The pressure figures on a vacuum pump datasheet can look confusing at first glance. The same table often carries numbers in both mbar and mmHg, and some of those numbers appear with a minus sign in front of them — a combination that trips up many engineers and buyers who are new to vacuum technology. This article explains what these pressure values actually mean, the difference between absolute and relative pressure, and how to read a datasheet table line by line, using a real example.
Table Of Contents
- What Is Absolute Pressure, and How Does It Differ From Relative Pressure?
- What Does the mbar Unit Mean in Vacuum Measurement?
- How Should Values Given in mmHg Be Interpreted?
- How Do You Read a Datasheet Value Through a Worked Example?
- How Is Vacuum Level Calculated Relative to Atmospheric Pressure?
- What Do the 50 Hz and 60 Hz Columns Actually Show?
- How Do You Read the Other Values on a Datasheet?
- Frequently Asked Questions
What Is Absolute Pressure, and How Does It Differ From Relative Pressure?
Pressure can be expressed relative to two different reference points. Absolute pressure uses a perfect vacuum as its reference — when absolute pressure in an environment reaches zero, no gas molecules remain, meaning a theoretically perfect vacuum has been achieved. Relative pressure (gauge pressure), on the other hand, uses the current atmospheric pressure as its reference, and typically answers the question "how much lower or higher than atmosphere?"
Values on vacuum pump datasheets are almost always given in absolute pressure. The reason is that absolute pressure is unaffected by local atmospheric conditions — altitude, air temperature, humidity — so it provides a consistent, comparable reference across different locations. When a pump is said to reach "33 mbar vacuum," that means the pump can pull the ambient absolute pressure down to 33 mbar — far below standard sea-level atmospheric pressure, roughly 1013 mbar.
What Does the mbar Unit Mean in Vacuum Measurement?
The millibar (mbar) is one-thousandth of a bar, so 1 bar equals 1000 mbar. Standard atmospheric pressure at sea level is taken as approximately 1013.25 mbar. On a vacuum pump datasheet, the "maximum vacuum pressure" figure given in mbar represents the lowest absolute pressure the pump can achieve — the smaller that number, the deeper the vacuum the pump reaches.
For example, if a pump's datasheet reads "33 mbar," that pump can bring the ambient pressure down from roughly 1013 mbar atmospheric level to 33 mbar absolute — meaning it removes roughly 97% of atmospheric pressure. The mbar unit is the most widely used unit in industrial vacuum datasheets worldwide, and because it aligns with the SI system, it is generally the preferred unit for engineering calculations.
How Should Values Given in mmHg Be Interpreted?
The millimeter of mercury (mmHg) is a historical unit that expresses pressure as the height of a mercury column in a manometer, and it is still common in some datasheets, particularly those with American engineering roots. Standard atmospheric pressure equals 760 mmHg. On vacuum pump datasheets, mmHg values are usually given with a minus sign — as a relative figure that answers "how many mmHg below atmospheric pressure?"
For instance, a value of "−735 mmHg" indicates the pressure has dropped 735 mmHg below atmospheric (760 mmHg), leaving roughly 760 − 735 = 25 mmHg of absolute pressure in the environment. These two conventions — absolute in mbar, relative in mmHg — describe the same physical state from two different angles, and the next section walks through exactly how these two values correspond to one another with a concrete example.
How Do You Read a Datasheet Value Through a Worked Example?
In Gücüm Pompa's monoblock vacuum pump table, the "Maximum Vacuum Pressure" column lists 33 mbar, and the adjacent mmHg column shows −735 mmHg. We can verify that these two figures describe the same physical condition as follows:
- Standard atmospheric pressure is 760 mmHg.
- 33 mbar absolute pressure corresponds to roughly 24.75 mmHg absolute (using a conversion factor of approximately 1 mbar ≈ 0.75 mmHg).
- Converting that absolute value into a relative (atmosphere-referenced) figure: 24.75 − 760 ≈ −735 mmHg.
As this shows, the two columns in the table express the same vacuum level through two different units and two different reference points — one absolute in mbar, the other relative in mmHg. Understanding that these two figures do not contradict each other, and are simply two conventions from different engineering traditions, prevents a common misreading of vacuum pump datasheets.
How Is Vacuum Level Calculated Relative to Atmospheric Pressure?
When evaluating the actual vacuum level achievable at a given plant, the local atmospheric pressure should also be taken into account. The standard figure of 1013 mbar applies at sea level; at a high-altitude facility, atmospheric pressure runs lower than that figure, which can theoretically shift the absolute vacuum level a given pump achieves. This is why datasheet values are typically stated under standard atmospheric conditions and a specific reference temperature — the further actual field conditions drift from that reference, the more the real result can deviate from the catalog figure.
Sharing the plant's altitude and climate conditions with the supplier before purchase helps avoid unexpected performance gaps, particularly in critical processes.
What Do the 50 Hz and 60 Hz Columns Actually Show?
Gücüm Pompa's datasheet tables carry two separate column groups for every model — one for 50 Hz grid frequency, one for 60 Hz. Grid frequency directly determines the rotational speed of an asynchronous electric motor; a motor on a 60 Hz supply spins roughly 20% faster than the same motor on a 50 Hz supply. That means the same pump casing delivers higher suction capacity and requires higher shaft power at 60 Hz.
The maximum vacuum pressure figure (mbar/mmHg) generally stays the same across both frequencies, because that value is primarily a function of the casing design and the service liquid's properties, not rotational speed. Rotational speed, suction capacity, and shaft power figures, however, do change with frequency. Turkey and Europe run on a 50 Hz grid standard, while most of North and South America and some Asian countries run 60 Hz — so selecting the correct column matters for export orders.
How Do You Read the Other Values on a Datasheet?
Beyond pressure figures, a vacuum pump datasheet typically carries these additional columns:
- Shaft power (kW / HP): The power the motor consumes to drive the pump. This differs between the 50 Hz and 60 Hz values for the same model.
- Rotational speed (RPM): Revolutions per minute, determined by grid frequency and motor pole count.
- Maximum suction capacity (m³/s or m³/h): The gas volume the pump can draw in at a given vacuum level.
- Maximum service liquid capacity (L/min): The service liquid flow rate the liquid ring pump requires.
- Noise level (dBA): Typically measured at a specific operating pressure, such as 80 mbar.
Taken together, these values give a complete technical picture of whether a pump suits a given process. Looking only at vacuum level and ignoring capacity or power requirements can lead to a poor match — reading every column of the datasheet together is the safer approach.
Frequently Asked Questions
Why Are Absolute and Relative Pressure Expressed With Different Numbers for the Same Condition?
Both figures describe the same physical state, but they're measured from different reference points. Absolute pressure treats a perfect vacuum as zero, while relative pressure treats the current atmospheric pressure as zero. That's why the same vacuum level can appear as a small number in absolute units and as a large negative number in relative units.
Why Do Datasheets Give Both mbar and mmHg Together?
Different countries and industries have historically adopted different pressure units. mbar aligns closely with the SI system and is common in Europe, while mmHg remains preferred in parts of the Americas and in certain traditional engineering fields. Giving both units together makes the datasheet readable for buyers across different markets.
Can a Pump Rated for 50 Hz Be Used Directly on a 60 Hz Grid?
Because rotational speed, capacity, and power requirements all shift with motor characteristics, this needs to be assessed on a project-by-project basis. Datasheets usually list separate rows for each frequency; using the correct frequency column and consulting the supplier when needed is the recommended approach.
Are Datasheet Values Always Matched Exactly in Field Conditions?
No. Datasheet values are obtained under defined test conditions — standard atmospheric pressure and a specific reference service liquid temperature. Field factors such as altitude, actual service water temperature, and line losses can shift real-world results somewhat from the catalog figure.
