What a test bench actually asks of the air system
Pneumatic suspension rigs, air-spring test stands, valve and actuator test benches, hose and component leak benches and end-of-line functional testers all have the same basic pattern. They draw a burst of air to fill a known volume, regulate to a target pressure, wait for temperature and pressure to settle, measure, and then vent. Between those steps the bench may use almost no air at all.
That pattern matters because a plant air system is usually sized on average demand. A bench that fills a large air spring or fixture in a few seconds can draw several times the plant’s spare capacity for that moment, pull down the header and disturb other users. The opposite mistake is also common: a dedicated compressor is bought for the peak and then runs mostly unloaded.
A good specification separates four things:
- Fill demand: the volume of free air needed per fill and the time allowed for it.
- Hold demand: the small make-up flow during stabilisation and measurement, including fixture leakage.
- Cycle rate: fills per hour, which sets average demand.
- Pressure and quality: the supply pressure needed upstream of the bench regulator, and the dryness and cleanliness the instruments require.
Calculating fill air from the pressurised volume
The free air needed to raise a closed volume from one pressure to another is approximately:
Free air volume ≈ V × (p2 − p1) ÷ p0
where V is the internal volume being pressurised (fixture, air spring, hoses and manifold), p1 and p2 are the start and end pressures in absolute terms, and p0 is atmospheric pressure, about 1.013 bar(a). Gauge pressures can be used for the difference as long as both are gauge.
Worked example (illustrative only): an air-spring test fixture plus hoses holds 50 litres. It fills from atmospheric to 6 bar(g), 87 psig. Free air ≈ 50 × 6 ÷ 1.013 ≈ 296 litres per fill. If the fill must take 10 seconds, the average flow during the fill is about 1.8 m³/min (about 63 cfm). If the bench cycles 30 times an hour, the average demand is only 30 × 0.296 ≈ 8.9 m³ per hour, about 0.15 m³/min (about 5 cfm).
The gap between a 63 cfm fill and a 5 cfm average is exactly why test benches are best served with local storage. Two refinements are worth noting:
- Compliant elements such as air springs, bellows and hoses change volume with pressure, so measure or calculate the volume at test pressure, not at rest.
- Fast fills heat the air. When it cools, the pressure falls and the regulator adds more air. Allow for this in the hold phase.
Local receivers and the bench supply arrangement
A local receiver next to the bench supplies the fill peak and lets the plant line recover slowly between cycles. The usable air in a receiver depends on how far its pressure can fall while still staying above what the bench regulator needs:
Usable free air ≈ receiver volume × (receiver start pressure − minimum pressure) ÷ p0
Continuing the example, if the bench regulator needs at least 7 bar(g) upstream to deliver 6 bar(g) cleanly, and the receiver is charged to 9 bar(g), each litre of receiver gives about 2 ÷ 1.013 ≈ 2 litres of free air. To supply 296 litres per fill with margin, a receiver well above 150 litres would be required. Check the result with the receiver transient storage calculator and confirm the receiver is designed, certified and inspected for its pressure.
| Arrangement | When it suits | Watch-outs |
|---|---|---|
| Direct from plant header | Small volumes, low cycle rate, stable header with spare capacity | Fill peaks disturb other users; bench results vary with header pressure |
| Plant header plus local receiver and check valve | Most benches with short, large fills | Receiver must be sized for usable air, not just volume; drain condensate |
| Dedicated compressor and receiver | Higher test pressures, remote benches, or strict pressure stability | Compressor may run lightly loaded; choose control mode accordingly |
| Booster from plant air | Test pressure above plant header | Booster ratings, heat, and safety relief must be engineered for the duty |
Regulation, valves and the stabilisation period
The bench’s regulator and valves decide how quickly the volume fills and how cleanly it settles. A precision regulator with too small a flow capacity will stretch the fill time; an oversized fast valve can overshoot. Many benches use a two-stage fill: a fast coarse fill to near the target, then a fine regulator or proportional valve for the last part.
After the fill, temperature and pressure must settle before a reading is meaningful. The settling time depends on volume, fill speed, materials and ambient temperature. In Indian summer conditions, with workshop ambient often 40–45 °C, the gap between hot compressed air and cooler fixtures can be larger than a test procedure assumed, so include temperature measurement where readings are used for decisions.
Points to record for each bench:
- Regulator type, set range and flow capacity at the required pressure drop.
- Fill and vent valve size and response time.
- Pressure transducer range, accuracy and calibration date.
- Stabilisation rule: fixed time, or pressure change below a stated rate.
- Vent path, silencer and where the exhaust air is discharged.
Air quality for test benches
Test instruments, precision regulators and proportional valves are sensitive to water, oil and particles. Water condensing inside a fixture also changes the test result, because water vapour and liquid behave differently from dry air as temperature changes. A practical baseline for most benches is:
- A refrigerated dryer giving a pressure dew point typically around +3 °C, or a desiccant dryer (typically −20 °C or −40 °C PDP) where the bench or its surroundings are cold, or where water would affect results.
- General-purpose and fine coalescing filters upstream of the bench, with differential-pressure indication.
- A particle filter on the bench inlet sized for the fill peak, not the average.
- An agreed air-quality class to ISO 8573-1 written into the bench specification.
Where the bench tests components used in food, medical or oxygen service, the component standard may set additional requirements for oil content and cleanliness. Those requirements belong to the test owner and the component specification.
Safety boundaries the air supply must respect
Compressed gas stores much more energy than the same volume of water at the same pressure. A failure during a pneumatic test can release that energy suddenly. The air supply design must support the test owner’s safety measures, not replace them. As a minimum the supply side should provide:
- A pressure relief device on every receiver and on any supply that could exceed the bench or component rating.
- A lockable isolation valve and a means of venting the bench to a safe state.
- Clear separation between the supply pressure and the test pressure, with the regulator set and locked by authorised staff.
- Guarding, enclosure or distance as required by the test owner’s risk assessment.
Proof, burst, fatigue and leak-rate tests need an approved method, hazard controls, stop criteria and acceptance rules from the accountable test owner or the equipment manufacturer. A description such as “pressurise and hold” is not a test procedure. Our separate pages on pressure-testing boundaries and leak-testing fixtures cover those limits in more detail.
Step-by-step: building a test bench air requirement sheet
- List each bench, its test type and the volume it pressurises at test pressure.
- Record start and end pressures in bar and psig, and the allowed fill time.
- Calculate free air per fill and the fill flow rate.
- Record cycles per hour and calculate average demand.
- Estimate hold make-up flow, including fixture and connector leakage.
- State the minimum supply pressure upstream of the bench regulator.
- Define the air-quality class and any special cleanliness needs.
- Choose the supply arrangement: header, local receiver, dedicated compressor or booster.
- Size the receiver for usable air and confirm its pressure rating and relief.
- Confirm with the test owner that the supply arrangement suits their approved procedure.
Common problems and their usual causes
| Symptom | Likely cause | First check |
|---|---|---|
| Fill time longer than specified | Undersized regulator, valve or hose; receiver too small | Log pressure upstream of the regulator during a fill |
| Other machines fault when the bench fills | Bench draws directly from a header with little spare capacity | Add local receiver with check valve and flow restriction on recharge |
| Readings drift during the hold | Thermal settling, fixture leaks, wet air | Measure temperature; leak-check fixture; check dew point |
| Results vary between shifts | Header pressure or ambient temperature changes | Log header pressure and ambient alongside test results |
| Water or oil inside fixtures | Missing dryer or filters, saturated elements, no drains | Inspect drains, filter elements and dryer operation |
How Reddy Compressors helps
Reddy Compressors has supported industrial compressed-air users since 2002 from our office at Tarnaka, Secunderabad. We are an independent multi-brand specialist, not an authorised dealer of any brand, and we sell new and used/refurbished compressors, rent from our own fleet, and service ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other makes. We have tie-ups with all major and reputed brands. For test benches and suspension rigs we can:
- Review the bench air requirement and advise on receivers, dryers, filters and the supply arrangement.
- Supply a dedicated compressor, receiver and treatment package, new or used/refurbished.
- Log pressure and flow during bench cycles to find the real peak and average demand.
- Provide rental compressors (50–2,000 cfm, 2–30 bar) for trials, commissioning or temporary test areas.
- Service the compressor, dryer and filters under AMC, and supply spare parts identified from the machine’s nameplate.
Our team of more than 20 service engineers and technicians covers all of Telangana and Andhra Pradesh, and we supply equipment and spares across India. The test method itself stays with you or your equipment supplier.
What to send us
- Type of bench and what it tests.
- Pressurised volume, test pressure and allowed fill time.
- Cycles per hour and shifts per day.
- Air-quality needs and any component standard that applies.
- Present compressor, dryer and header pressure, and the distance from the compressor room.
- Photos of the bench supply connection, regulator and any receiver.
Frequently asked questions
How much compressed air does a test bench need?
Calculate it from the volume being pressurised and the pressure rise. Free air per fill is roughly the volume multiplied by the pressure rise in bar, divided by atmospheric pressure. Divide by the fill time for the peak flow, and multiply by cycles per hour for average demand. Add the small make-up flow during the hold and any fixture leakage.
Do I need a separate compressor for a pneumatic test bench?
Often not. Many benches work well on plant air with a local receiver and check valve that supply the fill peak. A dedicated compressor makes sense when the test pressure is above the plant header, the bench is remote, or pressure must stay unaffected by other users. A measured demand profile helps decide which arrangement is more economical.
Why does my test pressure keep dropping after filling?
Air heats up during a fast fill, and as it cools the pressure falls even with no leak. Fixtures, seals and hose connections can also leak, and wet air behaves less predictably. Allow a defined stabilisation period, measure temperature where it matters, leak-check the fixture and confirm the dryer is delivering the expected dew point.
Can I pressure test with compressed air instead of water?
Pneumatic testing stores far more energy than hydrostatic testing at the same pressure, so failures can be violent. Whether it is allowed depends on the applicable code, the component and an approved procedure from the responsible engineer. We can supply and set up the air source, but the test method, hazard controls and acceptance criteria must come from the accountable test owner.
What air quality is needed for a leak or function test bench?
Most benches need dry, filtered air to protect precision regulators, valves and transducers and to keep water out of fixtures. A refrigerated dryer with typical pressure dew point around +3 °C and coalescing filters suits many cases. Use a desiccant dryer where cold conditions or the test result demand drier air, and specify an ISO 8573-1 class.
What size receiver should I fit next to a test bench?
Size it for usable air, not just volume. Usable free air is the receiver volume multiplied by the allowed pressure drop, divided by atmospheric pressure. The drop is from the charged pressure down to the minimum the bench regulator needs. Make the usable air comfortably larger than one fill, and use a receiver rated, certified and inspected for its working pressure.
Related pages
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. No prices or tariffs are assumed: enter your own to see costs. Efficiencies and other figures shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by a service engineer against the machine data sheet and the site.
Test-bench fill air, demand and receiver
Free air per fill, flow during the fill, average demand from the cycle rate, and the local receiver that can supply a fill between its charge pressure and the pressure the bench regulator needs.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Volume entered in: litres
- Internal volume being filled (in the unit above): 50
- Start pressure (bar g): 0
- Target pressure (bar g): 6
- Atmospheric pressure (bar a): 1.013
- Allowance for cooling after the fill, hoses and leaks (%): 0
- Fill time allowed (s; 0 if not set): 10
- Fills per hour (0 if not relevant): 30
- Make-up flow during the hold, fixture leakage (L/min free air): 0
- Local receiver charge pressure (bar g; 0 if no receiver): 9
- Lowest receiver pressure the user accepts (bar g): 7
Result
- Volume: 50 L = 0.05 m³
- Pressure rise: 6 bar (87 psi); end pressure 7.01 bar a
- Free air to add (pressure rise basis): 0.2962 m³ = 296 L
- Free air on the absolute-ratio basis: 0.3462 m³ = 346 L (includes one volume already inside)
- Free air per fill used below: 0.2962 m³ = 296 L (pressure rise basis)
- Fill time: enter a supply flow to get the time
- Average flow needed to fill in 10 s: 1.78 m³/min = 1,777 L/min = 62.8 cfm
- Average demand: 8.88 m³/h = 0.148 m³/min = 148 L/min = 5.23 cfm
- Fill flow ÷ average demand: 12 times: the reason a local receiver pays
- Usable free air per litre of receiver: 1.97 L (9 → 7 bar g)
- Receiver that gives one full fill on its own: 150 litres (minimum; choose larger and a rated, inspected vessel)
Method. Free air to add = V × (p₂ − p₁) ÷ p₀, with p₁ and p₂ the start and end pressures (a gauge difference equals an absolute difference) and p₀ the atmospheric pressure, about 1.013 bar a. The “absolute pressure ratio” basis, V × (p₂ − p₁ + p₀) ÷ p₀, adds one volume of free air: for a volume that starts at atmospheric pressure it equals V × p₂(abs) ÷ p₀. It counts the air already inside, so it overstates what must be pumped in by one vessel volume; some procedures use it as a conservative figure. Fill time = free air ÷ supply flow (the supply flow is compressor FAD at the final pressure, or regulator flow capacity, as free air). Flow for a required time = free air ÷ time. Average demand = free air per fill × fills per hour + make-up flow. Receiver able to give one whole fill on its own = free air per fill × p₀ ÷ (receiver charge pressure − lowest pressure the user accepts). Isothermal and ideal gas: a fast fill heats the air, which then cools and loses pressure, so allow extra; real fill time through a regulator is longer than the minimum shown because flow falls as the pressure difference shrinks near the set point.
What to take from it. A bench that fills in seconds draws many times its hourly average: serve the fill from a local receiver with a check valve, and size the compressor for the average.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.
Receiver storage sizing
Estimate the receiver volume needed to ride through a short burst of demand without the pressure falling below the limit.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Burst demand above compressor output (m³/min free air): 5
- Burst duration (s): 20
- Allowed pressure fall (bar): 0.7
- Receiver volume you already have (litres; 0 to skip the hold-up check): 1520
Result
- Receiver volume needed: 2,412 litres
- 2.41 m³
- Practical size (with 25% margin): 3,016 litres
- Hold-up time of the receiver you have: 12.6 s before the pressure falls by 0.7 bar
Method. V = Q × t × P_atm / ΔP, where Q is the shortfall in free air (m³/min), t the time (min), ΔP the allowed pressure fall (bar) and P_atm = 1.013 bar. The same relation gives the hold-up time of an existing receiver: t = V × ΔP / (Q × P_atm). Isothermal; for a real system add a margin.
What to take from it. A receiver buys time. It does not create capacity: once it is spent the pressure keeps falling until the compressors catch up.