Compressed Air Demand Profiling: Measure Before You Buy

1What gets measured, and for how long

A demand profile runs on three logged signals, and all three matter. Drop any one and you’re back to guessing.

Compressor electrical input, in kW. Not the nameplate number. The kW in a compressor’s name is the nominal motor rating, and your meter sees the whole package: motor, cooling fan, drive and control losses together. Total package input runs higher, and by how much varies by model, so the CAGI performance data sheet is the reference: Atlas Copco’s GA37, for example, pairs a 37.3 kW nominal motor with a 43.3 kW package input at full load, about 16 per cent higher. Our energy audit guide works to 15-minute logged intervals on electrical input as the audit baseline.

Header pressure. Sampled fast enough for the transient you’re investigating, which for dip diagnosis means sampling of about a second. A pressure dip that trips a packaging line lasts seconds; a weekly spot reading with a gauge has next to no chance of catching it. One-minute stored records are only acceptable where the logger samples faster internally and preserves the minimum and maximum excursions within each interval.

Flow. Measure it with a calibrated flow meter, and be clear about what the meter is telling you. Flow at the compressor discharge is compressor supply, what the machines deliver, and on most sites it’s the practical place to put the meter. Plant demand is what your production equipment draws, and where storage sits between the compressors and the plant, main-header or downstream flow is the measurement that represents it. During a short burst the receiver gives up part of the air, so discharge flow on its own understates the instantaneous peak. Match the measurement point to the question. Receiver sizing needs plant-demand flow and the compressor’s contribution for the same event, because the receiver is charging and discharging between the two. CAGI-comparable specific power needs compressor or package kW paired with flow at compressor discharge. A downstream demand meter and compressor kW can’t simply be read together instant by instant across those charge and discharge transients. Where flow can’t be instrumented, it can be estimated from compressor loaded hours and nameplate free air delivery, but only on fixed-speed or load/unload machines running near their rated loaded output. It doesn’t hold for VSD or modulating compressors, whose delivered flow varies continuously while the machine runs. Even where it does hold, it gives you volume totals, not a time-resolved demand trace, and the profile should say so.

How long? Long enough to capture a representative cycle: one to two weeks for a single-shift site, two to three weeks for multi-shift operations. Cover at least one representative operating cycle plus a genuine low-load or non-production window, because that floor is where your leak load identifies itself. On a Monday to Friday plant the weekend is what provides that window, so include one. On a 24/7 plant, a seven-day roster, or a site that takes its downtime midweek, it’s whatever period production actually stops or drops away.

What does a good profile look like? Picture a week of flow data on one chart. Overnight, a flat low line that shouldn’t be there but is. At 6 am, a ramp as production starts. Through the day, a sawtooth of peaks riding on a steady base, with a dip at lunch. Friday night, everything shuts down and the trace falls, but not to zero. The gap between that weekend floor and zero is the number this whole page is about.

3The decisions a real profile unlocks

Each feature of the chart maps to a capital or operating decision, and without the chart you’re making that decision on a salesperson’s assurance.

VSD or fixed speed. A variable speed drive matches motor speed to demand, so it earns its premium on sites where demand genuinely swings, and buys little on a site running flat out at steady load. That’s why a blanket “VSD saves 35 per cent” claim deserves a raised eyebrow: demand variability is the major screening input, and that’s exactly what the logger measures before anyone quotes you. What you actually save also depends on the full-load and part-load efficiency curves of the machines you’re comparing, your pressure settings and control band, and how your existing compressors are sequenced. The profile tells you whether a VSD compressor is your answer or an expensive irrelevance.

Receiver sizing. The profile gives you two of the inputs straight off the trace: the excess burst flow above what the compressor supplies, and how long the burst lasts. Correct receiver sizing combines those with the usable pressure band between your normal operating pressure and the minimum your processes accept, and with how your compressor and its controls respond during the event. Sized from measured inputs, receivers come out right. Sized from folklore, they come out wrong by up to a factor of ten, and our energy audit guide shows the arithmetic. The air receiver guide covers selection once the profile has given you real numbers to size from.

Set-point reduction. The pressure trace shows the pressure minima that actually occurred while you were logging, and how the system behaves as pressure falls. It doesn’t tell you what your processes could tolerate. That allowable minimum has to come from your process and equipment requirements, then be verified by a controlled trial at the lower set-point. Reducing the set-point by 1 bar typically saves 6 to 7 per cent of compressor energy, per our energy audit guide, and lower pressure shrinks every leak on site at the same time. The profile turns “can we lower the pressure?” from an argument into something you can test.

Staging and sequencing. On multi-compressor sites, sequencing problems look like machines fighting each other: two compressors trading load where one would do, or a large machine idling to cover a demand a small one should carry. Seeing that takes per-compressor logging, load state and power on each machine, because one system total can’t separate them, so specify it up front. Sequencing fixes cost little, and per-machine data is the evidence they’re needed.

System design. If the profile points at a distribution restriction, or shows demand that has outgrown the installed system, that’s a system design conversation, and the profile is the first document a good designer will ask for.

5Profiling versus the walk-through audit

Plenty of things get sold as compressed air audits. The distinction that matters is simple: was anything logged?

A walk-through audit is a person, a clipboard and an opinion. It can spot open blowing, hear the worst leaks and read your set-point off the gauge, and that has some value. What it cannot do is tell you your base load, your peak behaviour, your off-hours flow or your specific power, because none of those exist without a logger. When a walk-through ends in a proposal to buy equipment, you’re reading a quote dressed as an assessment. A vendor’s free audit isn’t free if it steers the answer before any data is collected; an independent audit optimises for your lowest-cost outcome regardless of brand, which is the whole case for measurement-first auditing.

Measured baselines also underpin some project-based incentive pathways, while other scheme activities are calculated on deemed, benchmark or prescribed methods instead. Some Australian schemes reward measured, verified savings, on a per-state, per-scheme basis. In Victoria, the Victorian Energy Upgrades program runs a project-based activities pathway in which savings are measured before and after the project under the Essential Services Commission’s published measurement and verification method, a public methodology you can read. A demand profile can contribute useful baseline evidence to a VEU project like that, but the formal baseline and the certificate evidence must meet the Commission’s applicable measurement and verification requirements. In New South Wales, the Energy Savings Scheme is a continuous certificate scheme administered by IPART, with new scheme rules that commenced on 1 July 2026, and energy.gov.au’s business equipment upgrades listing names compressed air systems among the upgrades available under it. Eligibility, rules and incentive values are scheme-specific and change; our grants and incentives map tracks each scheme’s current status with the date we last verified it. The practical point stands on its own: a logged demand profile is evidence you can take to a scheme, a lender or your own finance team. A walk-through opinion is not.