tube, since it would have to measure absolute pressure if it doesn't have an external reference to ambient pressure.    Automotive TPMS monitors are used with tubeless tires and the portion of the monitor (valve stem) exposed to the ambient pressure can provide the reference.    I'm not sure how it would be done in a tube except to use a calibrated absolute pressure monitor and convert everything based on current ambient static pressure conditions.   Could be done, but kinda sounds like a pain.

1 hour ago, tigers2007 said:

When folks fill with nitrogen, do they vacuum out the room air from the tube in order to completely fill with nitrogen?

In racing applications the wheels have two valve stems, one to fill and the other to evacuate the air until the nitrogen fill is complete.   Some pro teams do this to get the last little eek out of performance, mostly so that the fill pressure stays a little more stable over the heat cycle of the tire.    I don't know anybody (that I've ever raced with, anyway, including people who bring the big rigs to the track) who bother with nitrogen filling.   It usually makes very little difference, unless you're something extreme like an airliner or an SR-71.

[Hank]

The tire shop I patronize fills my truck tires with "nitrogen." At home, I top off when needed with standard 78% Nitrogen from my garage compressor. My Mooney tires always are 78% Nitrogen. I really don't care which is used, as long as there is no additional charge . . . .

[jaylw314]

AFAIK, nitrogen is only useful inasmuch as it is non-reactive.  The difference in expansion/pressure between air and pure nitrogen should be negligible--all gases obey the ideal gas law within a couple percent, and the ideal gas law actually becomes more accurate as temp increases.

For racing applications, this makes some modest sense, since the tires/wheels can get super hot, and I suppose pure nitrogen may prevent oxidation of the wheel and inside of the tire.  On the other hand, those get changed out so frequently it's hard to imagine this could help a lot.

[Planegary]

I always thought that the nitrogen didn’t leak thru the tires as quickly as regular air so it was just a maintenance convenience issue. That was my understanding of the reason for nitrogen

[jaylw314]

10 minutes ago, Planegary said:

I always thought that the nitrogen didn’t leak thru the tires as quickly as regular air so it was just a maintenance convenience issue. That was my understanding of the reason for nitrogen

"People" suggest oxygen leaks out 3-4 times more quickly through rubber than nitrogen, but I've not seen good data out there on it.  Consumer Reports suggested after 12 months of a tire sitting inflated (not a realistic test), the difference in pressure loss was something like 3 psi vs 2 psi.  Even assuming it's true, think about this:

• Fill with air
• You lose some of the oxygen over time
• Check the pressure--it's low, add more air.  Now you have >80% nitrogen in the tire.

 

[EricJ]

3 hours ago, Planegary said:

I always thought that the nitrogen didn’t leak thru the tires as quickly as regular air so it was just a maintenance convenience issue. That was my understanding of the reason for nitrogen

The best explanations I've heard from people I had the most confidence might actually have some idea were that it's mostly just that it's drier, and that if there *is* moisture from whatever source (tire mounting lube, whatever), that it will be less likely to affect pressure changes with pure N2 than air.   I think the affect is a small one regardless.

Many years ago there was a scandal in F1 where one of the race teams was accused of using espionage to find out that a competitor was using CO2 instead of N2 and that it gave them an advantage of some kind.   I never did hear anything definitive on it, but some of the scuttlebutt is still around:

https://www.apexspeed.com/forums/showthread.php?25913-CO2-tire-gas-re-McLaren

 

[jaylw314]

50 minutes ago, EricJ said:

The best explanations I've heard from people I had the most confidence might actually have some idea were that it's mostly just that it's drier, and that if there *is* moisture from whatever source (tire mounting lube, whatever), that it will be less likely to affect pressure changes with pure N2 than air.   I think the affect is a small one regardless.

Many years ago there was a scandal in F1 where one of the race teams was accused of using espionage to find out that a competitor was using CO2 instead of N2 and that it gave them an advantage of some kind.   I never did hear anything definitive on it, but some of the scuttlebutt is still around:

https://www.apexspeed.com/forums/showthread.php?25913-CO2-tire-gas-re-McLaren

 

I suspect they were hoping since an equivalent volume of CO2 weighs 60% more than N2, that it could absorb or transfer 60% more heat.  Totally bunk science if you ask me, but hopefully there are some more engineering types who could answer this.

I get the moisture explanation, and that at least makes sense, but again, it's hard to imagine this being an issue except in racing where the tire temps are routinely near or above boiling temp.  

Edited May 2, 2019 by jaylw314

[carusoam]

N  14g/mol

O  16g/mol

C  12g/mol

 

Add up all the atoms in the molecule... to get their total weight... N2 would be 28g

Why not go with He?  Consider H2, Hydrogen is an incredibly tiny atom...

The lighter molecules also are much tinier... if leaking through a porous rubber membrane is a problem... He will find its way out quickly... memory of latex balloons and helium... they would last a day... metalized Mylar balloons... go on for weeks...

So I went out to the hangar today... Thanks to this thread... and measured the air pressure in my tires... it’s been about six months... I was expecting to need to add some...

I’ll check again when the plane goes in for maintenance...

Thanks for sharing all the details... 

Best regards,

-a-

 

[mikefox]

I think I'll avoid H₂ - think "Hindenburg"!

[ilovecornfields]

I use N2O in my tires. Good for at least 5 knots in cruise.

[Mufflerbearing]

On our racing tires, we start at 18 lbs but within two laps (2.5 miles a lap), they grow to 25 lbs where we want them.  During a yellow flag, they cool down a lot and loose pressure again.  There is a saying that yellow flags creates more yellow flags.  Some teams use the nitrogen to keep the tires from growing in pressure.  Does it really work?  I have heard from these teams that it is good for a few pounds.  Just not worth it for our cars.

[skydvrboy]

On 5/4/2019 at 12:27 PM, ilovecornfields said:

I use N2O in my tires. Good for at least 5 knots in cruise.

You should try it in your cannula.  It'd add 1500 ft/min to your climb rate.  You will be amazed how fast you get high!

[carusoam]

MB,

What type of racing is that?

Sounds like drag racing where a good tire patch improves grip for launch and acceleration...

 

25psi sounds kind of low for a road course where the sidewalls would be flexing more...

Banked ovals might be good for a well distributed tire patch across the width of the tire...

 

PP thoughts only, no first hand race car experience...

Best regards,

-a-

[EricJ]

58 minutes ago, carusoam said:

25psi sounds kind of low for a road course where the sidewalls would be flexing more...

Depending on the weight of the car, the specific tire type, etc., that's actually not at all unusual for a cold (starting) pressure for road racing.    It's pretty typical to gain about 8psi or more with temperature and hot pressures in the mid-30s are common.

Guidelines for one type of common race tire:   https://www.tirerack.com/images/pdf/warranty/HO0914.pdf