Question

The air pressure of car tires should be checked regularly for safety reasons and for prevention of uneven tire wear. Find out the units of measurement on a typical tire gauge, and determine how gauge pressure relates to atmospheric pressure.

Short answer

Units of measurement include psi, kPa, and bar. Gauge pressure is the tire pressure above atmospheric pressure, calculated as \[ P_{\text{gauge}} = P_{\text{absolute}} - P_{\text{atmospheric}} \]

Step-by-step solution

- Identify Units of Measurement

A tire gauge commonly measures pressure in units such as psi (pounds per square inch), kPa (kilopascals), or bar. Look at a typical tire gauge and note the units it displays.

- Define Gauge Pressure

Gauge pressure is the pressure measurement relative to atmospheric pressure. It shows how much pressure is in the tire above the ambient atmospheric pressure.

- Understand Atmospheric Pressure

Atmospheric pressure is the pressure exerted by the weight of the atmosphere and at sea level, it is approximately 101.3 kPa or 14.7 psi.

- Relate Gauge Pressure to Atmospheric Pressure

Gauge pressure can be calculated by subtracting atmospheric pressure from absolute pressure (the total pressure inside the tire). Mathematically, \[ P_{\text{gauge}} = P_{\text{absolute}} - P_{\text{atmospheric}} \]

Key concepts

Gauge Pressure

Gauge pressure is a crucial concept when measuring tire pressure.
It tells us the pressure inside the tire that is above atmospheric pressure.
When you use a tire gauge, you are typically reading the gauge pressure. This value helps determine if the tire is properly inflated.
Unlike absolute pressure, which considers both the internal pressure of the tire and the atmospheric pressure, gauge pressure focuses only on the difference.
Mathematically, gauge pressure is represented as:
\( P_{\text{gauge}} = P_{\text{absolute}} - P_{\text{atmospheric}} \)
This formula highlights that to find the gauge pressure, you subtract the atmospheric pressure from the absolute pressure in the tire. It's important to understand this to avoid confusion and ensure safe tire maintenance.

Atmospheric Pressure

Atmospheric pressure is the pressure exerted by the weight of the atmosphere surrounding us.
At sea level, this is about 101.3 kPa or 14.7 psi.
This pressure changes with altitude, weather conditions, and other factors.
When you measure tire pressure, you always deal with atmospheric pressure.
For instance, when checking tire pressure with a gauge, the atmospheric pressure acts as a baseline.
Understanding the constant nature of atmospheric pressure at sea level helps comprehend how gauge pressure calculations work.
In gauge pressure measurements, atmospheric pressure is the reference point, essentially zeroed out to focus only on the excess pressure inside the tire.
This simplifies the process of ensuring tires are neither under-inflated nor over-inflated.

Pressure Units

When measuring tire pressure, you will come across several units:

• psi (pounds per square inch)
• kPa (kilopascals)
• bar

Psi is commonly used in the United States, while kPa and bar are more prevalent in other parts of the world.
Understanding these units is crucial for accurate tire pressure measurement.
For example, recommended tire pressures might be given in psi, kPa, or bar, depending on the manufacturer and region.
Most tire gauges will show the reading in one or more of these units, and you should be comfortable converting between them if necessary.
It's always important to follow the tire pressure specifications given by the vehicle manufacturer, and knowing these units will help ensure your tires are properly inflated, leading to safer driving conditions and better tire wear.