Measuring Compressor Performance in the Field with a Sonic Nozzle

At the Industrijska konferenca iz pnevmatike 2026 in Slovenia, CALMS experts Gorazd Bregar and Rok presented our approach to measuring actual compressor performance directly in the field.

Their presentation, “Compressor Performance Measurements in the Field with a Sonic Nozzle,” focused on a fundamental question in compressed air optimization:

Is the compressor still performing the way it was designed to perform?

Manufacturer data provides an important reference point, but real operating conditions change over time. Compressor age, operating pressure, inlet conditions, control strategy and system losses can all influence actual performance.

That is why field measurement is an important step before making decisions about control optimization, servicing, overhaul or replacement.

Manufacturer Data Is the Starting Point

A compressor manufacturer’s data sheet provides the reference factory performance of the machine.

Typical values include:

• Free Air Delivery (FAD)
• Discharge pressure
• Electrical input power
• Specific power
• Isentropic efficiency

Reference performance is commonly determined according to standardized methods such as ISO 1217, which defines compressor performance testing and acceptance tolerances.

These values tell us how the compressor was designed to perform under defined reference conditions.

But they do not necessarily tell us how the compressor performs today.

Actual field performance can differ because of operating conditions, ageing, maintenance condition and control behavior.

Why System Monitoring Alone Is Not Enough

A compressed air audit can tell us a lot about the overall system.

We can measure total power consumption, system flow, pressure and demand patterns. However, system-level data does not necessarily identify the performance of each individual compressor.

Two different combinations of compressors can produce almost the same flow while requiring very different amounts of electrical power.

For example, during system analysis we may observe almost identical air demand while one operating profile requires approximately 600 kW and another approximately 800 kW.

The difference can come from:

• Operating pressure
• VSD operating point
• Unloaded losses
• Blow-off losses
• Compressor sequencing

This creates an important diagnostic question:

Is poor efficiency caused by the compressor itself, or by the way the system is being controlled?

Before optimizing the complete compressed air system, we need to understand the actual performance characteristics of the individual machines.

A Performance Test Is More Than One Measurement

A compressor performance test should not provide only one operating point.

The objective is to create an actual performance curve showing how the compressor behaves across its operating range.

During a field test, we measure or determine values such as:

• Discharge pressure
• Actual flow / FAD
• Electrical input power
• Specific power
• Isentropic efficiency
• Available capacity
• Unloaded power

The exact test procedure depends on the compressor type.

VSD Compressors

For a variable-speed compressor, several stable operating points can be measured across the complete operating range.

This provides a much better understanding of how efficiency changes depending on compressor load.

Fixed-Speed Compressors

For fixed-speed machines, measurements at full load and unloaded operation are particularly important.

Unloaded power can represent a significant energy loss if the compressor spends too much time running without producing useful compressed air.

Centrifugal Compressors

For centrifugal compressors, measurements can be performed across the complete safe operating range.

Depending on the machine, this may include:

• Minimum stable flow
• Different IGV positions
• Rated operating point
• Full flow
• Blow-off operation
• Surge limit, where safe to test

The result is a practical compressor performance curve that can be used for benchmarking, diagnostics, simulation and further system optimization.

Using a Sonic Nozzle for Compressor Flow Measurement

One of the central topics of the presentation was the use of a sonic nozzle according to ISO 9300.

A sonic nozzle provides a stable and repeatable method for measuring gas flow.

Its geometry consists of a smoothly converging inlet, a precisely machined throat and a diffuser.

Pressure and temperature are measured upstream of the nozzle.

When the pressure ratio across the nozzle is sufficient, the flow at the throat becomes choked, meaning the gas reaches sonic velocity at the nozzle throat.

Under these conditions, mass flow depends primarily on:

• Upstream absolute pressure
• Upstream temperature
• Nozzle throat area
• Gas properties
• Discharge coefficient

This makes the sonic nozzle a useful reference method for compressor flow verification.

Instead of relying on an installed flowmeter of unknown accuracy, we can create an independent reference measurement of the compressor’s actual delivered flow.

How a CALMS Field Performance Test Works

The CALMS field measurement concept combines the existing compressor installation with a portable measurement section containing the required sensors, data acquisition equipment and sonic nozzle.

During the test, CALMS measures several key parameters.

Flow / FAD

Actual compressor flow is determined using the sonic nozzle together with measured operating conditions.

The measured mass flow can then be converted to standardized Free Air Delivery conditions.

Total Package Power

Electrical input power is measured directly at the compressor.

This is essential because flow alone does not tell us whether the compressor is operating efficiently.

Discharge Pressure

Pressure is measured at a defined measurement point during every operating condition.

Inlet and Ambient Conditions

The measurement also considers:

• Inlet or ambient temperature
• Atmospheric pressure
• Humidity, where correction is required

Together, these measurements allow us to create the actual compressor performance curve under real operating conditions.

We can then compare two very different datasets:

Factory performance - How was the compressor designed to perform?

Field performance - How does the compressor actually perform today?

That comparison provides significantly more information than a simple system audit alone.

Field Case Study: Testing a 110 kW Compressor

During the presentation, CALMS also shared a practical field case from the chemical industry in Slovenia.

The tested machine was an Ingersoll Rand RS110ie, rated at 110 kW and capable of operating at pressures up to 14 bar.

The objective was to verify the actual delivered flow and validate the sonic-nozzle measurement concept.

The measurement setup included:

• ISO 9300 sonic nozzle
• Pressure measurement
• Temperature measurement
• Power measurement
• CALMS Edge / data acquisition equipment

The compressor was tested across several operating pressures.

During the test, measured flow remained very stable at approximately 17 m³/min.

The measured value was within approximately 2% of the expected compressor performance, demonstrating that the portable measurement concept can provide repeatable and technically useful results in real field conditions.

The estimated measurement uncertainty of the tested setup was approximately ±0.3 Nm³/min, or around ±2%.

Turning Measurements Into Decisions

Measurement itself does not save energy.

Its value comes from using the results to make the right technical decision.

Once actual flow, power and pressure are known, we can determine whether the compressor requires:

• Service
• Overhaul
• Replacement
• Control optimization

For example:

FAD Below Expectations

Lower-than-expected delivered flow can indicate:

• Compressor wear
• Internal losses
• Valve issues
• Filter problems
• Other performance degradation

The appropriate response may be service or overhaul.

High Specific Power

High specific power indicates that the compressor requires more energy than expected to produce a given amount of compressed air.

Depending on the cause, the solution may involve:

• Optimization
• Service
• Overhaul
• Replacement

High Unloaded Power

A compressor consuming significant power while unloaded may indicate unnecessary control losses.

In this situation, improving sequencing or compressor control can often be more valuable than modifying the compressor itself.

Unstable Operating Points

Unstable operation can indicate either mechanical limitations or an unsuitable control strategy.

Further diagnostics can help determine whether control adjustments or mechanical intervention are required.

Better Measurements Also Mean Better Control

Field performance measurements are not only useful for maintenance decisions.

They also improve the quality of compressed air system simulations.

Instead of using theoretical compressor curves, we can use measured values for:

• Flow
• Power
• Pressure
• Available capacity
• VSD operating range
• Unloaded power

This creates a more realistic model of the compressed air system.

More realistic compressor models allow us to improve:

• Compressor sequencing
• Pressure setpoints
• VSD load sharing
• Control algorithms
• Simulation accuracy

In other words:

better measurement data leads to better control decisions.

A Portable Reference for Future Projects

One of the main goals behind the CALMS sonic-nozzle concept is portability.

The complete measurement section can be transported between sites and used as a mobile reference for different applications.

Potential use cases include:

• Compressor performance verification
• Verification after servicing
• Verification after compressor overhaul
• Energy audits
• Commissioning
• Acceptance testing
• Installed flowmeter verification
• Compressor benchmarking
• Improving simulation inputs
• Improving control algorithms

The concept therefore goes beyond a single compressor test.

It creates a repeatable method for independently verifying compressor performance across different sites and equipment.

Future development will focus on extending the concept to additional compressor sizes and flow ranges, developing additional sonic-nozzle measurement sections and further standardizing the test procedure within the CALMS platform.

Measure First. Optimize Second.

Compressed air optimization should not start with assumptions.

Before changing compressor sequencing, replacing equipment or investing in a new control system, it is important to understand how the existing compressors are actually performing.

A reliable field performance test helps answer two very different questions:

Is the compressor itself inefficient?

or

Is the system controlling an otherwise healthy compressor inefficiently?

The answer can completely change the recommended investment.

At CALMS, our goal is to combine measurement, system analysis, simulation and control optimization so that compressed air decisions are based on actual operating data rather than assumptions.

Want to Know How Your Compressors Are Actually Performing?

Contact the CALMS team to learn more about field compressor performance testing, compressed air audits and system optimization.