This section covers 9 core dimensions: vacuum pressure rating, operating temperature, wiring medium, sealing structure, electrical performance, material, usage frequency, standard compliance, and quantity allowance. It encompasses two types of structures: rubber-pressurized sealing ports and KF vacuum feedthrough ports.
I. Vacuum/Minimum Pressure Rating (First Priority, Determines Port Type)
The higher the negative pressure and altitude, the more stringent the sealing requirements. This directly differentiates between two types of test ports:
1. Normal Operating Conditions (≥5kPa, Altitude ≤20000m, Automotive/Consumer Electronics/Battery UN38.3)** Optional **Rubber-pressurized sealing lead-in port (Φ20/25/30/50/80)**; slight leakage is permissible, pressure can be maintained by double-layer flange compression of the fluororubber plug.
2. Extreme Low Pressure (0.1～5kPa, Aerospace/Military/GJB150A/DO-160, Altitude 20000～30000m)** Ordinary rubber stoppers are prohibited. **KF glass/ceramic sintered airtight feeders (KF16/KF25/KF40/KF50)** must be used, with airtight metal welding, a leakage rate ≤1×10⁻⁹ Pa・m³/s, and no organic adhesive creep leakage issues.
3. Key Judgment: Repeated pressure increase/decrease and long-term pressure holding tests are required. The leakage rate must match the equipment's pressure control accuracy. Leakage will result in substandard pressure increase/decrease rates and step pressure holding, and will not be accepted by third-party testing.
II. Equipment Temperature Range (Determines Sealing Rubber Material)
Low-pressure chambers generally feature a wide-temperature composite cycling range of **-70℃～+150℃**. Ordinary silicone rubber hardens at low temperatures and releases gas at high temperatures. Selection criteria are strictly differentiated:
1. -40℃～+85℃ (Conventional Vehicle-Mounted):** Ordinary silicone rubber can be used for short-term testing; FKM fluororubber is still recommended for long-term cycling.
2. -70℃～+150℃ (Aerospace, High/Low Temperature/Low Pressure Composite):** **Forced FKM fluororubber sealing plugs** are required, offering resistance to high and low temperatures, low gas release, and resistance to negative pressure tensile stress without collapse.
3. Feedthrough flanges should uniformly be made of 304 stainless steel. Plastic flanges deform under thermal cycling and leak through gaps under negative pressure.
III. Cable Medium and Total Cross-sectional Area (Determining Hole Size)
1. Cable Type Differentiation
- Fine Signal Cables: Thermocouples, RS485, CAN, Low-Voltage Acquisition (Φ0.3～3mm fine wire)
- Power Cables: 24V/220V power supply for the entire device, battery high-voltage harnesses (Φ4～12mm thick sheathed wire)
- Special Media: Coaxial RF, Fiber Optic, Pneumatic Tubing, Hydraulic Pipelines (requires a separate large hole to avoid puncturing the rubber stopper)
2. Hole Size Selection Principles
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Allow a ≥5mm margin for the total outer diameter of all cable sheaths; do not fill completely. Overloading the harness will damage the sealing surface and cause negative pressure leakage.
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Specification Matching Reference:
- Φ20: Only multi-channel thermocouples, single-set communication thin wire
- Φ25: Multiple signal lines + low-power power lines
- Φ30: Standard wiring harness for electrical control system (most common standard configuration)
- Φ50: Power battery, high-power drive thick cable
- Φ80: Large system assembly, air pipe + cable run together
3. KF feeder selection by core count/current: KF25 multi-core signal, KF40 high-current single-core, KF16 BNC RF coaxial.
IV. Sealing Structure and Anti-Negative Pressure Design (Distinguishing Core Points from Ordinary Constant Temperature and Humidity Chamber Test Holes)
Ordinary constant temperature and humidity chamber test holes lack an anti-negative pressure structure. Low air pressure must meet two requirements:
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Double-layer bidirectional clamping flange**: The inner and outer flanges clamp the rubber plug, counteracting the inward pulling force of atmospheric pressure and preventing the rubber plug from being vacuum-sucked and permanently deformed by creep; simple single-layer hand-tightening flanges are not suitable.
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The rubber plug features a thickened support step and a high-hardness elastomer, withstanding tens of thousands of cycles of alternating "vacuum-to-normal pressure" fatigue; thin, soft silicone plugs develop gaps after only a few dozen cycles.
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KF sintered feedthroughs have no rubber components, relying on a metal-glass hermetic seal to completely avoid negative pressure deformation issues.
V. Electrical Conditions: Voltage, Current, Signal Type
1. Rubber-sealed lead holes
Suitable only for **low-voltage, low-current applications (≤10A, DC/AC220V); no insulation, unsuitable for high-voltage or radio frequency applications.
2. KF Airtight Feeder Detailed Electrical Specifications
- Multi-core Signal Feeder: 3-48 cores, 3-5A per core, compatible with thermocouples and bus acquisition;
- High Current Power Feeder: Single core 30A/50A/70A, high-voltage power supply for the entire system;
- BNC Coaxial Feeder: 50Ω impedance, suitable for RF, high-speed communication, and radar testing;
- Withstand Voltage Requirements: Aviation system feeder withstand voltage ≥1500V, insulation resistance ≥1000MΩ.
3. High-Voltage Lithium Battery Testing Precautions
Live low-pressure testing carries the risk of arcing and breakdown. High-voltage harnesses should preferably have independent KF feeders and should not share rubber holes with signal lines.
VI. Outgassing and Sample Cleanliness Requirements
1. Optics, PCBs, Chips, Lenses, Photovoltaic Modules**: Ordinary silicone rubber is prohibited, as small molecule volatilization contaminates the sample surface, causing atomization and reduced insulation; low-outgas FKM or all-metal sintered feedthroughs are required.
2. Military, Semiconductor, High-Precision Testing: Use baked-grade KF ceramic feedthroughs, free of organic precipitates.
3. Ordinary Plastics, Battery Casing Appearance Inspection: Silicone rubber sealing holes can be used, resulting in lower costs.
VII. Usage Frequency and Disassembly/Removal Requirements
1. Frequent Sample Changes, Daily Wiring Harness Insertion and Removal (R&D Screening)**: Choose **Removable Rubber Compression Test Holes**, allowing for flexible cable addition/removal, simple maintenance, and low cost.
2. Long-Term Fixed Installation, Minimal Wiring Disassembly, Long-Term Reliability Certification (Aerospace/Automotive Final Testing)**: Choose KF integrated airtight feedthroughs, allowing for long-term use with a single connection, offering significantly better sealing stability than rubber plugs.
VIII. Compliance Requirements for Testing Standards
Different industry standards mandate corresponding test port specifications; the selected type must match:
1. GB/T2423.21, UN38.3 Low-Pressure Batteries: Standard Φ30/Φ50 fluororubber sealing ports;
2. GJB150A, MIL-STD-810, DO-160 Airborne Equipment: Mandatory KF sintered airtight feedthrough, meeting extremely low leakage rate requirements;
3. Third-Party Testing Laboratories: Equipment test ports must provide vacuum seal leakage rate parameters; simple rubber ports are not accepted based on high-altitude extreme test data.
IX. Quantity, Location, and Expansion Reserves
1. Zoning: **Signal small ports + power large ports are separated**; thin signal wires use Φ20/25, thick cables/air pipes use independent Φ50 to avoid wire harness entanglement and compression sealing.
2. Spare Hole: Reserve at least one extra blind flange sealing hole of the same specification. This avoids secondary modifications to the enclosure when adding sensors or tooling later.
3. Installation Location: Avoid hot and cold air circulation ducts and directly above the sample to prevent condensation from seeping into the test hole and exacerbating leakage.
Quick Selection Summary
1. Atmospheric pressure ~ 5kPa, frequent line changes, consumer electronics/batteries → Double-layer flange FKM fluororubber sealing hole (Φ25/Φ30 standard)
2. 0.1～5kPa, aerospace and military, RF, long-term live-line testing → KF25/KF40 glass sintered hermetic feedthrough
3. Wide temperature range -70~150℃, precision optical samples → All prohibit ordinary silicone rubber, use FKM or metal feedthroughs exclusively
4. Coarse power harnesses, power battery assemblies → Independent lead-out of Φ50 large-diameter sealing hole
