The question of how much is pressure In one unit of measurement relative to another, often occurs in motorists when servicing a vehicle. When the driver approaches the compressor at the gas station or uses his own gaugeIt may be confusing to the scales. Some devices show the value of barothers in kgf/cm² or atm. Understanding the ratio of these values is critical for the safe operation of the car.
Many people mistakenly believe that bar and technical atmosphere (kgf/cm2) are absolutely identical concepts that can be changed in places without taking into account the error. Although the difference between the two is less than 2%, in precision technique or with fine tuning. pneumosystem It might make a difference. In the context of car tires, such an error is usually permissible, but it is necessary for every competent driver to know the exact recalculation formula.
In this article, we will examine the physical essence of these units, provide accurate conversion rates and explain why different numbers are often written on pressure gauges. You will learn to quickly convert values in your mind and understand what exactly the arrow of your measuring instrument is showing at a particular time.
Physical difference between Bar and CHS/cm2
First, it is necessary to clearly define what these values are. barbar It is a non-systemic unit of pressure that is approximately equal to one atmosphere. It was introduced for the convenience of calculations in meteorology and technology. On the other hand, kg/cm2 A kilogram force per square centimeter is a unit from the ICGSS system, often referred to as the "technical atmosphere." It shows a force of 1 kilogram acting on an area of 1 square centimeter.
The mathematical ratio is as follows: 1 bar equals 100,000 Pascals. At the same time, 1 kgf/cm2 is 98,066.5 Pascals. This means that 1 bar is slightly larger than 1 kgf/cm2. The exact conversion factor shall be 1 bar = 1.0197 kgf/cm². Backwards: 1 kgf/cm² = 0.9806 bar.
⚠️ Attention: When calculating in the household sphere (for example, tire pumping), this 2% difference is often neglected, considering 1 bar equal to 1 kgf / cm2. However, when working with high-precision industrial equipment or high-pressure hydraulic systems, such an error is unacceptable.
In the automotive industry, manufacturers often indicate recommended pressures in the vehicle. technical documentation It's in bars or atmospheres. Understanding that kg/cm2 This is actually the same as the “atmosphere” in the colloquial speech of mechanics, helps to avoid errors. If the instructions say "2.2 atmospheres", and the gauge scale in bars, you can safely focus on the value of 2.2.
Use a digital pressure gauge with the ability to switch units of measurement – this will save you from having to make calculations manually and reduce the risk of error.
Exact table of pressure unit translation
In order not to get confused in numbers and coefficients, it is most convenient to use ready-made tables of correspondence. Below are the main values that are most commonly found in car, motorcycle and truck maintenance. Here we compare. bar, kg/cm2Technical atmosphere and popular unit in the United States PSI.
| Bar (Bar) | CGV/cm2 (kgf/cm2) | Atmosphere (atm) | PSI (pound/sq. inch) |
|---|---|---|---|
| 1.0 | 1.02 | 0.99 | 14.50 |
| 1.5 | 1.53 | 1.48 | 21.76 |
| 2.0 | 2.04 | 1.97 | 29.01 |
| 2.2 | 2.24 | 2.17 | 31.91 |
| 3.0 | 3.06 | 2.96 | 43.51 |
As you can see from the table, the difference between the columns barbar and CGV/cm2 minimal. For a value of 2.0 bar, the difference is only 0.04 units. In the conditions of a garage or track, it is almost impossible to notice such a difference on a conventional hand gauge due to the error of the device itself. However, engineering These values are fixed precisely.
column PSI Pounds per square inch is given for those who are faced with American machinery or imported tires. Translation is carried out here at a more complex coefficient, so the presence of a table or calculator in the phone will be very useful. Remember that 1 bar is approximately 14.5 PSI.
Why are Bar, Atmosphere and CGS/cm2 confused?
The main reason for the confusion lies in the historical development of the units of measurement and their close numerical value. Physical atmosphere Atm is the pressure at sea level, which is 101,325 Pa. Technical atmosphere (a) equal to 1 kgf/cm2 is 98,066.5 Pa. barbar It was defined as 100,000 Pa to simplify the calculations.
As a result, we have three values that differ from each other by less than 3-4%. In everyday speech, especially among motorists and masters of service, all these values are often called simply “atmospheres”. When a mechanic says, “Pump up to two atmospheres,” he may mean 2 bars or 2 kgf/cm2. For vehicle This terminological freedom is permissible.
- 🚗 Motorists: The terms "atmosphere", "bar" and "kgs" are used as synonyms, rounding the values to tenths.
- 🔧 Engineers: These units are distinguished strictly, since in hydraulics and pneumatics even a small pressure difference affects the efficiency of the system.
- 🌍 International standard: In Europe and Russia, bars and kgf/cm2 are more commonly used, while in the US and UK the PSI system is dominant.
It is also important to note that in modern Digital pressure gauges often have a unit switching button. This allows the user to choose the usual format of displaying data. If you are used to kgf/cm2, there is no point in translating values in your mind if the device can do it automatically.
⚠️ Attention: Never confuse pressure units (bar, kgf/cm2) with units of volume or mass. Attempt to pump into the tire "kilograms" of air without reference to the area (cm2) is physically incorrect, although in everyday life they often say "add a pound of pressure."
Practical application in tyre pumping
When you go to the compressor to pump tireYou need to know the recommended value for your car. It is usually indicated on a plate in the opening of the driver's door or in the instructions for use. Most often, you will see a value in bars (for example, 2.3 bar). If your pressure gauge shows in kgf/cm2, you can safely put 2.3, since the difference of 2% (0.046 units) for traffic safety does not play a role.
However, there is a nuance associated with temperature. The pressure in the tires depends on the air temperature. Heat-up rubber In motion, the pressure is increasing. Therefore, the value should be measured and adjusted on a “cold” tire when the car stood for several hours. In this case, the accuracy of the instrument is more important than the accuracy of the conversion of units of measurement.
☑️ Tyre pressure check
If you use an old hand gauge, its error can reach 0.2-0.3 atmosphere. In this case, it makes no sense to look for the difference between 1 bar and 1.02 kgf/cm2. It is much more important to monitor the uniformity of pressure in all wheels and its compliance with the load on the wheel. car-axle.
For trucks and SUVs, pressure requirements may be higher, up to 3-5 bars or more. There is also a rounding rule. If the instruction is 4.5 kgf/cm2 and you pump 4.5 bar, the difference is less than 0.1 units, which is within the tolerance for most truck tires.
Features of working with manometers of different types
There are many types of pressure measuring devices, and each of them can have its own scale. Hand gauges (mechanical) often have a double scale, where the external is marked in bars, and the internal - in PSI or kgf / cm2. Take a close look at the designations so as not to confuse the external and internal scales.
Digital pressure gauges They are more accurate and often allow you to switch units of measurement at the touch of a button. This is an ideal option for a modern driver. However, they have their drawback - dependence on the element of nutrition. If the battery is dead, the numbers may disappear or start to “lie” by showing the wrong values.
- 🔘 Mechanical: Reliable, do not require batteries, but are afraid of shocks and vibrations, eventually losing accuracy.
- 💻 Digital: Accurate, comfortable, have lighting, but require food and can freeze in severe frost.
- 📱 TPMS systems: Sensors inside the wheels transmit data to the car screen, showing real-time pressure in selected units.
When choosing a pressure gauge, pay attention to the range of measurements. For cars, a device up to 5-7 bar is enough. For trucks, a range of 10-12 bar and above is required. Using a passenger gauge on a truck can cause the device to break or incorrect readings at the top of the scale.
How to check the accuracy of the pressure gauge?
The easiest way is to compare the readings of your device with the readings of the reference gauge at the gas station or in the tire center. If the difference is greater than 0.2 atm, it is better to replace or calibrate the device.
Effect of error on traffic safety
Many drivers are wondering: does the difference between bar and kgf/cm2 affect safety? The answer is that the 2% difference is not. But ignoring pressure control in general is. Unproven tires lead to increased fuel consumption, sidewall wear and risk of aquaplanning. Over-pumped tires worsen traction and make the ride tough.
Security It depends on how much pressure in your tires corresponds to the operating conditions. In winter, the pressure drops and it needs to be adjusted more often. In summer, on the contrary, it grows. Under these conditions, fluctuations in the 0.1-0.2 atmosphere (the difference between bar and kgf/cm2) are lost against the background of temperature changes.
⚠️ Attention: Never release pressure from hot tires, focusing on cold norms. After the trip, the pressure is always higher than nominal. If you lower the air to normal on a hot tire, then when cooling, the pressure will fall below the permissible minimum.
The load should also be taken into account. If you are traveling alone, a full-time 2.0-2.2 bar is enough. If you load the car to the end for a trip to the country, the manufacturer recommends increasing the pressure to 2.4-2.6 bar (or kgf / cm2). In this case, it is more important to follow the manufacturer’s recommendations than to look for perfect accuracy in hundredths of a unit of measurement.
For everyday use of the car, 1 bar and 1 kgf / cm2 can be considered equal. A 2% difference does not affect safety, but regular monitoring of blood pressure is critical.
Frequently Asked Questions (FAQ)
Can the tires be pumped to 1 kgf/cm2 if the instructions say 1 bar?
Yes, you can. The difference between these values is only about 0.02 units, which is imperceptible to the operation of the car and is within the margin of error of any household pressure gauge.
How is the technical atmosphere different from the physical?
The technical atmosphere (1 kgf/cm2) is 0.98 bar, and the physical atmosphere (atm) is 1.013 bar. In automotive practice, the technical atmosphere is often implied, and it is almost equal to a bar.
How to transfer PSI to bars without a calculator?
For quick translation, you can divide the PSI by 14.5. For example, a 29 PSI divided by 14.5 would give about 2.0 bars. This is enough for an approximate estimate.
Why are compressors on different scales?
Different scales are determined by the standards of different countries. In Europe and Russia, bar and kgf / cm2 are accepted, in the USA - PSI. Compressor manufacturers apply multiple scales for the convenience of users from all over the world.
Should the difference between bar and kgf/cm2 be considered in racing cars?
In professional motorsport, where suspension and tyre settings are perfect, engineers use precise values in Pascals or bars to within a hundredths of a clock. For street driving, this accuracy is excessive.