As people get ready to start traveling again, some are discovering that their RV tires have lost inflation pressure over the past few months while it was parked. There are a number of possible contributors to a loss of pressure.
Some of these have been covered in my posts on leaking valve cores or metal valve O-Rings or possibly punctures or even rim corrosion. This post will cover just two of the possible causes. permeation and Temperature change.
The simple answer is that tire pressure will change about 2% for every 10°F change in temperature.
The complex answer and mathematical proof follows:
1. Normal pressure loss due to gas permeation. This is basically what happens when the molecules of gas "leak" or travel through the rubber of a tire. New passenger and LT tires sold to "Detroit" have to pass tests and meet a specification of a loss rate of less than 1% or 2% per month when averaged over a number of weeks or months, with different companies having different specs depending on the application. TBR (Truck-Bus-Radial) tires as seen on Class-A RVs will have similar performance goals. Low Cost import tires sold at "Billy-Jo-Bob's Cheep Tire Emporium and Bait Shop" may not have any such spec so might lose pressure faster because of the use of lower cost inner-liner rubber. This topic could be an entire post of its own but is quite technical and is not based on just the molecule size of Oxygen.
2. Tire inflation pressure is directly proportional to tire temperature. The "Ideal Gas Law" is a good approximation of what really happens. The general terms of the Ideal Gas Law are PV=NRT
P is the Absolute pressure, not gauge pressure, measured in pascals
V is the air Volume measured in cubic meters. While tires do change a little bit in volume the difference is not meaningful for the purposes of this discussion.
N is the amount of gas in the air chamber in moles. For the purposes of this discussion, we can ignore this number as we are not changing the amount of gas in the tire when we are comparing the pressure difference just due to temperature difference.
R is the ideal, or universal, gas constant. R for dry air is 287.1 for N2 296.8 and for O2 259.8 for water vapor it is 461.5 so you can see that moisture in your inflation gas can significantly affect the Pressure/Temperature ratio a change from air (79%N2) to 95% N2 is not significant for the purposes of this discussion. I did a post on how to make your own "air dryer" so you can approximate the properties of dry air or dry Nitrogen.
T is the absolute temperature in degrees kelvin.
After discarding the inconsequential terms we really end up with is a simple ratio P2/P1 = T2/T1 and we can even ignore the conversion of psi to pascals as the ratio of pressure is the ratio of temperature in degrees Kelvin. Sorry we do need to use Kelvin but you don't need to do a conversion as you will soon see.
Lets see how this woks out
If P1 is 100 psi and if T1 is 70°F or 294.261°K
and if the temperature drops to 60°F or 288.706 °K we can solve for P2/P1 =
288.706/294.261 which leaves us with the ratio ot the two temperatures as 0.9811 so we had a 2% drop in pressure for a 10°F drop in temperature
30°F would be 272.039/294.261 = 0.92448 or 8% drop in pressure for a 40°F drop in temperature from 70°F to 30°F.
You may see various web pages such as this one from TireRack saying 1psi for each 10°F change but you need to remember that this information is based on the assumption that the "normal" inflation is about 35 psi in a passenger tire.
Monday, March 3, 2014
Monday, February 24, 2014
Over the years as technology has advanced it becomes more and more easy to detect ever smaller quantities of material or forces. As an engineer I too often hear people expressing concern over some new discovery of x parts per Million or even x parts per Billion of some contaminant to air or water, or foreign material in some product. Some of these numbers are important and meaningful while others are not really significant so there should not be a cause for concern. The challenge is to know when some measurement is meaningful.
Some of you are asking yourself what this has to do with RV tires. Well if you have read many of my posts here in this blog or on various RV Forums you no doubt have seem me and others saying you need to know the actual load on each tire and not just guess. Also we tell you how important it is to have the proper inflation in your tires.
What made me think of a need for this topic is the occasional complaint I get from some that think we are suggesting a need to get their RV weighed every time they travel or add an item to their packing list.. They basically say they are not going to do that so ask "Why bother to weigh the RV at all?"
Lets step back and look at tire Load & Inflation tables. For highway use tires the tables have 5 psi increments and depending on the size tire that 5 psi increment can provide as little as 110# additional load carrying capacity for a small ST type tire to 320# or more additional capacity to a large Class-A tire. If we do some simple math that means on a single axle small Trailer you would need to adjust your inflation if you were to increase the load in the trailer by 220# but the Class-A RV with rear duals and a tag you might be able to tolerate (8x320#) or 2,560# additional load (assuming perfect load distribution).
If your hobby was going to yard sales and picking up cast iron lawn ornaments it might be possible for a couple good sized items at (115# each) to be meaningful so you would need to increase your tire inflation on your small teardrop trailer (assuming you were not already running the tire max). The owner of the 40' Class-A on the other hand could probably pick-up a couple dozen items that each weighed 105# and if the load was perfectly distributed still not be overloading the tires.
Now clearly we can count (measure) the difference in the additional load of the two 115# items and the load of twenty four 105# items. In both cases the additional load is easily measured but while 120# could be very meaningful to the small trailer owner an increase in load of 120# would certainly NOT be meaningful to the Class-A RV owner. Maybe you now have a better understanding of the difference between measurable and meaningful.
In a previous post, I provided information of the accuracy of pressure gauges, showing that I find that 10% to 15% of the gauges I have checked are off by 5 psi or more. While my personal gauges are certified accurate to +/- 0.5psi, that is way more accurate you you "need" if you are setting your cold inflation to give yourself a 10psi Safety Margin over what is needed for your normal full load. If however you are running loads that fall just below the load indicated for the cold inflation you are using, then BOTH your gauge needs to be more accurate than +/-5 psi and your scale reading needs to be accurate to less than half of the load increment in the table.
Example: Tables for a 255/80R22.5 show 4,805# @ 95 and 4,975@ 100 psi so your gauge needs to be at least +/- 2.5 psi from actual certified measurement and your load needs to be measured to an accuracy of at least +/- 85# if you are using 100 psi as your cold inflation.