TVAC chamber sizing starts with the largest approved test configuration, including fixture, harness, instrumentation, access and radiative clearance. The assembly must pass through the door and fit within the clear dimensions of the selected shroud and thermal plate. Nominal vessel volume is useful for product classification, but it is not the binding test envelope. Growth margin should be defined from a realistic programme forecast rather than an unspecified future maximum.
Nominal volume is not usable volume
Nominal volume describes the empty vessel. The usable envelope is reduced by shroud panels, thermal plate, support structure, ports, fixtures and required clearances. The reduction varies substantially with chamber geometry and options, so a generic percentage should not be used for final selection.
Request the clear internal width, height and depth, door passage, usable plate area, load rating and port intrusions for the selected configuration. Compare those dimensions with an as-tested CAD envelope, not only the bare flight hardware.
Clearance rules of thumb
Clearance supports radiative boundary conditions, installation, harness routing and instrumentation. The required value depends on surface temperatures, view factors, geometry, access tools and the facility's operating method. A fixed hand-width rule can be useful for an early sketch but is not a design criterion.
What clearance provides
- Radiative environment: verify view factors to the controlled shroud and limit exposure to warm openings or reflective obstructions.
- Handling: technicians need room for hands, tools and torque wrenches at the mounting points, and loading must work without scraping past shroud edges.
- Harness routing: service loops, connector savers and bend radii between item and feedthroughs consume more depth than drawings suggest.
- Instrumentation: thermocouple runs, purge lines and any camera or viewport lines of sight need unobstructed paths.
Door and access geometry: cubic vs cylindrical
Two vessel geometries dominate, and the trade-off is practical rather than aesthetic. Cylindrical chambers – like the C Series – carry the external atmospheric pressure efficiently, which favours large volumes and long test items; the price is a circular cross-section that clips the corners of boxy test envelopes. Cubic chambers – like the T Series – offer a full rectangular cross-section and a door that opens the entire face, which suits box-shaped units, shelf-style fixturing and frequent loading.
In every case the door aperture, not the volume, defines the largest loadable item. Check the pass-through dimensions with fixture and protruding connectors included – and plan how the item actually travels: trolley, rail system or crane access changes what a given door is worth.
Fixturing, thermal plate and harness space
Test items rarely float. They bolt to the thermal plate or sit in a fixture that reproduces the mechanical interface – and that fixture adds height, footprint and thermal mass of its own. The thermal plate needs enough area for the item's conductive interface, and if several units are tested in parallel, for all of them plus sensor routing.
Harnessing deserves its own line in the sizing exercise: flight connectors with savers, breakout boxes, dozens of thermocouple lines and possibly purge or fluid lines all live between item and feedthrough panel. Teams that size for the item alone routinely discover that the harness – not the hardware – is what actually fills the chamber.
Growth margin versus oversizing
Growth margin should cover credible programmes over the intended planning horizon. Oversizing increases vessel, pumping and thermal-system requirements, floor space, utilities and turnaround time. The trade should compare recurring cost for the expected campaign mix with the cost of using an external facility for exceptional oversized items.
A sensible compromise: size for the realistic envelope of the next several years – not for the largest item imaginable – and add a defined margin. If one exceptional future item dominates the requirement, testing that one campaign externally is often more economical than owning its chamber.
Standard size classes as orientation
Current Deepvac Standard Series reference configurations cover nominal sizes of 65, 125, 250, 500, 1000 and 2000 litres in cubic T Series and cylindrical C Series architectures. These classes are an orientation for requirements capture; binding clear dimensions, loads and performance depend on the selected configuration and quotation.
A custom vessel becomes relevant when geometry, aspect ratio, access, optical bench, port count or load cases cannot be satisfied within a standard platform. The decision should follow a documented fit check rather than a preference for standard or custom.
Takeaway
Chamber size should be selected from the usable test envelope, door passage, fixture, harness, radiative clearance and a defined growth case. Nominal volume is only an orientation value; the binding comparison uses clear dimensions in the selected configuration.
Frequently Asked Questions
Start from the largest realistic test item including its fixture, add harness space and clearance to the shroud on all sides, and verify that the assembly passes through the door aperture. Compare that envelope against usable interior dimensions – not nominal litres. Then add a defined growth margin for the coming years, and resist sizing for a single exceptional item; external testing can cover that case.
There is no dependable universal fraction. Shroud, plate, support, ports, fixture and clearances reduce the empty-vessel volume by an amount that depends on configuration. Use clear dimensions and door passage from the selected layout.
Neither is better; they optimise different things. Cylindrical vessels carry atmospheric pressure efficiently, which favours large volumes and long test items. Cubic chambers offer a full rectangular cross-section and a full-face door, which suits boxy units, shelf fixturing and frequent loading. The deciding questions are your test item shapes and your loading process – which is why both geometries exist as standard.
No. Unused volume can increase pump-down time, thermal-system size, utility demand, floor space and purchase cost. Define a realistic growth case and compare exceptional future items with external test capacity before increasing the permanent chamber class.
Technical references
Primary standards, agency material and current Deepvac product pages used to verify the technical statements in this article.