Charles’s Law….Expansion of Gases
Under equal pressure all gases expand at the same rate. A gas expands by the same proportion as the temperature rises, provided external pressure remains the same.
How does this relate to tire pressure warning lights? Simply put, as a tire warms up, the air pressure in the tire increases. As the tire cools, the air pressure in the tire decreases. See below for examples.
This little tid-bit of information explains why, on cold mornings, your tire pressure warning light may turn on. Yes, it is that simple; as the temperature of a tire changes so does tire pressure.
Attached to your tire valve stem is the tire pressure monitor, which contains the following and possibly more; a pressure sensor, analog-digital converter, micro controller, frequency transmitter, low frequency receiver, battery, voltage regulator for battery management, temperature sensor and a few other goodies depending on the vehicle manufacturer.
How much can it change?
If a tire’s temperature drops 10 degrees, your tire’s pressure would drop about 1 psi.
Say you set your tire pressure to 32 psi a month or 2 ago when it was warmer, say 50 degrees…now it is a single degree morning.
Guess what?,, your tire pressure has dropped 4 to 5 psi.
What Happens next?
The tire pressure computer in most passenger cars will allow for approximately 10% tire pressure change/drop before the tire pressure warning light comes on. A 4 psi drop will turn on the tire pressure warning light on many of today’s cars…
This also explains why after you drive your vehicle the warning light will go out. The tire warms up, up goes pressure, out goes the light. See below for more
Increasing the tire pressure in cold weather can also be found in your Owner’s Manual. See below ..With a few simple adjustments for the driving environment your tire pressure warning system is a good information and safety center.
How much can temperature change tire pressure? Let’s go for a drive…
A January 17th start up outside air temperature of 17 degrees.
Tire pressure warning light is on.
Let’s check our rear tire pressure
Before leaving the house my rear tire pressure was checked and off we go for a 5 mile drive to work in a snow storm.
.
5 miles later, look what we have now
Here you go. Outside air temperature is still 17 degrees. The roads are snow covered and the 5 mile drive to work brought tire pressure up by nearly 3 psi in both rear tires. Tires do heat-up as you drive even on cold days… It just shows how temperature changes pressure.
What should I set my tire pressure to?
This clip is from a Subaru Owner’s Manual. Check your vehicle’s Owner’s Manual for recommended pressure to temperature adjustment.
We’ve found that 3 to 4 psi seems to do the trick in our region.
Battery life, How long do they last?
Tire pressure monitor battery life expectancy is 6 to 10 years. When the battery gets low, the transmitter signal becomes too weak for the vehicle’s computer to read. This usually is indicated by a blinking tire pressure warning light.
The fix? A replacement sensor.
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