Why Compressor Current Logging Doesn’t Tell the Whole Story

Hero image: illustrative visual of compressed air monitoring equipment, not a photograph of a WMC site survey.

Compressor current logging can reveal useful operating patterns. It cannot, on its own, tell you everything about the air your factory receives or what that air costs to produce.

Before using a logging report to select a replacement compressor or justify an upgrade, ask which results were measured and which were calculated. A graph can look precise even when the conclusion depends on assumptions.

What current logging can tell you

A current logger records electrical current over time. On a suitable fixed-speed compressor, interpreted alongside its controls, this can help identify stopped, loaded and unloaded periods. It can highlight frequent cycling, long running hours and activity outside production shifts.

Software may estimate air production using these operating periods and the compressor’s performance data. That can be a useful starting point, provided the machine details, control behaviour and assumptions are appropriate. Estimated output is not the same as directly measured airflow.

Look beyond the compressor room

A useful assessment follows the air from generation to the equipment using it. Agree which parts of the system need investigation: compressor controls, treatment, receiver storage, distribution and production requirements.

Measurements need site context. A layout showing instrument positions helps explain what each reading covers. Review leaks and drains, required pressure and air quality, and whether each air use is necessary. Storage and control behaviour should be considered together when investigating demand peaks.

The scope should follow the problem and the decision being made. A pressure complaint, a capacity upgrade and an assessment of running costs may require different checks.

What WMC measures during a compressed air audit

WMC combines compressor current logging with airflow and pressure logging to build a fuller picture of how the system operates. We do not directly measure electrical power in kW during our audits. Any energy or running-cost figures derived from current and compressor data are estimates, with the assumptions clearly stated.

These describe available measurement types, not customer readings or site survey results. The scope depends on the purpose of the assessment.

How current relates to power and energy

Electrical current, power and energy are related. Current is measured in amps, real electrical power in kilowatts, and energy consumed over time in kilowatt-hours. Current alone does not establish real power: voltage and power factor also affect the relationship.

A current trace, suitable compressor performance data and clearly stated assumptions can support an estimate of energy use. It should not be presented as directly measured kW or kWh. Variable-speed operation makes a simple fixed conversion from amps particularly unreliable because operating conditions change.

WMC uses current, airflow and pressure logging together to assess operating patterns, air demand and supply performance. Where energy costs or potential savings are estimated, the report should identify the data and assumptions behind the figures. A decision requiring directly measured electrical energy would need a separate, suitably scoped power measurement exercise.

What combined logging helps you decide

Current, airflow and pressure readings are most useful when recorded over the same period and compared with what production was doing. If pressure falls, airflow shows how demand changed while current indicates how the compressor responded. Together, these readings help investigate the cause rather than simply record the symptom.

Choose capacity around actual demand

Minimum, average and peak airflow help establish how much air the site needs and how demand varies between shifts. The duration of peaks matters too: a brief surge and a sustained increase may need different solutions. This evidence supports compressor sizing and helps assess planned expansion, alongside the requirements of any new equipment.

It can also help avoid buying a larger compressor before addressing avoidable demand, storage or distribution problems. The aim is sufficient capacity at the required pressure, with an appropriate allowance for resilience and future needs.

Review compressor combinations and controls

Comparing the demand profile with compressor operating patterns helps assess whether the existing machines work together effectively. Long unloaded periods, frequent cycling or several machines running during low demand can highlight opportunities to investigate control settings and sequencing.

The same evidence helps assess whether a fixed-speed compressor, a variable-speed machine or a combination suits the site. A variable-speed compressor is not automatically the best answer: its operating range and performance need to match the measured demand.

Investigate pressure losses before increasing pressure

Logging pressure at two relevant points while recording airflow helps show whether pressure losses increase as demand rises. Depending on the measurement locations, this can guide investigation of pipework, filters, dryers or other restrictions.

Addressing a restriction may allow the required pressure to reach production without raising the compressor set point. Any pressure reduction should be checked against the needs of critical equipment during representative operating conditions.

When is dew-point measurement useful?

Dew-point checks are optional and should answer a specific question. They are useful when moisture is causing problems, dryer performance needs checking or a process has a defined air-dryness requirement. The measurement location and pressure must be considered when interpreting the result.

They can also support an assessment of dryer energy-saving opportunities. On compatible desiccant dryers, dew-point-based controls can extend drying cycles when conditions allow, reducing unnecessary regeneration and associated purge-air or energy use. Audit measurements can help assess this opportunity and verify that the required dryness is maintained after changes.

A temporary dew-point sensor does not itself control the dryer or measure energy savings. Recommendations depend on the dryer design, existing controls and operating conditions. A low dew-point reading alone does not prove that energy is being wasted.

Compressor output is not the same as production demand

Airflow measured in a main supply line describes the air crossing that point. It can include production use, leaks and other demand downstream. Receiver storage also means that compressor output and immediate factory consumption need not match at every instant.

For that reason, ask where the flow meter sits and what the reading represents. If air volume is calculated from a compressor’s nominal output instead, ask how the estimate accounts for operating pressure, control mode and actual machine condition.

A current trace cannot explain a pressure complaint

Consider a workshop reporting weak air tools while the compressor appears to run normally. Reviewing current alone leaves an unanswered question: what pressure reaches the tools during the problem?

A sensible investigation would compare pressure at relevant points while the demand occurs. That helps assess whether attention should go to supply capacity, controls, storage or a restriction in the distribution system. Increasing compressor size before understanding the complaint could leave the underlying issue unresolved.

Out-of-hours running needs context

A compressor running overnight is a reason to investigate, not automatic proof that every unit of air is leaking. Confirm which processes, equipment and air-treatment functions remain active. Where those legitimate uses are understood, measured non-production flow can help assess avoidable demand.

A compressed air leak survey then helps locate leaks for repair. Monitoring and leak detection answer related but different questions.

The logging period must represent your business

Ask your assessor to include the operating conditions that matter: busy shifts, start-up, changeovers and non-production periods. Note unusual shutdowns, temporary equipment and production changes alongside the readings. A convenient week is not necessarily a representative week.

For a site with several compressors, the assessment should also explain how their operation overlaps. Record which units were available and how they were controlled so that one machine’s current trace is not mistaken for the performance of the complete system.

What should the customer receive?

Agree the report contents before logging starts. A useful report should explain:

• The equipment covered, instrument locations and production conditions during monitoring.
• How demand and pressure vary, and how compressor operation relates to those changes.
• The findings supported by measurements, any remaining uncertainties and further checks needed.
• Prioritised actions, with the evidence for each recommendation.
• The assumptions behind any estimated costs, savings or payback, and how results will be checked.

The outcome should be a practical basis for deciding what to repair, adjust or investigate before committing to equipment. Ask for measured values and estimates to be labelled separately, so the confidence behind each recommendation is clear.

Questions to ask before accepting a recommendation

• Which values were directly measured, and which were calculated?
• Where were the instruments installed, and what equipment did they cover?
• Did the logging capture our normal shifts and important demand peaks?
• Are airflow reference conditions and operating pressures stated?
• What assumptions underpin the energy costs and projected savings?
• Have leaks, pressure problems and control settings been considered?
• How will the proposed improvement be checked after installation?

Check what improves after the work

Logging creates a baseline for checking the results of leak repairs, control adjustments or distribution improvements. Repeat measurements under comparable production conditions can show whether non-production air consumption has fallen, pressure is more stable or compressor cycling has changed.

Account for differences in production output, working hours and operating pressure when comparing results. Lower measured airflow demonstrates reduced air demand under those conditions; it does not, by itself, establish an exact electricity saving. Any kWh or running-cost saving estimated from current and compressor data should remain clearly labelled as an estimate.

Choose the assessment around the decision

Every investigation does not need every instrument. Current logging may be enough to answer a narrow question about running patterns. A major equipment purchase or a pressure problem may need a broader measurement plan.

Discuss the decision you need to make before agreeing the audit scope. WMC’s compressed air energy assessment services can help you consider the information needed. For equipment selection, see our compressor supply and installation service and our guide to compressor sizing with a variable-speed drive.

A useful report should make its evidence and limitations clear enough for you to make an informed decision.