Why Miniature Diaphragm Pump Manufacturers Use Different Pressure Units: 10 Key FAQs

You're comparing two miniature pump datasheets. One lists pressure in psi, another in bar, and a third in kPa. You're left wondering if they are even comparable, worried that a simple misinterpretation could lead you to specify the wrong component.

The reason for the variety is simple: manufacturers use different units to align with regional standards (like Imperial vs. SI) and industry conventions (like medical vs. industrial). Understanding these units is the key to confidently comparing any pump specification and choosing the right one for your project.

Having worked in the mini pump industry for over a decade, I can tell you this is one of the most frequent points of confusion. An engineer in Germany will instinctively think in bar, while their counterpart in the United States will design around psi. As a global manufacturer, BODENFLO has to be fluent in all of them. To help demystify this topic, I've compiled the ten most common questions we get and answered them one by one.

1. Why are there so many different units for pressure?

You look at pump specifications and see a confusing mix of units. This makes it hard to compare products and creates uncertainty. Why can't there just be one single, standard unit for pressure?

The variety of pressure units exists due to historical, regional, and industrial conventions. Different systems of measurement, like Imperial (psi) and Metric/SI (bar, kPa), were developed and adopted independently around the world, and many industries have stuck with their traditional units.

The core of the issue is the lack of a single, universally adopted measurement system throughout history. The Imperial system, with units like pounds and inches, evolved in the British Empire and remains standard in the United States. This is where psi (Pounds per Square Inch) comes from. In contrast, most of the world adopted the Metric system, which led to units based on meters and kilograms. The official scientific evolution of this is the SI system (Système International d'Unités), which gives us the pascal (Pa). Because a single pascal is a tiny amount of pressure, we commonly use kilopascal (kPa). The bar is another metric unit, not officially SI but widely used in Europe because it's conveniently close to atmospheric pressure. Different industries simply adopted the unit common to their region and discipline and have continued to use it for consistency.

2. What are the 'Big Three' pressure units: psi, bar, and kPa?

When looking at specs, three units appear more than any others: psi, bar, and kPa. Without a clear reference, it’s difficult to visualize how they relate to each other or which one is larger.

These are the most common units in engineering. Briefly, psi is the Imperial standard common in the US. Bar is a metric unit popular in Europe, roughly equal to atmospheric pressure. kPa, or kilopascal, is the official global SI unit for pressure.

To make confident decisions, it's helpful to understand the definition and typical application of each of the "Big Three." Each one has a specific context where it feels most intuitive to the engineers using it. Think of them as different languages for expressing the same physical property. Here's a quick reference table to help you distinguish between them:

Unit	Full Name	System	Definition	Primary Application Area
psi	Pounds per Square Inch	Imperial	Force of one pound over one square inch.	US Industrial, Automotive
bar	bar	Metric	Exactly 100,000 pascals (~1 atmosphere).	European Engineering, Meteorology
kPa	kilopascal	SI	Force of 1,000 newtons over one square meter.	Scientific, Medical, Global Tech

As a rough guide, remember this: 1 bar ≈ 14.5 psi ≈ 100 kPa.

3. How is vacuum pressure different from positive pressure?

You need a pump for suction, and the datasheet lists a "maximum vacuum" of -80 kPa. The negative sign is confusing. How can pressure be a negative value, and how does this relate to positive pressure?

Positive pressure is force exerted above the surrounding atmospheric pressure, like inflating a tire. Vacuum is any pressure below atmospheric pressure. The negative sign indicates it's a "gauge" pressure, meaning it's measured relative to the atmosphere as the zero point.

Think of atmospheric pressure as "sea level." Any pressure above this is a positive altitude, and any pressure below it is a negative altitude. This is the concept of gauge pressure. Our atmosphere exerts a constant pressure on everything, around 101.3 kPa (or 14.7 psi) at sea level. A standard pressure gauge is calibrated to ignore this and read "0" in open air.

• Positive Pressure: When our miniature pump compresses air, it creates a pressure higher than the atmosphere. On a gauge, this is a positive value (e.g., +150 kPa).
• Vacuum (Negative Pressure): When the pump removes air from a sealed space, it creates a pressure lower than the atmosphere. On a gauge, this is represented as a negative value (e.g., -80 kPa). A value of -101.3 kPa would theoretically be a perfect vacuum at sea level.

4. Why are special units like mmHg or inHg used for vacuum?

Just when you think you understand vacuum as negative kPa, you see a medical device spec that lists vacuum in mmHg, or a weather report in inHg. Why do these legacy units still exist for vacuum?

These units, millimeters of mercury (mmHg) and inches of mercury (inHg), are legacy measurements from the first barometers. They persist in specific fields due to deep-rooted convention. mmHg is the standard in medicine, while inHg is common in US aviation and meteorology.

These units date back to Evangelista Torricelli's invention of the mercury barometer in the 17th century. He discovered that atmospheric pressure could support a column of mercury about 760 millimeters high. This direct physical measurement became a unit of pressure itself.

• mmHg (Millimeters of Mercury): Also known as a Torr, this unit is firmly established in the medical field. Blood pressure, for instance, is measured in mmHg worldwide. It provides a fine level of precision ideal for physiological measurements.
• inHg (Inches of Mercury): The Imperial equivalent of mmHg, it's still used as the standard for barometric pressure in US aviation and weather reporting. Pilots rely on it to calibrate their altimeters.

While modern pumps don't use mercury, these units are so ingrained in certain industries that manufacturers continue to provide specifications in mmHg and inHg to match their customers' expectations.

5. What is the difference between 'absolute' and 'gauge' pressure?

The term "absolute pressure" appears on some scientific datasheets, while others use "gauge pressure." This distinction seems subtle but can have a major impact on choosing the right pump for a high-precision application.

Gauge pressure uses the surrounding atmospheric pressure as its zero point. Absolute pressure uses a perfect vacuum as its zero point. Therefore, an absolute pressure reading will always be higher than a gauge pressure reading by the amount of the current atmospheric pressure.

This is one of the most critical concepts in pressure measurement. Let's use a table to clarify the difference:

Feature	Gauge Pressure	Absolute Pressure
Zero Point	Ambient Atmospheric Pressure	Perfect Vacuum (0% atmosphere)
Vacuum Reading	A negative number (e.g., -50 kPa)	A positive number above zero (e.g., 51.3 kPa abs)
Atmosphere	Reads as "0"	Reads as ~101.3 kPa abs or ~14.7 psia
Common Use	Most industrial uses (tires, compressors)	Scientific, medical, high-altitude applications

The key takeaway is that gauge pressure can change if you go up a mountain (where atmospheric pressure is lower), but absolute pressure will not. For most standard miniature pump applications, gauge pressure is sufficient. However, for vacuum deposition or scientific analysis where precision is paramount, specifying in absolute pressure is essential.

6. How can I accurately convert between pressure units?

You need to compare two pumps, but one is rated in bar and the other in psi. You need to make a quick and accurate conversion, as a small mistake could lead to choosing an underpowered or overpowered pump.

While you can use rough estimates for a quick check (1 bar ≈ 14.5 psi), you should always use a reliable online conversion calculator or a detailed conversion chart for final specifications. Precision is critical in engineering.

Manual calculations can introduce rounding errors, which can be problematic in sensitive applications. However, understanding the conversion factors is important. Here is a more precise table for reference. To use it, find your starting unit on the left and multiply it by the factor in the corresponding column to get your desired unit.

From Unit	To Get psi	To Get bar	To Get kPa
psi	1	0.0689476	6.89476
bar	14.5038	1	100
kPa	0.145038	0.01	1

Pro Tip: When I'm working with a new client, I always confirm the units before diving into the technical details. I'll even put both sets of units on our initial quotes, for example, "Target Pressure: 2.1 bar (30.5 psi)," just to ensure everyone is on the same page from the very beginning.

7. Which pressure unit should I use for my project's documentation?

You are creating the specification sheet for a new device. You want to use the most appropriate pressure unit to avoid confusion for your team, your customers, and your component suppliers.

The best practice is to use the unit most common in your device's target industry and primary sales region. For global products, listing the primary unit followed by one or two common equivalents in parentheses is the clearest approach.

Choosing the right unit is about clear communication. Following conventions prevents misunderstandings that can lead to costly design errors. Here’s a simple decision-making guide:

• Step 1: Identify Your Primary Industry.
  • Medical? Default to kPa or mmHg.
  • Industrial Automation? Use psi in the US, bar in Europe.
  • Consumer Electronics? kPa is often a safe bet.
  • Automotive? A mix of psi (tires) and bar (engine sensors).
• Step 2: Identify Your Primary Market.
  • North America? Lead with psi.
  • Europe? Lead with bar.
  • Asia / Global? Lead with an SI unit like kPa or bar.
• Step 3: Combine and Clarify.
  • For a medical device sold globally, the best format would be: Maximum Vacuum: -80 kPa (-600 mmHg).
  • For an industrial tool sold in both the US and Europe: Required Pressure: 7.0 bar (101.5 psi).

This simple practice of providing conversions shows foresight and makes your documentation universally understandable.

8. Are certain pressure units standard in specific industries?

You're designing a device for a new market and want to "speak the language" of that industry. Using the wrong units on your datasheet could make your product seem out of touch or difficult to integrate.

Yes, absolutely. Adhering to industry-specific unit standards is crucial for clear communication and compatibility. For example, the medical field predominantly uses kilopascals (kPa) and millimeters of mercury (mmHg), while industrial pneumatics in the US heavily relies on psi.

Using the expected unit shows that you understand the application's context. It makes it easier for engineers to compare your product to established benchmarks. Here is a breakdown of common industry standards that we encounter regularly at BODENFLO:

Industry	Primary Pressure Unit(s)	Common Application Examples
Medical Devices	kPa, mmHg	Blood pressure monitors, vacuum therapy (NPWT)
Industrial Automation	psi (in US), bar (in EU)	Pneumatic actuators, robotic grippers
Automotive	psi, bar	Tire pressure, MAP sensors, fuel & oil pressure
HVAC	kPa, Inches of Water (inH₂O)	Air duct pressure, filter monitoring
Aerospace & Aviation	psi, inHg	Cabin pressure, hydraulic systems, altimeters

When we develop a new pump series, we often create datasheets with different primary units depending on which industry we are targeting with that specific model.

9. What does 'max pressure' on a datasheet really mean?

You see a pump listed with a "maximum pressure" of 3.5 bar. You assume this is the pressure the pump will deliver in your device, but in testing, you can't seem to reach that level.

"Maximum pressure" (or max vacuum) on a datasheet typically represents the pump's capability at zero flow, also known as dead-head. It's the highest pressure potential the pump can generate when its outlet is completely blocked. The actual operating pressure will be lower.

This is a critical point that can trip up even experienced engineers. A pump's datasheet shows its maximum potential, not its performance in a specific application. The relationship between pressure and flow is described by a performance curve.

• Maximum Pressure: This occurs when the flow rate is zero (the outlet is blocked). The pump is working its hardest but not moving any gas.
• Maximum Flow: This occurs when the pressure is zero (the outlet is completely open to the atmosphere). The pump is moving the most gas but not building any pressure.
• Operating Point: In your actual device, there is resistance (from tubing, valves, etc.), so the pump will operate at a point somewhere in the middle of the curve, delivering a certain flow rate at a certain pressure.

When selecting a pump, you must ensure your required "operating point" (e.g., 5 L/min at 1.5 bar) falls comfortably within the pump's performance curve, well below the maximum pressure limit.

10. Does altitude affect a mini pump's pressure and vacuum performance?

Your device needs to function correctly in Denver (high altitude) as well as in Shanghai (sea level). Will the pump's vacuum performance, specified in gauge pressure, be the same in both locations?

Yes, altitude significantly affects pump performance, especially for vacuum. A pump's gauge pressure and vacuum ratings are relative to the local atmospheric pressure. At higher altitudes, where atmospheric pressure is lower, a pump's maximum achievable vacuum (in gauge pressure) will decrease.

This happens because the pump works by creating a pressure differential relative to the outside air.

• At Sea Level: Atmospheric pressure is ~101 kPa. A pump rated for -85 kPa gauge vacuum is creating an absolute pressure of (101 - 85) = 16 kPa abs.
• At High Altitude (e.g., 3,000 meters): Atmospheric pressure drops to ~70 kPa. That same pump can still only reach the same absolute pressure of 16 kPa abs. Therefore, its measured gauge vacuum is now only (70 - 16) = -54 kPa.

The pump's "strength" in absolute terms hasn't changed, but because the starting atmospheric pressure is lower, its performance relative to the atmosphere (gauge vacuum) is reduced. If your device must achieve a specific gauge vacuum at high altitude, you must select a pump with a much deeper maximum vacuum rating at sea level to compensate for this effect.

🧩 Conclusion

Understanding pressure units is about ensuring clear communication and precise engineering. By knowing why they exist and how to convert them, you can confidently specify the perfect miniature pump for your application every single time.

At BODENFLO, we speak every unit of pressure — whether your project calls for psi, bar, kPa, or mmHg, our technical team ensures accurate interpretation and optimal product matching. Our miniature diaphragm pumps are available in a wide range of pressure and vacuum levels, with customizable specs, certified quality, and global compatibility.

👉 Need support with selecting the right pump or converting technical specs across regions?
Our experts are here to help.

📩 Contact us today at info@bodenpump.com
🔎 Explore more: bodenpump.com