In technical documentation, instructions for compressors, or when servicing vehicle hydraulic systems, different measurement systems are often found. Beginners and even experienced professionals can face confusion when it is urgent to understand kgf cm2 what is it in bar and how to quickly recalculate without errors. This is critical for setting up gearboxes, checking tire pressure on special equipment, or calibrating pressure gauges.
Understanding the physical essence of these quantities allows you to avoid emergency situations associated with equipment overload. In this article we will examine in detail the origin of the units, the mathematical relationships between them and the practical aspects of their use in modern automotive and industry.
The main difficulty lies in the fact that kgf/cm² is a non-systemic unit, rooted in the Soviet engineering school, while bar and Pascal belong to the international system. Despite this, old pressure gauges are still widespread in garages and factories.
Physical essence and origin of units of measurement
To gain a deep understanding of the processes, it is necessary to understand the definitions. Unit kgf/cm² (kilogram-force per square centimeter) is often erroneously called simply "atmosphere" or "technical atmosphere" (at). It means a force of one kilogram acting on an area of one square centimeter. This concept is intuitive to old-school engineers, as it is easily visualized by the weight of the load.
Unlike a technical unit, bar is a non-systemic unit, but is widely used in meteorology and technology, since it is very close to the same physical atmosphere. One bar is equal to 100,000 Pascals. The difference between the technical and physical atmosphere is about 3%, which is often ignored in everyday conditions, but can be critical in precision hydraulics.
It is important to note that the international SI system uses the Pascal as its base unit, but its values are too small for practical use in the automotive industry. Therefore, multiples are used. The relationship between them is based on the acceleration of gravity and the density of substances.
⚠️ Attention: When calculating pressure in brake systems or engine management systems, even a small error in unit conversion (for example, rounding a coefficient) can lead to incorrect operation of electronic control units.
Historically, Soviet pressure gauges were calibrated in kgf/cm², while imported equipment was calibrated in bar or psi. This creates the need for constant recalculation when servicing a mixed fleet of equipment or using adapters.
Why are they still using kgf/cm²?
In the countries of the post-Soviet space, a huge base of measuring instruments produced in the USSR has been preserved. Replacing them is not economically feasible, so engineers are forced to operate with familiar quantities, despite the global transition to the metric system.
Translation mathematics: exact coefficients and formulas
Converting one unit of measurement to another requires knowledge of the exact coefficient. Many people use an approximate value, believing that 1 kgf/cm² is equal to 1 bar. However, this simplification is only valid for a rough estimate. For accurate engineering calculations, it is necessary to use more detailed figures.
Conversion formula from kgf/cm² in bar looks like this: you need to multiply the value in kgf/cm² by a factor of 0.980665. Conversely, to get the technical atmosphere from the bar, you need to divide the value by the same coefficient or multiply by 1.0197.
Let's look at an example. If the pressure gauge shows 10 kgf/cm², then in bars it will be:
10 * 0.980665 = 9.80665 barAs you can see, the difference is almost 2%. In high-pressure systems, such as power steering or truck air suspension, this difference can be several atmospheres, which significantly affects diagnostics.
Remember a simple rule for a quick mental estimate: 10 kgf/cm² is approximately 9.8 bar. That is, the value in bars is always slightly less than in technical atmospheres.
To make it easier to understand the relationships, we present a table with the main values that are often found when servicing cars:
| kgf/cm² (at) | Bar | Pascal (MPa) | PSI (lb/in²) |
|---|---|---|---|
| 1 | 0.98 | 0.098 | 14.22 |
| 5 | 4.90 | 0.49 | 71.10 |
| 10 | 9.81 | 0.98 | 142.23 |
| 20 | 19.61 | 1.96 | 284.46 |
| 100 | 98.07 | 9.81 | 1422.33 |
Using accurate values is especially important when calibrating pressure sensors, where the error should not exceed 1-2%. The tabular data shows that with increasing pressure, the absolute error when using a simplified 1:1 ratio increases proportionally.
Automotive and industrial applications
In the automotive sector, unit kgf/cm² found everywhere, especially in documentation for domestic equipment (KAMAZ, MAZ, GAZ) and Soviet-made equipment. Truck tire pressure, operating pressure in the engine lubrication system, pressure in the fuel system of diesel engines - all these parameters are often indicated in technical atmospheres.
Modern imported cars and diagnostic scanners are more often used bar or kPa (kilopascals). For example, when connecting a professional scanner to a Common Rail system, you can see a pressure of 1600 bar, which corresponds to approximately 1632 kgf/cm². Understanding this correspondence helps you navigate the data stream faster.
Particular attention should be paid to the pneumatic brake systems of trucks. Normal operating pressure in the circuits is about 6.5–8.0 kgf/cm². If a mechanic is used to working with bars, he can expect a value of 6.5–8.0 bar, which is an underestimate and may lead to incomplete release of the wheels.
- 🚛 Pneumatic systems: The pressure in truck receivers is usually 8–10 kgf/cm² (about 7.8–9.8 bar).
- 🚗 Fuel systems: In gasoline distributed injection systems, the pressure is 2.5–3.5 kgf/cm², and in direct injection systems (GDI/FSI) it can reach 150–200 bar.
- 🏗️ Hydraulics: Working pressure in hydraulic boosters and hydraulic presses varies from 70 to 200 kgf/cm² and higher.
It is also important to take into account the thermal expansion of liquids and gases. When heated, the pressure in a closed volume increases. If the system is calibrated in kgf/cm² at 20°C, then when used in hot weather, the readings may change, and it is important to understand in what units the valve response threshold values are measured.
Working with pressure gauges and diagnostic equipment
When purchasing a new pressure gauge for a garage or service station, you may encounter a double scale or, conversely, a device where only one value is indicated. Often on the dials of Chinese manufacturers the scale can be graduated in MPa (Megapascal), which also requires recalculation.
If your device has a scale in MPa, then 0.1 MPa is equal to 1 bar or approximately 1.02 kgf/cm². This is a convenient guideline: to convert Megapascals to technical atmospheres, you need to multiply the value on the scale by 10 and add a small correction of 2%.
⚠️ Attention: When using digital pressure gauges, be sure to check in the setup menu in which units the data is output. Many devices allow you to switch modes (Unit), but the default settings may be factory settings (often PSI or kPa).
Diagnostic adapters connected to the OBDII connector transmit data to software on the computer. In the software settings (for example, OpenDiag, Scanmaster) it is often possible to select a preferred system of units. This eliminates the need to make mental calculations while working.
When working with analog pointer instruments, it is important to consider the accuracy class. Old pressure gauges with a scale in kgf/cm² can have an error of up to 4%, which makes them unsuitable for fine-tuning modern injectors, but quite sufficient for monitoring tire pressure.
☑️ Checking the serviceability of the pressure gauge
Typical errors during conversion and operation
One of the most common mistakes is completely ignoring the difference between kgf/cm² and bar in responsible nodes. For example, when inflating aircraft or special equipment tires, a difference of 2% may be unacceptable. In passenger cars, the difference between 2.2 bar and 2.2 kgf/cm² is about 0.04 bar, which is difficult to notice by eye on the pressure gauge scale, but for road grip it is noticeable.
Another mistake is the confusion between absolute and gauge pressure. Pressure gauges usually show gauge pressure (the difference between system pressure and atmospheric pressure). The conversion formulas work specifically for excess pressure. If we are talking about absolute pressure (for example in vacuum systems or in thermodynamic calculations), it is necessary to take into account atmospheric pressure, which is approximately 1.033 kgf/cm² or 1.013 bar.
Some technicians try to use online converters on the phone right next to the car. This is acceptable, but it takes time. It is much more effective to learn basic correspondences or have a reminder handy.
- 📉 Underpressure: If the instructions indicate 3.0 kgf/cm², and you inflated 3.0 bar, you underinflated the tire by about 0.06 atm. This leads to increased wear on the sidewalls and increased fuel consumption.
- 📈 System overload: If the safety valve is set to 10 bar and you apply 10 kgf/cm² (which is more than 10 bar), the valve may not operate in time, creating the risk of a line rupture.
- 🔧 Incorrect calibration: Setting the fuel pressure regulator to incorrect units will result in a lean or rich mixture and unstable engine idle.
It is also important to consider that standards may vary from country to country. American system PSI (pound-force per square inch) found on imported compressors and pressure gauges. 1 bar is equal to approximately 14.5 PSI. Confusion between these systems is even more common than between bar and kgf/cm².
A basic safety rule: If you are unsure of the units on the meter or the instructions, always use a lower pressure value for the initial test of the system, gradually increasing it under control.
Practical recommendations for specialists
To simplify the work, it is recommended to have a laminated conversion table of basic quantities in the glove compartment or at the workplace. This will eliminate the need to search for information on the Internet, especially in a garage where the connection may be unstable.
When purchasing new equipment (compressors, pumps, hydraulics), try to choose models with a dual scale or the ability to switch units. This is a universal solution that will last longer and is suitable for working with any equipment, be it an old tractor or a modern crossover.
If you are engaged in chip tuning or in-depth diagnostics, configure the interface of your diagnostic program so that it displays data in the units you are familiar with, but with the understanding that “under the hood” the ECU operates with its own internal codes.
Remember that 1 kgf/cm² is exactly equal to 0.980665 bar, and this is the only constant physical relationship that should be relied upon in controversial situations.
How does a technical atmosphere differ from a physical one?
The technical atmosphere (1 at = 1 kgf/cm²) is based on a mass of 1 kg and an area of 1 cm². The physical atmosphere (1 atm) is the normal atmospheric pressure at sea level at 0°C. 1 atm ≈ 1.033 kgf/cm². Engineering almost always involves a technical atmosphere.
Can 1 bar be considered equal to 1 kgf/cm²?
For household needs (inflating bicycle tires, checking the pressure in the system), an error of 2% is insignificant. However, for setting up fuel equipment or press hydraulics, such rounding is unacceptable.
How to convert PSI to kgf/cm²?
To convert PSI to kgf/cm², you need to divide the PSI value by 14.223. For quick translation in your head, you can divide by 14.
Why are there two scales on the pressure gauge?
Two scales (usually black and red) are provided for the convenience of users from different regions or industries. Black often represents kgf/cm², and red represents bar or PSI, or vice versa. Always check the markings on the dial.
Does temperature affect pressure gauge readings?
Yes, mechanical pressure gauges have temperature errors. With strong heating or cooling, the pressure gauge spring can change its properties, and the pressure in the measured medium (gas) will change according to Gay-Lussac's law. It is best to take measurements at the operating temperature of the system.