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Old 03-05-2007, 05:33 AM   #1
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I've just started using my Pressure Pro system. I know the main reason for this is to spot a drop in pressure. However, as you travel, air pressure increases due to the heat. Is there any general rule that tells you how high you should expect the pressure to rise? With SmartTire, you get pressure and temperature. As I understand it, high temps can indicate a serious problem too. I'm wondering if my Pressure Pro reads too high, I may have a heat problems. I just really don't know what constitutes too high in terms of a difference from cold to hot psi.
Thanks
Ron
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Old 03-05-2007, 05:33 AM   #2
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I've just started using my Pressure Pro system. I know the main reason for this is to spot a drop in pressure. However, as you travel, air pressure increases due to the heat. Is there any general rule that tells you how high you should expect the pressure to rise? With SmartTire, you get pressure and temperature. As I understand it, high temps can indicate a serious problem too. I'm wondering if my Pressure Pro reads too high, I may have a heat problems. I just really don't know what constitutes too high in terms of a difference from cold to hot psi.
Thanks
Ron
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Old 03-05-2007, 06:56 AM   #3
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Ron, I have the SmartTire set up. The temperature generally runs as high as 130 F. and pressures increase around 15% or more.

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Old 03-09-2007, 11:26 AM   #4
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Ambient temperature has an effect on tire pressure. Cold air will lower the pressure and heat will increase it. Warm tires (driven 20 minutes or so) will have a higher internal temperature and pressure than when cold. This can give a false sense of security if you check tire pressure while on a trip. Cold pressure is the best way as it will give you the best indication of the true inflation pressure before you heat up the tire and get a misleading reading.

If your tire requires 30psi cold and you measure 25psi cold you are underinflated by 5psi. If you take a measurement after driving for over 20 minutes the pressure might be up to 30psi and you would think it is fine. Wrong! The tire is still underpressurized and this creates too much sidewall flex which will eventually damage the tire structure.

Measure your tires cold and always bring them up to the minimum cold pressure. Being a little too high is better than being too low. The reason is that a tire weakened by miles of low pressure flexing will have internal damage that once stressed under higher pressure can fail. A good tire can withstand the moderately higher pressures without failure.

The Pressure Pro is a good option as are other tire pressure monitoring devices becasue they will tell you if a tire is underinflated sooner than by manually checking it. Also, an automatic system is far better than a manual gage since it will not forget to check the tires.
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Old 03-10-2007, 05:17 AM   #5
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The formula for pressure increase in a constant volume container (tires are pretty close to a constant volume)is:

P1/T1= P2/T2

Where P1 is the initial pressure, T1 is the initial temperature.
Where P2 is the new pressure, T2 is the new temperature.
The temp must be in degrees kelvin.

Or P2 = P1 X T2/T1

For P1 = 85 psi, T1 = 294.26' kelvin, and T2 = 305.37' kelvin. (T1 = 70'F, T2 = 90'F)

P2 = 85 X 305.37/294.26 P2 = 85 X 1.038 = 88.21

So for a 20' F increase in temp the pressure should increase by about 3 psi.

I have checked this with a infrared thermometer and a pressure gauge and the results are very close to the expected numbers.

The Pressure Pro numbers are not quite as accurate. I suspect the sensors may be temperature sensitive to some degree.
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Old 03-17-2007, 03:13 PM   #6
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Hey Rappar, If you have checked the weight on each of your 4 corners on the motor home and have adjusted the pressure to that suggested by the tire manufacture for that much weight, I would say that you should not worry about it as the tire manufactures have taken into consideration that the tires will build up pressure as you travel. Good Motoring Gary.
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Old 03-17-2007, 04:22 PM   #7
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A large motorhome tire with normally high pressures (75-125 psi cold) will likely increase 10-15 psi on a hot day. That represents a large increase in tire internal temperature but is not uncommon when traveling at interstate speeds on a hot afternoon.

The rule of thumb (which is derived from the formula that Clay posted) is that tire pressure will increase or decrease about 2% (not 2 psi) for every 10 degrees Fahrenheit of temperature change. Thus a tire at 75 psi will gain about 1.5 psi if its temperature increases 10 degrees F.

You can't tell much from a tire's absolute pressure or temperature, but if one tire gets noticeably hotter than the others then it's time for a closer inspection. Especially if they are tires that are normally at the same pressure and on the same axle. So if your Pressure Pro shows that one tire is increasing in pressure significantly faster than the others, stop and check it out.
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Old 03-20-2007, 12:41 PM   #8
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The only way I can get the sensors for the Pressue Pro to fit my front tires is to add 90 degree extensions. The sensors are about 1 inche in diameter and will stick out about 3/4 inch above the valve stem so I need some room for them and don't have that now with the wheel covers on the front. I am thinking about going with the extensions but only if I can find some really good stainless ones. Any ideas?
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