In the operation of the vehicle drivers regularly face the need to control the pressure in the tires, but the units of measurement on the pressure gauges are often perplexing. On some devices, the scale is marked in bars, on others - in atmospheres or PSI, which creates confusion and requires recalculation of values for the correct pumping of the wheels.
Understanding the physical essence of these values is necessary not only for passing a technical inspection, but also for ensuring safety on the road, since even a slight deviation from the norm affects rubber wear and fuel consumption. Let’s see if there is a real difference between these concepts or if they can be neglected in everyday life.
In automotive practice, these values are considered conditionally equal, but from the point of view of exact physics and metrology, there is a certain delta between them, which should not be ignored with serious loads or professional diagnostics.
Physical essence and origin of units of measurement
To begin with, it is worth looking at definitions to understand where these units come from and what they actually mean in the physical world. Atmosphere Atm is a non-systemic unit of pressure measurement that has historically evolved as the average pressure of the Earth's atmosphere at sea level. It was widely used in engineering and science before the introduction of international standards.
In turn, bar It is also a non-systemic unit, but it was introduced specifically for the convenience of calculations in meteorology and technology, since its value is close to the atmospheric, but is tied to the decimal system. One bar is equal to exactly 100,000 Pascals, making it convenient for engineering calculations.
The main difference lies in the exact numerical values: the technical atmosphere (at) is defined as the pressure produced by a force of 1 kgf, evenly distributed over a surface area of 1 cm2, while the bar is tied to the SI system through Pascali. Physical atmosphere The atm is different from the technical and is 101,325 Pa, which further complicates the picture for the layman.
⚠️ Warning: Confusion between technical (atm) and physical (atm) atmospheres can lead to an error of about 3%, which is critical for high-precision industrial systems, but is acceptable for household use in tire installation.
Exact ratio of magnitudes and mathematical difference
If you omit the complex physical formulas and move on to the dry numbers that are important for the driver, the difference between units becomes apparent on detailed examination. The bar and the technical atmosphere (which is most often implied in car pressure gauges) are not equal to each other.
One bar is 1.019716 kgf/cm2 (technical atmosphere), and one atmosphere is approximately 0.986923 bar. At first glance, the error seems negligible, but when pumping tires of a truck or working with pneumatic systems, this delta is summed up.
For clarity, consider the exact conversion rates:
- 📏 1 bar = 1.0197 at (technical atmosphere)
- 📏 1 atom = 0.9807 bar
- 📏 1 bar = 0.9869 atm (physical atmosphere)
- 📏 1 atm = 1.0133 bar
So if you pumped the wheels up to 2.0 bar, you actually got 2.04 atmospheres. Otherwise, when the pressure gauge is 2.0 at, the pressure in the bars will be 1.96. The difference of 0.04 units per value may seem insignificant, but over a long distance it affects the uniformity of tread wear.
Comparative table of value translation
For quick orientation in numbers and avoiding errors when setting pressure, it is useful to have a correspondence table at hand. It will help convert values instantly if your pressure gauge shows one value and the instructions for the car indicate another.
| Bar (bar) | The atmosphere is techn. (at) | The atmosphere of the physique. (atm) | Pound/sq. inch (PSI) |
|---|---|---|---|
| 1.0 | 1.02 | 0.99 | 14.5 |
| 1.5 | 1.53 | 1.48 | 21.75 |
| 2.0 | 2.04 | 1.97 | 29.0 |
| 2.5 | 2.55 | 2.47 | 36.25 |
| 3.0 | 3.06 | 2.96 | 43.5 |
As you can see from the table, with increasing pressure, the absolute error between the bar and the atmosphere increases. If for a passenger tire with a pressure of 2.2 bar the difference with the atmosphere is only 0.044 units, then for a truck with a pressure of 8.0 bar the difference will already be almost 0.16 units.
Practical application in motor transport
In real life, the question of "bar or atmosphere" is solved quite simply: most modern mechanical and electronic pressure gauges are calibrated in bars or have a double scale. Automakers in their operating manuals most often indicate recommended pressure in bars or kilopascals (kPa).
However, on old Soviet pressure gauges or devices produced for the domestic market of the CIS countries, you can still find the marking "atm" or "kgf / cm2". In this case, it is necessary to remember that 1 kg/cm2 This is the technical atmosphere, which is almost equal to the bar with an error of about 2%.
When using automatic tire pumping stations at refueling stations, you should be careful: some of them allow you to choose units of measurement, and accidental switching can lead to under-pumping or pumping of wheels. Always check the sticker on the body rack or the gas tank hatch of your car.
☑️ Pressure check before travel
Experts recommend using the numeric gauge with the ability to switch units, since they exclude the human factor when reading reading from handguns. In addition, electronics often have a higher accuracy class.
⚠️ Note: only measure the pressure on the "cold" tires (after parking for at least 2-3 hours), as heating the rubber when driving increases the pressure inside, distorting the real readings.
Impact of unit difference on safety and flow
It would seem that it does not matter what to measure – the main thing is that the wheels were pumped. However, systematic under-pumping or over-pumping caused by unit confusion or inattention has specific consequences for the wallet and driver safety.
Insufficient pressure (even 0.2-0.3 bar below normal) leads to an increase in rolling resistance, which directly affects fuel consumption. The engine has to spend more energy to overcome the resistance of the deformed tire.
- 📉 The wear of the side parts of the tread increases.
- 📉 The risk of aquaplaning increases due to the deterioration of water drainage.
- 📉 The likelihood of damage to the cord and the occurrence of hernias increases.
- 📉 Controlling is deteriorating and the braking distance increases.
On the other hand, the excess pressure makes the suspension stiffer, transferring all the blows from the roughness of the road to the body and suspension elements. The center of the tread wears out faster, and traction deteriorates due to a decrease in the area of contact of the spot.
Remember the simple rule: 1 BAR ≈ 1 ATM. For everyday driving, a difference of 0.02-0.03 units will not have a critical impact on safety, but try to adhere to the values specified by the car manufacturer.
The American PSI System and Other Nuances
If we have dealt with the bar and atmosphere, then owners of American-made cars or imported tires may face labeling. PSI (Pounds per Square Inch – Pounds per Square Inch) It is the main unit of pressure measurement in the United States and the United Kingdom.
Translating PSI into bars or atmospheres is quite difficult, as the odds are not integers. One PSI is approximately equal to 0.069 bar or 0.07 atmosphere. The formula can be used to translate the value in PSI divided by 14.5 to get the bars.
Many modern pressure gauges have a switch button. UnitIt changes the mapping between BAR, PSI and kPa. If your appliance only shows PSI and you need 2.2 bars, look for a value of about 32 PSI on the scale.
Why does the pressure in tires rise in summer?
In summer, when heated from hot asphalt and the sun, tire pressure can increase by 0.2-0.3 bar. It is a normal physical process (Gay-Lussac’s law), so it is better to check and adjust the pressure in the morning while the car is in the shade.
Frequently Asked Questions (FAQ)
Can tires be pumped to 1.9 atm instead of 2.0 bar?
Yes, you can. The difference is only 0.02 bar (about 1%), which is within the permissible margin of error of any household pressure gauge and will not affect the performance of the car.
What does kPa mean on a pressure gauge?
kPa is the kilopascal, the basic unit of pressure measurement in the SI system. 1 bar equals 100 kPa. If the instructions indicate 2.2 bar, then you need to pump up to 220 kPa.
How often should I check my blood pressure?
It is recommended to check the tire pressure at least once a month, and before each long trip. Sudden temperature changes also require mandatory inspection.
Why are the gas station gauge readings different from my own?
Manometers at public gas stations often have a large margin of error due to heavy operation and the lack of regular verification. It is always better to have your own calibrated appliance.
Does altitude affect tire pressure?
Atmospheric pressure drops with height, but the pressure inside the tire (excess pressure) remains virtually unchanged, as the tire is sealed. However, the external pressure drop may have a slight effect on the volume of the tire, but this effect can be neglected in road trips.