1. More advanced drive method:
The Hilintec SCPV adopts FOC (Field-Oriented Control) vector drive with sinusoidal current commutation, ensuring smooth operation, low heat generation, millisecond-level response speed, and excellent acoustic performance. In contrast, conventional stepper motors use open-loop step drive, which suffers from step loss, resonance, and noticeable noise.

2. Higher control accuracy:
The Hilintec SCPV incorporates a high-precision position sensor that provides micron-level closed-loop feedback control of the spool position, enabling real-time deviation correction and achieving repeatability within ±0.1% F.S. Conventional stepper proportional valves, however, rely on open-loop micro-step control, where the actual spool position is affected by load and friction, making accuracy difficult to guarantee.

3. Power-loss position memory:
The Hilintec SCPV retains the spool position memory even after power-off. Upon re‑powering, it can resume operation directly without the need for homing or zero‑seeking, thereby enhancing system safety and convenience. Conventional stepper valves lose position information after power-off and require a homing routine at every restart.

4. Integrated drive-and-control (all-in-one) design:
The Hilintec SCPV integrates the driver, control circuitry, and sensor entirely within the valve body. It accepts 4‑20 mA, 0‑5 V, or Modbus (RS485) control signals directly, ready for use without an external driver, saving both cost and installation space. Conventional stepper valves require a separate dedicated driver and controller, adding extra bulk and system cost.

Summary:
The Hilintec SCPV comprehensively outperforms conventional stepper proportional valves in drive efficiency, response speed, acoustic noise, closed-loop accuracy, power-loss memory, and integration level. It represents a complete generational upgrade over stepper-motor proportional valve products, and is particularly well‑suited for applications demanding high precision, high dynamic response, and high reliability. For upgrading existing stepper-motor proportional valve solutions, the SCPV series is the ideal choice.

In the testing and application of positive‑displacement pumps such as diaphragm pumps, you may often encounter this question: under the same operating conditions, the same pump gives a notably higher reading with a rotameter, a different value with a mass flowmeter, and yet another with a soap film flowmeter. In practice, the rotameter reading is commonly referred to as the “peak flow rate,” the soap film flowmeter reading as the “average flow rate,” and the mass flowmeter directly provides the “mass flow rate.” Three instruments, three numbers—which one truly represents the pump’s actual output? Why do these differences arise, and how do they affect your equipment selection and process control? This article will examine each of these factors in detail.

I. Root Cause of the Differences: The Interaction Between Airflow Pulsation and Measurement Principles

Diaphragm pumps are a typical type of positive‑displacement pump. Their operating principle involves periodic suction and discharge of gas via the reciprocating motion of the diaphragm. This intermittent operation inherently generates a strongly pulsating outlet flow—velocity fluctuates rapidly, and pressure rises and falls accordingly. When this unsteady airflow enters flowmeters based on different principles, each meter yields a different reading.

1. Why Does a Rotameter Show “Peak Flow”?

A rotameter consists of a tapered tube with a float inside. It indicates flow rate based on the lift force exerted on the float by the passing gas. Under ideal steady conditions, the flow rate corresponds one‑to‑one with the float’s equilibrium position. However, in a pulsating flow, when the instantaneous velocity surges, the float is pushed upward quickly; when the velocity drops, the float cannot promptly fall back to the position corresponding to the average flow, owing to its own inertia and wall friction. At the same time, the float responds more readily to high velocities than to low ones, so its time‑averaged suspended position is always biased toward the peak of the instantaneous flow. Therefore, the rotameter reading under pulsating flow is not the true average flow, but a higher value close to the peak—hence the term “peak flow rate.”

2. Soap Film Flowmeter – A Natural “Pulsation‑Immune” Average Flowmeter

The principle of the soap film flowmeter is extremely simple: a soap film acts as a piston that is pushed by the airflow through a glass tube of precisely known volume. By measuring the time it takes for the soap film to travel between two scale marks, the average flow rate is directly obtained as volume divided by time. In this process, the soap film itself has virtually no inertia and causes negligible pressure drop. Regardless of how the flow pulsates, it accurately integrates the total volume of gas delivered by the pump over a period and then averages it, thus providing the physically true average volumetric flow rate. The pulsation energy is completely smoothed out, giving a stable reading that represents the actual output.

3. Mass Flowmeter: A Hidden Prerequisite for an Accurate Average – Sampling Rate

Thermal mass flowmeters directly measure mass flow by sensing the heat carried away by the gas. In principle, they are not affected by instantaneous density changes caused by pressure pulsation, and most models offer an averaging function, making them seem ideal for pulsating flows. However, there is a critical and easily overlooked prerequisite for obtaining a truly accurate average mass flow rate: the sensor’s sampling rate must be sufficiently high.

According to the Nyquist sampling theorem, to fully capture a periodic signal of frequency *f*, the sampling rate must exceed 2*f*. For example, for a pump running at 600 rpm, the fundamental frequency is 10 Hz, meaning the sensor sampling rate should theoretically be greater than 20 Hz. However, in engineering practice, to reliably capture the details of the pulsation waveform and avoid aliasing errors, the recommended sampling rate is often at least 10 times the pulsation frequency. In other words, if the pump speed is 600 rpm, the sensor sampling rate should be at least 100 Hz before averaging to obtain a credible average mass flow rate.

If the sampling rate is insufficient, the sensor will “undersample” the rapidly varying flow, incorrectly folding high‑frequency pulsation signals into low‑frequency spurious signals. In that case, even if the averaging function is turned on, the calculated value will be severely distorted and cannot represent the true mass flow at all. Only when the sampling rate is high enough to accurately capture the instantaneous mass flow waveform and average it can a reliable average mass flow rate be obtained.

Even after obtaining an accurate average mass flow, another “conversion trap” remains: when comparing mass flow with volumetric flow under operating conditions, conversion must be performed using gas temperature, pressure, and molecular weight. In the pulsating flow of a diaphragm pump, pressure and temperature fluctuate continually, so using static parameters to convert a dynamic process inherently introduces deviations. In addition, humidity and differences between the actual gas composition and the instrument’s calibration gas can cause discrepancies between the converted volumetric flow and the actual humid‑air volumetric flow measured by the soap film flowmeter. Therefore, even if the mass flowmeter provides an accurate average mass flow, “translating” it into operating‑condition volumetric flow will still likely not match the soap film flowmeter result.

II. Practical Impacts of Inconsistent Readings

The differences among these three flowmeter readings are by no means just a numbers game. When you use a rotameter to evaluate pump performance, the inflated “peak flow” may lead you to overestimate the pump’s capacity margin, potentially causing insufficient gas supply downstream or incorrectly reducing pump speed, which affects process outcomes. If you rely on a mass flowmeter’s converted volumetric flow without proper correction, you might misinterpret the pump’s nominal displacement and mistakenly think the pump has degraded abnormally. The average volumetric flow given by the soap film flowmeter directly reflects the actual volume of gas delivered to the piping per unit time, and has a clear relationship with the product of pump stroke volume and rotational speed—making it the most straightforward and reliable indicator for evaluating positive‑displacement pump performance.

In short, inconsistent testing standards lead to confusion in acceptance criteria between suppliers and users, misaligned equipment commissioning, and ultimately increased communication and time costs.

III. Why Should the Soap Film Flowmeter Be the Standard?

Faced with these three distinct readings, the industry widely adopts the soap film flowmeter as the reference standard, for clear and compelling reasons:

• Direct principle, immune to pulsation – The soap film flowmeter is the only positive‑displacement flowmeter based on direct volume‑time measurement. Its measurement process is “naturally compatible” with the pulsating flow of diaphragm pumps, requiring no assumptions about dynamic response; it directly gives the true average volumetric flow.
• Benchmark accuracy – The soap film flowmeter is recognized as one of the primary standard devices for low gas flow rates. Its accuracy depends only on the tube volume and timing precision, both easily traceable to length and time standards. Systematic errors are minimal and controllable, which is why many metrology laboratories use it to calibrate other flowmeters.
• No conversion complexities – Measurements are taken directly at the actual temperature and pressure of the pipeline, yielding the operating‑condition volumetric flow that intuitively corresponds to the pump’s actual exhaust capacity. If comparison is needed, conversions to standard conditions are transparent and introduce none of the uncertainties due to sensor dynamic response or concerns about sampling rate adequacy.
• Industry consensus and ease of use – In fields such as environmental monitoring, pump R&D, and medical devices where small gas flow accuracy is critical, the soap film flowmeter has always been the “gold standard” for calibration and arbitration. Its physical process is clear and straightforward, requires no complex electronic corrections, and is more likely to gain trust from all parties.

IV. Final Thoughts: Make Good Use of Differences, but Unify the Standard

We are not suggesting that rotameters and mass flowmeters have no value. Rotameters are simple, intuitive, and excellent for online quick checks and for applications where pulsation is relatively mild. Mass flowmeters are indispensable when precise gas mass control is required (e.g., chemical reaction feed). The key is to understand their respective behaviours under pulsating flow and to establish a comparison chain with the soap film flowmeter as the benchmark.

Recommendations: For applications where a mass flowmeter must be used to measure pulsating flow, be sure to verify that the sensor’s sampling rate meets the engineering requirements for the pulsation frequency corresponding to the pump speed, ensuring that the source data for the average value is authentic and accurate. Likewise, a rotameter should be understood as an “indicating instrument” rather than an absolute measuring device. We suggest that in daily use, you use the soap film flowmeter to determine the pump’s actual average volumetric flow as the basis for acceptance and calibration, then use this benchmark to calibrate or correct rotameters on the production line, or to provide reliable operating‑condition conversion parameters for mass flowmeters. In this way, each flowmeter can serve its own purpose, ending the numerical confusion and solidifying the foundation for your process quality and equipment management.

To facilitate customers in selecting and estimating product flow rates, Hailin Technology’s air pump series are all marked with both the “peak flow rate” measured with a glass rotameter and the “average flow rate” measured with a soap film flowmeter. The two values may differ across different products, but both are actual measured data. If you need further assistance in model selection, please contact Hailin Technology’s pre‑sales engineers.

In fact, this is neither a malfunction nor false advertising—it is a common yet easily misunderstood phenomenon in fluid machinery. To understand it, we need to revisit a fundamental concept: the rated maximum output pressure is not equal to the ultimate pressure the pump can physically generate.

Rated Pressure: The “Red Line” for Safe Operation

The “maximum output pressure” listed on the specification sheet typically refers to the maximum output pressure that the pump is allowed to withstand or maintain under long-term, continuous, and reliable operating conditions.

In other words, this is the “safety red line” set by engineers. Within this red line, the pump’s diaphragm, valves, seals, motor, and other components can operate stably within their expected service life. Once exceeded, the pump may face risks such as diaphragm rupture, valve failure, increased leakage, or even motor stall and burnout.

Therefore, the 0.1/0.3/0.5 MPa indicated on the specification sheet is the recommended operating upper limit, not the physical ceiling of pressure.

Incompressibility of Liquids: The “Infinite Amplifier” of Pressure

Why can pressure easily exceed this upper limit? The key lies in a fundamental property of liquids: incompressibility.

Gases can be compressed. When pumping air, it becomes increasingly difficult as you approach the end, and the pressure rise has a clear physical limit because the gas volume is decreasing. However, liquids are different—under conventional pressures and temperatures, the volume of a liquid hardly changes with pressure.

When the outlet of a diaphragm liquid pump is blocked (e.g., a valve is closed or the tubing is clogged), the pump continues to run, and the diaphragm keeps pushing forward against the liquid. Since the liquid has nowhere to go and is nearly incompressible, the pressure surges dramatically in an instant.

Theoretically, if the pump structure is strong enough, the motor torque is sufficient, and the tubing and seals are completely leak-free, the pressure can keep rising until:

• The tubing bursts
• The diaphragm tears
• The fittings leak
• The motor stalls due to insufficient torque

In other words, the pressure can theoretically increase indefinitely—until some weak link in the system fails.

Measured Pressure Exceeding the Rated Value Is a Manifestation of Physical Laws

Therefore, when you block the outlet and measure the pressure, a reading that exceeds the rated value precisely proves that:

• The pump’s seals and structure can temporarily withstand higher pressures.
• The motor has not yet completely stalled and is still delivering torque.
• The incompressibility of the liquid is taking effect.

This “pressure beyond the rated value” is essentially an instantaneous or short-term pressure value generated when the pump is challenging its own limits. It does not represent the pump’s normal operating capacity, but rather a warning signal approaching the failure boundary.

Why Don’t Manufacturers Set the Rated Pressure Higher?

Because the setting of the rated pressure must balance lifespan, reliability, power consumption, and cost.

• Diaphragm materials, valve plates, pump head structures, and seals all have fatigue lives. The higher the pressure, the shorter the lifespan.
• Higher pressure increases motor heat generation and reduces efficiency.
• The pump body and fittings require stronger structures, significantly increasing cost and weight.

The rated value provided by the manufacturer is the result of balancing “how long it can last” against “how much pressure it can withstand.”

How to Prevent Uncontrolled Pressure Overshoot?

In many practical applications—such as precision instruments, medical devices, and analytical testing equipment—excessively high pressure can damage downstream components. There are three common methods to address this issue:

1. Install a Pressure Relief Valve (Overflow Valve)
Connect an adjustable pressure relief valve in parallel to the pump’s outlet line. When the pressure exceeds the set value (e.g., 0.3 MPa), the valve opens, diverting part of the liquid back to the reservoir or to atmosphere, thereby limiting the maximum system pressure. This is the most common and reliable approach.

2. Use a Backpressure Safety Valve
Connect a backpressure valve in series in the outlet line. It acts like a “pressure fuse”: under normal pressure, it maintains the path; when pressure exceeds the limit, it automatically opens to relieve pressure. Compared to a relief valve, a backpressure valve is more suitable for applications that require maintaining a certain backpressure.

3. Choose a Low-Torque Motor with Stall Protection
If the pump itself uses a low-torque motor that is designed to allow short-term stalling (with current controlled within a safe range), then when the pressure becomes too high and the motor stalls, the pressure will naturally stop rising. This approach eliminates the need for external valves, but you must confirm whether the motor specification allows frequent or prolonged stalling; otherwise, there is still a risk of burnout.

Summary

The core logic to remember in one sentence:

The rated maximum output pressure of a diaphragm liquid pump is “how high it can safely operate,” not “how high it can actually hold.” Liquids are incompressible—block the outlet, and the pressure will race toward the failure boundary.

Once you understand this, you will no longer be alarmed by the phenomenon of “measured pressure exceeding the rated value.” Instead, you will recognize it as the pump telling you: it’s time to install a pressure relief valve.

Further Insight: Safety Margins and Engineering Lessons in Design

From another perspective, this characteristic also reminds engineers that when selecting a pump, you cannot look only at the rated maximum pressure; you must also pay attention to the pressure peaks under abnormal operating conditions. Without a pressure relief path, the “self-pressurizing” capability of a diaphragm pump is sufficient to damage precision valves or sensors. Therefore, when integrating a diaphragm liquid pump, it is advisable to reserve a safety valve port on the outlet line, or directly select a pump model with a built-in pressure relief circuit, to fundamentally avoid overpressure risks.

📌 Engineering Recommendation: For any liquid system involving closed chambers, valve switching, or the possibility of accidentally closing the outlet, be sure to install a mechanical pressure relief valve between the pump and the downstream components (set at a pressure slightly higher than the maximum system working pressure but lower than the pump’s burst pressure). This is both a measure to protect components and a key measure to extend pump life.

2. Heavy Loads Can Cause Stalling
If the pump is operating under a heavy load—for instance, due to high discharge pressure, high vacuum or positive pressure output, or increased pipeline resistance—the torque available from the motor may become insufficient to keep the pump rotating. As a result, the pump head can come to a forced stop, leading to a locked-rotor condition.

3. The Driver Triggers a Latched Protection Mode
Motor drivers continuously monitor the system’s status through current sensing, back-EMF detection, speed feedback, and other diagnostic methods. Once a stall is confirmed, the driver immediately activates a hardware-level protection mechanism and latches this fault state. Most drivers employ a non-self-recovering latching scheme. This means that even if you subsequently increase the PWM command to request higher speed, the protection state will not clear automatically, and the motor will remain unresponsive.

4. Only a Power Cycle Can Clear the Latched Fault
To exit the stall-induced latched state, a complete power cycle—fully disconnecting and then reapplying power—is required. This operation clears the driver’s fault memory and reinitializes the control logic. Therefore, simply adjusting the PWM signal is not sufficient to restore motor operation; a power-cycle reset is the only reliable way to restart the system.

The equipment used typically includes a Buchner funnel, a filtration flask, and a vacuum pump. When selecting an appropriate micro vacuum pump, two main factors should be considered:
(1) Consider the pressure resistance of the funnel and filter paper to determine the required vacuum level for the pump.
(2) Consider the power supply for the application scenario. For example, when used in the field without mains power, a low-power micro vacuum pump powered by batteries can be chosen.
Furthermore, when using a micro vacuum pump in this scenario, customers need to prevent liquids and other solid impurities from entering the pump chamber, as this can affect the pump’s service life.

Both aeration and gas washing usually require the exhaust tube to be submerged below the liquid surface to achieve optimal results. Therefore, this application requires an air pump capable of delivering positive pressure. Depending on the required flow rate, our series of combined pressure/vacuum pumps, such as the D15, D23, D35, D50, D60 models, are most suitable.

When selecting a pump, it’s crucial to consider the water pressure at the exhaust port (Pump Output Pressure = Actual Required Pressure + Water Pressure). The water pressure is determined by the depth of the gas outlet below the liquid surface; the deeper the outlet, the higher the output pressure required from the pump. When the pump’s output pressure equals the water pressure, the pump will deliver zero flow, as the flow rate decreases with increasing pressure.