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Old 03-31-2013, 01:22 PM   #43
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If you /we want to be accurate then we should all be using nitrogen.
Here is a discussion about that. So the part affected is the O2 which is about 18% of Air.

More research show that 1psi/10F is a better approximation, although Longacre says .8psi/10F. One of the discussions mentioned that radials were better at maintaining contant volume than bias ply. Are racing slicks bias or radial? That might account for some of the difference.

In any case, using the temperature compensation, I can avoid getting up before the sun and running my compressor when other are sleeping.
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Old 03-31-2013, 01:26 PM   #44
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Ok. I've read all of the responses as of this time and I'm confused. In a perfect world, tire pressure would be exact. Physical conditions cause changes in tire pressure which means you are only accurate at the time you measure tire pressure. Do tire manufacturers allow for pressure fluctuation in their tire design? I am reading in the instruction manual that came with my new Pressure Pro monitoring system that the device only warns me when there is a 12.5% drop in pressure and then a 25% drop. Is a 12% drop significant while driving or is it built into the tire to allow for this? When a tire has low pressure and heats because of this, does the gain in air pressure from heat expansion compensate for the low pressure? A can of worms! I plan to use an American Made tire gauge (which MAY be off a pound or two) and get my tires to manufacturer specs (as defined by my fairly accurate American Made gauge) and use my TPMS hoping that it is as accurate as it boasts and if I have a blowout due to tire pressure I am blaming KARMA! Never liked him anyway.
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Old 03-31-2013, 01:46 PM   #45
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OldRXRer,
Here's an article on guages- it's accepted that a couple of pounds one way or another won't hurt although once it gets this way I correct it. Certainly 12% is dangerous and if you drive very long like that your tire will either fail or become compromised, particularly within the sidewall. You can also see other psts by Roger Marble by looking in the right hand margin. There's lots of data there-

Gauge accuracy


If you are reading this blog you know that along with others, I constantly complain that people do not always maintain their tire pressure.

One part of the process is using a sufficiently accurate gauge. I consider +/- 2 to 3 percent or +/- 1psi when measuring passenger tires to be sufficiently accurate.

Over the past few years I have checked more than 40 different gauges and have observed a 14 to 18 percent failure rate in two groups of 20 or so gauges.

I have two digital "Master " gauges as seen to the right, which read to 0.5psi increment. Both have been checked against ISO certified laboratory gauges and one reads 0.5 psi high and the other 0.5 low. I compare these gauges to each other to confirm they have not gone out of calibration before I conduct my gauge test. I figure the chance of both going out of calibration the same amount and same direction at the same time is very unlikely.

This year while at the Gypsy Journal Rally in Celina, OH, at my Tire Basics seminar, I offered to check gauges. Here are the results.

Actual Indicated Type Gauge Error
82.0 81 Dial -1%
70.0 69 Dial -1%
77.0 75 Stick -2.6%
76.0 81 Dial +6.6% Fail
77.0 92 Short Stick +19.5% Fail
74.5 81 Long Stick +8.2% Fail
79.5 78 Long Stick -1.8%
77.5 78 Long Stick +0.6%
68.5 72 Long Stick +5.1% Fail
68.0 68 Dial =
68.5 74 Long Stick +8%

As you can see we observed a 36 percent failure rate with all the "fail" gauges giving a high reading which could result in the operator unknowingly operating their tires in an under-inflated state yet thinking the tires were properly inflated. Given the general tendency for RVs to be overloaded in the first place or to be operated right at the limit of the load capability for the specified inflation this is definitely not the better way for a gauge to be wrong.

Having your own accurate "master gauge" is not unreasonable given the low cost of good quality digital gauges. You don't even need a special dual foot master gauge.

Do your own "calibration check" by using the "master single head gauge to check a front tire. Then use your "truck" dual foot gauge (as seen on the left in the top picture above ) to get a reading on the same tire to confirm the reading. If the readings match you can be confident all your readings will be sufficiently accurate when you use your dual foot gauge on the rear tires.

I would suggest you get a digital gauge and then visit the tire store from one of the major tire companies and ask if they would check your digital against their master certified gauge. You could also ask when their master was last calibrated. A competent certification includes affixing a dated calibration sticker to a master gauge. If they cant tell you when it was calibrated just say "thank you" and go to a more competent tire dealer.

Finally don't just throw your master gauge in the bottom of your tool box. I suggest you keep it in a box of some type. Even a Tupperwear type container can protect your master gauge.

Also don't forget to always use a cap or screw on TPM sensor to keep dirt out of the valve core area of your valves. If you don't understand why you need a cap on the valve I suggest you subscribe to this blog as I will be posting a story showing exactly why you should always protect the valve core.
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Old 04-03-2013, 03:06 PM   #46
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I see they have one that will temperature compensate. Do you have any experience with that one?
Pressure is pressure no matter the temperature unless you are trying to estimate what the pressure will be at some other condition or you are trying to run a lab experiment that calls for "standard temperature & pressure" conditions.

Tire pressures are always to be measured at ambient and you do not need to do any adjusting for temperature, or barometric pressure or altitude or direction of the wind or even the color of the Rv
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Old 04-03-2013, 03:09 PM   #47
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If you /we want to be accurate then we should all be using nitrogen.
Sorry but 100 psi of N2 is the same pressure as 100 psi of air or even 100 psi of Argon. I suppose you could even try and use Mercury at 100 psi.
Pressure is Force per unit area.
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Old 04-04-2013, 06:08 AM   #48
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Quote:
Originally Posted by Tireman9 View Post
Sorry but 100 psi of N2 is the same pressure as 100 psi of air or even 100 psi of Argon. I suppose you could even try and use Mercury at 100 psi.
Pressure is Force per unit area.

True pressure is pressure, the benefit of Nitrogen is more uniform pressure tire to tire as tire temp increases, due to less moisture. Also less leak down over time due to larger molecule size.


Here is a decent article, I don't personally use it because it is a PITA to maintain if you don't want to add air when needed. http://www.tirerack.com/tires/tirete...jsp?techid=191
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Old 04-04-2013, 11:35 AM   #49
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Originally Posted by OldRXRer View Post
Ok. I've read all of the responses as of this time and I'm confused. In a perfect world, tire pressure would be exact. Physical conditions cause changes in tire pressure which means you are only accurate at the time you measure tire pressure.
Do tire manufacturers allow for pressure fluctuation in their tire design? I am reading in the instruction manual that came with my new Pressure Pro monitoring system that the device only warns me when there is a 12.5% drop in pressure and then a 25% drop. Is a 12% drop significant while driving or is it built into the tire to allow for this? When a tire has low pressure and heats because of this, does the gain in air pressure from heat expansion compensate for the low pressure? A can of worms! I plan to use an American Made tire gauge (which MAY be off a pound or two) and get my tires to manufacturer specs (as defined by my fairly accurate American Made gauge) and use my TPMS hoping that it is as accurate as it boasts and if I have a blowout due to tire pressure I am blaming KARMA! Never liked him anyway.
Tire manufacturers know that tires heat up when used and with that temperature increase there is a pressure increase. The Load Inflation tables are based on Ambient temperature. This means the tire is the same temperature as outside air.
Yes a 12% drop in pressure is significant. In fact a 20% drop and tire companies consider the tire "flat" and should NOT be driven on.
The heat increase to the air in the tire due to running low does NOT result in enough pressure increase to offset the loss of load capacity of the tire.
The inflation recommended by the RV MFG and shown on your Placard is based on an assumption of how much stuff you carry, like how many bowling balls are in your collection. Since I have more balls than you my coach is heavier so would need more air than you. That's why is is strongly suggested you learn the actual tire load so you know the minimum inflation you should use when setting your tire pressure.
You can learn about Hot Pressure HERE and in the links on that post.

You have seen the results of my gauge tests and my solution to knowing your gauge accuracy. Being off by a couple PSI is not an issue especially if you also follow the recommendation to run +5 psi above the minimum needed to carry the load.
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Old 04-04-2013, 11:50 AM   #50
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Originally Posted by Tireman9 View Post

Tire manufacturers know that tires heat up when used and with that temperature increase there is a pressure increase. The Load Inflation tables are based on Ambient temperature. This means the tire is the same temperature as outside air.
Yes a 12% drop in pressure is significant. In fact a 20% drop and tire companies consider the tire "flat" and should NOT be driven on.
The heat increase to the air in the tire due to running low does NOT result in enough pressure increase to offset the loss of load capacity of the tire.
The inflation recommended by the RV MFG and shown on your Placard is based on an assumption of how much stuff you carry, like how many bowling balls are in your collection. Since I have more balls than you my coach is heavier so would need more air than you. That's why is is strongly suggested you learn the actual tire load so you know the minimum inflation you should use when setting your tire pressure.
You can learn about Hot Pressure HERE and in the links on that post.

You have seen the results of my gauge tests and my solution to knowing your gauge accuracy. Being off by a couple PSI is not an issue especially if you also follow the recommendation to run +5 psi above the minimum needed to carry the load.
This is really good information presented clearly which helps me to understand my obligations in tire care and maintenance of my tires. I'm a bit more mechanically inclined than a rubber ducky, but knowledge in king. I'm sure I have what I need to be 'highway safe' now. Thanks for sharing your knowledge and expertise with me.
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Old 04-04-2013, 01:59 PM   #51
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Tireman, a question for you. First I am in agreement that 100 psi of standard compressor air is the same as 100 psi of N2 BUT is not the expansion rate due to temperature increase different from compressed air than N2? if you initially filled each tire in the morning when the ambient was lets say 60*F with 100 psi of compressed air and a 100 psi of N2 then drove until the tires both heated up to 115*F now are the pressures going to both be the same? I say they will not but I maybe wrong. If not then we should have Load Inflation tables for regular air from the compressor and a Load Inflation Table iwhen using nitrogen. I don't believe nitrogen expands at the same rate as compressed air so therefore at 115*F the tire using nitrogen would be underinflated.
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Old 04-04-2013, 09:00 PM   #52
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Tireman, a question for you. First I am in agreement that 100 psi of standard compressor air is the same as 100 psi of N2 BUT is not the expansion rate due to temperature increase different from compressed air than N2? if you initially filled each tire in the morning when the ambient was lets say 60*F with 100 psi of compressed air and a 100 psi of N2 then drove until the tires both heated up to 115*F now are the pressures going to both be the same? I say they will not but I maybe wrong. If not then we should have Load Inflation tables for regular air from the compressor and a Load Inflation Table iwhen using nitrogen. I don't believe nitrogen expands at the same rate as compressed air so therefore at 115*F the tire using nitrogen would be underinflated.
Mike First the simple answer. You would be hard pressed to measure the difference in pressure between Hot Air vs Hot N2 if both had same moisture content to start with. There is a mathematical difference in expansion of the dry gas but it is not meaningful. Now having said that if you inflate a tire with real wet air (shop doesn't drain their compressor tank or have a functioning dyer then yes the wet air increases more than the dry N2.
I have a couple of posts on N2 and hot inflation and even how to build your own air dryer.

Now to mess with your mind. Increased Rolling Resistance results in decreased fuel economy. Lower pressure means increased Rolling Resistance. So if dry N2 doesn't increase in pressure as much as air then the rolling resistance of N2 inflated tire would be greater than the rolling resistance of the tire with higher pressure air.

Now do go an inflate with water as there are down sides. I point this out just ot show that the system is a lot more complex than most people think and clearly more complex than someone selling "magic gas".

Read my blog posts and you will learn more.
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Old 04-04-2013, 09:01 PM   #53
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This is really good information presented clearly which helps me to understand my obligations in tire care and maintenance of my tires. I'm a bit more mechanically inclined than a rubber ducky, but knowledge in king. I'm sure I have what I need to be 'highway safe' now. Thanks for sharing your knowledge and expertise with me.

You are welcome. Hope you enjoy my blog and find more stuff to learn there.
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Old 06-18-2013, 11:06 AM   #54
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Thumbs up tire gauge

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Originally Posted by Mike Canter View Post
This is the absolute Rolls Royce of digital truck gauges. It cost $132 but it works and comes in a beutiful padded aluminum case and goes from 5 to 125 psi. We use Longacre tire gauge on our race car as do a lot of drag and NASCAR racers. They will put the dually head on it if you call them and ask them. Call Longacre Racing at 800.423.3110 and make sure you ask for Missy so you get the correct setup. Like I said this is expensive but is well worth. Can be used accurately at 30 psi for your cars or 110 psi for the motorhome. This will not break after one trip.

Click here for the specs: Longacre Racing - Online Catalog: Electronic Wheel Scales, Gauges, Pyrometers, Chassis Setup and More!
Mike thanks for posting this. I called misty and she understood what a dually head was bc of you. I am a first time owner of a 40 foor class a diesel so i am a novice but want to make sure i have a dood gauge for the tire pressures. I tlod misty about this post and she was thrilled. Thank you for your service to the country
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Old 06-18-2013, 03:29 PM   #55
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Here is one to avoid. Harbor Freight "bargain" for $6.77 back on '02. Used it once or should I say attempted to use it. Leaks air around the trigger mechanism, hose connection, and gauge connection. With all those leaks I couldn't tell you if the gauge goes over 20psi even though I had 130psi compressor input.

The saying "you get what you pay for" is not always correct. In this case I didn't get close to the cost in returned value.
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Old 06-18-2013, 03:52 PM   #56
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I would not trade my Milton for any other manufacturer.

Made in USA, very robust.

Not digital but you can take it to any truck tire shop and compare it to their calibrated gauge.

BTW, most of the truck shops will be using the Milton....reasonably priced as well.

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