What Base Pressure Means for Roots Vacuum Pumps

In vacuum equipment procurement and acceptance testing, pressure metrics are often the parameters of greatest concern for both buyer and seller. Yet the concept of ultimate pressure, long used across the industry, suffers from ambiguous definition and impractical measurement. As an experienced Roots vacuum pump manufacturer, we frequently encounter customer questions about pressure specifications in export projects, particularly how to define performance when a Roots pump is combined with a backing pump. This article explains the concept of base pressure, its influencing factors, and measurement methods, drawing on ISO standards and manufacturing practice to help international B2B decision-makers understand and accept equipment more accurately.

What Is the Difference Between Ultimate Pressure and Base Pressure

Ultimate pressure has long been regarded as a core performance metric for vacuum pumps, but the concept itself has inherent limitations. As noted in ISO 21360-1, ultimate pressure is an asymptotic pressure value in the test dome, theoretically the lowest pressure a pump can achieve, yet there is no practical measurement method or specification to determine it. In other words, ultimate pressure is a value that is approached infinitely but never truly reached, lacking operability in actual acceptance testing.

ISO 21360-1 therefore introduced the new concept of base pressure. It can be understood as the pressure value that tends to stabilize in a standard test dome when the pump operates under specified conditions without gas introduction, measured in pascals. Compared with ultimate pressure, base pressure offers several practical advantages:

• It removes the notions of lowest and limit, instead emphasizing the observable state of tending toward stability

• It is a practically measurable value rather than a theoretical asymptote

• It provides a reproducible reference point for both manufacturers and users during acceptance testing

• It reduces disputes between buyer and seller by establishing a clear, testable performance benchmark

From a manufacturing and acceptance perspective, replacing ultimate pressure with base pressure carries clear practical significance. Especially for vacuum pumps used alone and discharging directly to atmosphere, such as slide valve pumps, rotary vane pumps, liquid ring pumps, piston pumps, scroll pumps, claw pumps, wet-type Roots pumps, and air-cooled Roots pumps, adopting base pressure in performance labeling significantly reduces acceptance disputes. Given that industry habit takes time to shift, current Roots pump standards still list base pressure as a main performance indicator, but the direction is clear.

Why Base Pressure Is Not an Independent Metric for Roots Pumps

It is particularly important to emphasize that for Roots vacuum pumps, base pressure is not an inherent performance metric of the pump itself. Roots pumps typically function as booster pumps in a system, and the base pressure they can achieve depends on several external factors:

• The type of backing pump configured and its own base pressure level

• The temperature conditions under which the system operates

• The matching ratio between Roots pump pumping speed and backing pump pumping speed

• The sealing performance of the entire system, including piping and connections

This means that when labeling a Roots pump's base pressure, the type of backing pump configured must be specified simultaneously. Discussing a Roots pump's base pressure in isolation from its backing pump has no practical meaning. In our product technical documents, we clearly list system base pressure data for different backing pump combinations, allowing customers to select the appropriate configuration for their operating conditions.

How the Backing Pump Affects a Roots Pump's Base Pressure

The backing pump's performance directly determines the pressure level on the Roots pump's discharge side, which in turn affects the ultimate vacuum of the entire system. The lower the backing pump's base pressure and the better matched its pumping speed, the better the base pressure the Roots pump system can achieve.

We encountered a typical case in an early export project. A customer reported that our Roots pump's base pressure was inferior to comparable foreign products. After on-site investigation, we found the problem lay in the rotary vane backing pump. The investigation followed several steps:

• We first measured the Roots pump alone and confirmed its internal clearances and rotor condition were within specification

• We then tested the backing pump separately and found its sealing performance and oil separation efficiency were below expected levels

• We replaced the backing pump with a higher-quality unit and re-tested the complete system

• The Roots pump system's base pressure then fully reached the advanced level of comparable foreign products

This case fully demonstrates that base pressure is not an inherent characteristic of the Roots pump itself but a reflection of the system's combined performance. For industrial vacuum pump users, purchasing a Roots pump should not focus solely on the pump's own parameters, but also on backing pump selection and matching. During project consultation, we recommend the appropriate backing pump type and size based on the customer's target vacuum level and pumping speed, ensuring overall system performance meets requirements.

The table below summarizes common backing pump types and applicable scenarios:

How to Measure Base Pressure Accurately

The instrument for base pressure measurement has undergone a technical upgrade. Previously, standards specified calibrated thermocouple vacuum gauges for total pressure measurement, but international standards have long mandated capacitance diaphragm gauges. Capacitance diaphragm gauges are now widely used and offer far higher precision than thermocouple gauges, so updated standards uniformly specify calibrated capacitance diaphragm gauge readings for total pressure measurement.

Key points for measurement include:

• Test dome volume and connecting piping must comply with standard specifications to avoid affecting pressure stabilization time due to improper system volume

• Before measurement, ensure the pump has sufficiently removed residual gas and water vapor from the system, and take readings only after pressure stabilizes

• The gauge range should match the pressure range being measured, with low-range sensors used for high-precision measurement

• The entire measurement system must be reliably sealed, as any micro-leak will cause elevated readings

• Ambient temperature should be stable during measurement, as temperature fluctuations can affect gauge readings

What to Watch for When Using Capacitance Diaphragm Gauges

Capacitance diaphragm gauges are structurally divided into reference-type and relative-type, with different operation and maintenance requirements.

Reference-type gauges contain an internal high-vacuum reference capsule. The reading is actually the result of comparing the sensor measurement against the reference capsule pressure. Key characteristics include:

• The reference capsule's vacuum slowly degrades over time due to outgassing and leakage

• The gauge zero must be calibrated frequently, especially for low-range 100 Pa and 10 Pa sensors

• The reference capsule should undergo high-vacuum treatment or replacement at intervals based on usage conditions

• Calibration intervals should be shorter for gauges used in corrosive or dusty environments

Relative-type gauges replace the reference capsule with a high-vacuum pump, such as a molecular pump. Their advantages include:

• Zero calibration is not required because the reference is continuously maintained by the pump

• Long-term stability is generally better than reference-type gauges

• They are preferred for laboratories and facilities with the necessary infrastructure

Regardless of type, the complete gauge, including sensor, display unit, and cables, must be periodically sent to an authorized metrology authority for verification to ensure traceability and accuracy of measurement data.

As a vacuum pump manufacturer, we can provide technical guidance for base pressure measurement, including test dome design, instrument selection, and calibration interval recommendations, helping customers establish standardized acceptance measurement procedures. For customers purchasing multiple heavy duty vacuum pumps, we also offer on-site measurement training to ensure customer maintenance teams master correct measurement methods.

Conclusion

The base pressure of a Roots vacuum pump is a more practical performance metric than ultimate pressure, but it is not an independent parameter of the pump itself. Rather, it reflects the combined performance of the backing pump and Roots pump system. Understanding the concept of base pressure, the influence of the backing pump, and proper measurement methods helps customers make more accurate judgments during procurement and acceptance. We believe the value of a vacuum pump supplier extends beyond equipment delivery—it lies in standard interpretation and technical support that help customers establish a scientific vacuum performance evaluation system.