A thermal vacuum chamber is accepted twice: once at the builder's works in the factory acceptance test (FAT) and once after installation in the site acceptance test (SAT). Both events end with a signature that transfers risk, triggers payments and starts the warranty, yet the tests behind that signature are often defined late and loosely. This article answers a practical question: which tests and records prove that a chamber meets its specification, and what should a buyer witness rather than accept on paper? It works through the acceptance documents, vacuum and leak performance, thermal performance, controls and interlocks, cleanliness evidence and the checks that only make sense on site.
Acceptance Starts with the Specification, Not the Test Day
No ECSS standard defines how a test chamber is accepted. ECSS-E-ST-10-03C Rev.1 governs the testing of space segment equipment and explicitly excludes tests of facilities and ground support equipment from its scope. The acceptance criteria therefore come from the procurement specification and the contract, and they have to be translated into an acceptance test procedure (ATP) well before manufacturing ends. Each specified requirement needs a verification method (test, analysis, inspection or review of design), a measurable pass criterion and the conditions under which it is measured.
ECSS-Q-ST-20-07C, which sets quality and safety assurance requirements for space test centres, is useful here because it describes what the future operator of the facility must be able to show: calibrated instrumentation, controlled procedures and traceable records. A chamber accepted without those records leaves the test centre to reconstruct them later. Agreeing the ATP early also prevents the most common FAT dispute, namely a criterion that was never stated together with its measurement conditions.
Documents to agree before the FAT
- A verification matrix linking every specification clause to a test step, an inspection or an analysis, with the responsible party named.
- Pass criteria with units, measurement location and chamber state, for example empty, clean and dry, with or without a preceding bake-out.
- A list of witness points and hold points, so that the buyer knows which steps may proceed only after attendance.
- Calibration certificates for the gauges, temperature sensors, flow meters and data acquisition used as acceptance evidence.
- The configuration under test: pump set, shroud and plate circuits, controller firmware and software version.
Vacuum Performance: Pump-Down Time and Base Pressure
Pump-down time and base pressure are the figures most often quoted and most often misread. Below roughly 1×10⁻³ mbar the gas load in a clean chamber is dominated by water desorbing from the walls, so the result depends on how long the vessel was open, the humidity of the room air and whether it was vented with dry GN₂. A criterion that gives only a pressure and a time, without these conditions, cannot be verified unambiguously.
Measurement uncertainty matters as much as the reading. Ionisation gauges are gas-type dependent and are usually calibrated for nitrogen, and ECSS-E-ST-10-03C Rev.1 allows a default tolerance of ±80 % for chamber pressures below 1.3×10⁻³ hPa in hardware tests, which shows how coarse high-vacuum measurement is. Distinguish the operating pressure held during a test with cold shrouds from the ultimate pressure reached after extended pumping; a cold LN₂ shroud cryopumps water and usually lowers the reading considerably.
What to record during the pump-down demonstration
- The full pressure curve from atmosphere to base pressure with time stamps, not only the end value.
- Chamber history before the test: duration at atmosphere, vent gas, room temperature and relative humidity.
- Gauge type, position on the vessel and calibration date, plus a cross-check between two gauges where their ranges overlap.
- Pressure with shrouds at ambient and with shrouds cold, so that both operating and ultimate values are documented.
- Pump status during the run: foreline pressure, turbomolecular pump speed or cryopump stage temperature.
Leak Tightness: Rate-of-Rise and Helium Tracer Gas Testing
A rate-of-rise test isolates the chamber from its pumps and records the pressure increase. The total gas load follows from Q = V × Δp/Δt: a 1,000 litre chamber whose pressure rises by 1×10⁻³ mbar in 100 s has a gas load of 1×10⁻² mbar·l/s. The test cannot separate leaks from outgassing on its own, but the shape of the curve helps. Outgassing flattens over time, while a real leak produces a linear rise for as long as the chamber pressure remains far below atmospheric pressure.
Localising and quantifying leaks requires a tracer gas method. ISO 20485:2017 describes the techniques for leak detection with a tracer gas and a tracer gas specific leak detector, including the vacuum method in which helium is sprayed on the outside while the detector samples the evacuated chamber. The detector response should be checked against a calibrated reference leak before and after the test. Cryogenic circuits deserve particular attention, because thermal contraction can open leaks at LN₂ temperature that are invisible at ambient.
Leak test points worth witnessing
- Door seal and any secondary seals, sprayed systematically from top to bottom so that helium does not drift to an untested joint.
- Electrical, thermocouple and fluid feedthroughs, including LN₂ and GN₂ inlets where they pass through the vessel wall.
- Viewports and their clamping flanges, which combine dissimilar materials and are sensitive to thermal stress.
- Welds on the vessel and on the shroud circuits, the latter ideally tested after at least one cold cycle.
- An integral leak rate in mbar·l/s, recorded together with the detector sensitivity and the reference leak value.
Thermal Performance: Range, Rates, Uniformity and Stability
The thermal demonstration should prove four things: the extreme temperatures of the thermal plate and shrouds, the achievable heating and cooling rates, the spatial uniformity across the plate and the stability during a hold. Each figure needs a defined thermal load. An empty chamber ramps faster and holds more uniformly than one carrying a fixture and a test item, so a dummy mass or a resistive heater load that represents the intended use gives more meaningful evidence than an empty run.
The required stability follows from the hardware tests the chamber will support. ECSS-E-ST-10-03C Rev.1 sets default thermal vacuum tolerances of +0/−4 K at the minimum and −0/+4 K at the maximum test temperature for equipment, and defines dwell time from the moment the targeted temperature is reached within that tolerance. Shroud and plate control must stay comfortably inside that band, including any overshoot at the end of a ramp. Measuring LN₂ consumption over a full cycle also gives the operator a first realistic figure for running costs.
Thermal acceptance checks
- Minimum and maximum plate and shroud temperatures, each held long enough to show a stable reading rather than a passing value.
- Heating and cooling rates with the dummy load defined in the ATP, measured both on the load and on the plate.
- A uniformity map from several sensors across the plate surface, with the sensor positions documented in a sketch.
- Overshoot and settling behaviour at the end of each ramp, compared with the agreed tolerance band.
- LN₂ or refrigeration consumption over a complete cycle, with supply pressure and line condition noted.
Controls, Interlocks and Data Acquisition
Many chamber faults that put hardware at risk are control faults rather than vacuum or thermal shortcomings. The FAT should therefore exercise every automatic sequence listed in the ATP: pump-down, venting with GN₂, bake-out, thermal cycling and controlled warm-up after a cold phase. Each sequence is run as an operator would run it, from the operator interface, and the logged data are reviewed afterwards rather than only the screen display.
Interlocks are tested by fault injection. Simulating a pump failure, a loss of LN₂ supply, a power interruption or a sensor break shows whether the system reaches a safe state, and whether that state protects the test item as well as the chamber. Over-temperature protection should act independently of the control loop. The data acquisition deserves the same scrutiny: channel mapping, sample rate, time stamping and export format decide whether a later test report can be reconstructed.
Functional tests by fault injection
- Pump or compressor trip during operation, with the expected valve positions and alarm messages recorded.
- Power failure and restart: which state the system resumes and whether any valve moves unexpectedly.
- A venting attempt while cold surfaces are below the dew point, which should be blocked or warned against to prevent condensation.
- Independent over-temperature cut-off triggered by a simulated sensor value and verified at hardware level.
- Each measurement channel checked against a known reference, with channel names matching the documentation.
Cleanliness Evidence: The Chamber Must Not Be the Contaminant
A new chamber carries machining fluids, cleaning residues, fingerprints and outgassing from seals and cables. ECSS-E-ST-10-03C Rev.1 requires cleanliness and contamination control in test programmes to conform to ECSS-Q-ST-70-01, so the facility has to be able to show that it does not add molecular or particulate contamination to the test item. That evidence is best produced at the factory, before the chamber leaves a controlled environment.
A practical approach is an empty-chamber run after cleaning and bake-out, monitored with a quartz crystal microbalance or witness samples and a residual gas analyser. The RGA spectrum shows whether water, air components or hydrocarbon fragments dominate the residual gas; recurring peaks at higher masses point to oils or polymer outgassing. The acceptance criterion must be agreed in advance, because a figure measured in an empty chamber is a property of the facility, not of a future test item.
Cleanliness records to request
- Cleaning procedure and the agents used for vessel, shrouds and fixtures, with dates and responsible staff.
- Materials list for all parts inside the vacuum, in particular seals, cable insulation and lubricants.
- Bake-out profile of the empty chamber with temperatures, duration and pressure trend.
- QCM or witness sample results for an empty-chamber run, with the collector temperature stated.
- RGA spectrum of the empty chamber at base pressure, with the mass range and instrument settings.
Site Acceptance: What Transport and Infrastructure Change
The SAT verifies that the chamber still meets its specification after transport, rigging and reconnection, and that it works with the site's infrastructure. Vacuum joints may have been opened for shipping, shroud circuits disconnected and sensors removed. The site's utilities are new variables: electrical supply, LN₂ delivery pressure and line insulation, cooling water temperature and flow, compressed air, GN₂ for venting and the exhaust routing of the pumps.
A full repeat of the FAT is rarely necessary. A reduced programme usually covers a helium leak test of all joints opened during installation, a pump-down to the specified pressure, at least one thermal cycle with the site's cryogen supply and the interlocks that connect to site systems, such as oxygen deficiency monitoring and emergency stops. Documentation handover belongs to the SAT as well: as-built drawings, P&ID, wiring diagrams, manuals, spare parts lists and training records.
Closing the SAT
- A punch list that classifies open items by their impact on test capability, with an owner and a target date for each.
- Re-test records for every joint and connection disturbed during transport and installation.
- Thermal cycle results with the site's LN₂ supply, compared with the FAT values under the same load.
- Verified integration with site safety systems, including oxygen monitoring and emergency stop circuits.
- Handover of as-built documentation and software backups, and a clear statement of when the warranty starts.
Takeaway
A chamber acceptance is only as strong as the criteria behind it. Defining measurement conditions, loads and pass values in the acceptance test procedure before manufacturing ends turns the FAT into a verification rather than a demonstration. The most valuable witnessed steps are those that cannot be checked on paper: leak tests after cold cycles, thermal stability under load, interlock behaviour under injected faults and an empty-chamber cleanliness run. The SAT then confirms that transport and site infrastructure have not changed the result.
Frequently Asked Questions
The core set is a pump-down to the specified pressure under documented conditions, a rate-of-rise test and a helium tracer gas leak test, a thermal demonstration of range, rates, uniformity and stability with a defined load, functional tests of sequences and interlocks by fault injection, and cleanliness evidence from an empty-chamber run. The SAT repeats a reduced programme after installation and adds the integration with site utilities and safety systems.
No. ECSS-E-ST-10-03C Rev.1 covers the testing of space segment equipment and explicitly excludes tests of facilities and ground support equipment. Its tolerances and its definition of dwell time are still useful, because they describe what the chamber must be able to support. ECSS-Q-ST-20-07C sets quality and safety assurance requirements for space test centres, including their facilities and personnel. The acceptance criteria themselves come from the procurement specification and the contract.
Both have their place. Vacuum performance and cleanliness are usually demonstrated with the empty chamber, because the result then characterises the facility itself. Thermal rates, uniformity and stability should be demonstrated with a dummy mass or a heater load that represents the intended use, since an empty plate ramps faster and settles more evenly than a loaded one. The ATP should define the load by mass, material, heat dissipation and mounting.
It is a useful screening test but not sufficient on its own. The pressure rise combines real leaks and outgassing, and in a new chamber outgassing often dominates. A helium tracer gas test, as described in ISO 20485:2017, localises individual leaks and quantifies them with a detector checked against a calibrated reference leak. Leaks that only open at cryogenic temperature additionally need a test during or after a cold phase.
The failure is recorded as a nonconformance with the measured value, the test conditions and the suspected cause. The parties then decide whether the item is corrected and re-tested, accepted through a formal deviation or waiver, or carried to the SAT as a punch list item. The decision should follow the procedure agreed before the FAT, and any re-test should be witnessed under the same conditions as the original attempt.
Technical references
Primary standards, agency material and current Deepvac product pages used to verify the technical statements in this article.
- ECSS-Q-ST-20-07C: Quality and safety assurance for space test centres (1 October 2014)European Cooperation for Space Standardization (ECSS)
- ISO 20485:2017 Non-destructive testing: Leak testing, Tracer gas methodInternational Organization for Standardization (ISO)
- ECSS-E-ST-10-03C Rev.1: Testing (31 May 2022)European Cooperation for Space Standardization (ECSS)
- Standard Series TVAC chambersDeepvac GmbH