Choose a waveguide pressure inlet by the waveguide and flange drawing, exact gas connection, complete assembly pressure limit and required gas dryness. Then size the supply and protection system for purging, leakage makeup and credible regulator failure. The inlet’s catalog pressure is not automatically the operating setpoint for the line.
Use clean, dry air or another gas approved for the equipment. Specify contamination limits where oil, water or particles could enter the RF passage. Nitrogen systems also need a suitable discharge location and ventilation assessment for the installation.
| Selection input | What must be confirmed |
|---|---|
| RF interface | WR size, operating band, flange designation, orientation and RF limits. |
| Gas connection | Thread standard, size, gender, valve, tube dimensions and sealing method. |
| Pressure boundary | Lowest permitted component pressure at temperature, controls and relief capacity. |
| Moisture control | Gas dew point at a stated pressure, coldest wall temperature and purge procedure. |
| Acceptance | Leak-test method and limit, pressure records and full-band RF measurements. |
Gas Connection
Table of Contents
Seal Type Selection
Separate the gas-fitting thread seal, valve seal and waveguide flange gasket. They perform different jobs and may use different materials. In a pressure-only flange seal, the interface must provide the required RF boundary independently; a conductive gasket is needed only where the design calls for it.
Dolph’s pressure inlet model table lists 1/8-27 NPTF Schrader-valve configurations. Its brochure separately shows 1/8-27 NPT inlet data. Resolve that distinction on the approved drawing before ordering a mating fitting.
ASME B1.20.3 describes dryseal threads designed to close the spiral leakage path through controlled metal contact. That behavior depends on conforming, undamaged mating threads. The standard also permits compatible lubricant or sealant where functionally appropriate to reduce galling.[1]
Follow the fitting manufacturer’s assembly instructions. Sealant cannot correct a mismatched thread; excess tape or paste can shed into the gas path. Keep the specified sealant and lubricant away from the RF aperture.
Fitting Size Matching
Do not substitute G1/8 for 1/8 NPTF on the basis of nominal size. G threads are parallel pipe threads, while NPTF uses a tapered dryseal geometry. Confirm pitch, thread form and sealing arrangement rather than judging compatibility from a few turns of engagement.
Match tubing outside diameter and wall thickness to the fitting, then check its pressure rating at the service temperature. The smallest bore through the valve, fitting and tube can limit purge flow even when the external connection appears large enough.
The inlet must not project into the RF aperture or leave an internal burr. Confirm thread engagement, boss dimensions, section length and the clearance needed for a gauge or valve. The published inlet table includes 19, 108 and 152 mm configurations; the brochure shows different dimensional fields for some models, so use the released drawing for the ordered part.
Support the gas line so vibration or tubing weight does not load the inlet. Avoid sharp bends and water traps, and check access to the fitting after the waveguide supports are installed.
Leak Detection Methods
Choose the test method from the required sensitivity and access. Define the test boundary, gas, pressure, temperature, stabilization time and acceptance limit before testing.
- Bubble emission: locate leaks at accessible joints with an approved liquid or an approved immersion procedure. ASTM E515 treats this as leak detection or location; it is not a quantitative measurement of total system leakage.[2]
- Pressure decay: isolate the volume and measure pressure over a defined period after temperature stabilization. Include gauge resolution, test volume and gas temperature in the record.
- Tracer gas: use a calibrated detector and a procedure suited to the leakage limit. ISO 20485 describes techniques using a tracer gas and a detector specific to that gas.[3]
Temperature drift can mimic a leak. For a sealed rigid volume with approximately ideal gas behavior, Pabsolute/T remains constant when the gas quantity is unchanged; T must be in kelvin. A calculated cooling example from 303 K to 293 K reduces absolute pressure by about 3.3% without gas loss. Do not apply that percentage directly to gauge pressure.
Use the approved test pressure and source regulation. After locating and repairing a leak, repeat the test and remove any detection-liquid residue. “No visible bubbles” is only meaningful with the stated test conditions and observation period.
Pressure Control
Maximum Pressure Limit
Review every component inside the pressure boundary, including windows, flexible sections, seals, switches, gauges, valves and tubing. The complete assembly limit must respect the weakest applicable rating and any additional joint or installation limitation.
The Dolph inlet table displays 45 PSIG for its WR650–WR28 rows. The same product page reproduces brochure ranges of 15–45 PSIG for those size groups and 15–20 PSIG for the listed WR22–WR10 groups. These are separately published configuration data, not an instruction to operate every inlet at 45 PSIG. Confirm the model, temperature and rated pressure in the order documentation.
45 PSIG is approximately 310 kPa gauge. At 101.3 kPa atmospheric pressure, that corresponds to about 412 kPa absolute. Record which pressure basis applies to each specification and instrument; also check the differential pressure across each pressure window.
Keep normal operating pressure, alarm thresholds, relief set pressure and proof/test pressure as separate fields. Do not infer a proof pressure by multiplying a catalog value by an unsupported safety factor.
Relief Valve Setting
A regulator controls normal pressure; it does not by itself establish adequate overpressure protection. Size the relief path for credible failure inflow and assess trapped-gas heating, downstream backpressure and any isolating valve that could block protection.
Select the set pressure and discharge capacity together. Account for opening tolerance, pressure rise while flowing and reseating behavior so the protected assembly remains within its applicable allowable conditions. A nominal setting below a component rating is insufficient if the relief device cannot pass the failure flow.
Use a device intended for the gas and pressure range, with documented performance. Keep its outlet unobstructed and routed to a suitable location. Repeated venting calls for diagnosis of the regulator, setpoints, thermal conditions and capacity; raising the setting is not a substitute.
Stable Flow Range
For preliminary purge sizing, Q = N × V / t, where N is the specified volume-exchange count, V is the internal volume and t is the allowed purge time. Express volume and flow at the same pressure and temperature reference. The required exchange count depends on mixing, dead volumes and the measured dryness target.
Steady supply capacity must cover maximum expected leakage and intentional bleed with the specified margin. A sealed holding system and a continuous-purge system have different normal flow patterns; continuous operation is not automatically a fault in the latter.
Check pressure at the remote end, purge completion time, source duty and gas consumption. A source that maintains pressure by replacing a large leak can hide the defect. Compare consumption with the established baseline and investigate changes.
Moisture Protection
Seal Material Selection
Choose the exact seal compound using its published temperature limits, gas compatibility, permeability and compression-set data. Silicone, FKM, EPDM and NBR each contain multiple grades; a material-family name does not establish suitability for a particular gas or outdoor exposure.
For conductive elastomers against aluminum, review filler and flange-finish compatibility under expected moisture conditions. Match groove dimensions and compression to the approved compound. Stacked gaskets or improvised sealant layers cannot correct an incorrect groove.
Drain Hole Design
Do not add an open drain to a pressurized waveguide. A normally closed service drain or a purpose-designed condensate device may be appropriate only when its pressure retention and RF effects are validated.
An outward-facing, low-cracking-pressure check valve may release pressurizing gas as soon as its opening pressure is exceeded. It does not inherently distinguish liquid from gas and should not be specified as a gas-retaining drain without supporting design data.
Any approved drain feature needs a defined location, sealing arrangement, internal finish and inspection procedure. Repeat leak and RF tests after modification. Repeated water collection calls for investigation of gas dryness, ingress and shutdown conditions.
Anti-Condensation Measures
Dew point is the temperature at which water vapor begins to condense under the stated conditions.[4] Specify the gas dew point at its measurement pressure and keep it below the coldest expected internal surface, with an agreed margin for startup, nighttime cooling and shutdown.
Evaluate external insulation or controlled heating where needed. An internal paint, foam or dielectric layer can change RF loss, field distribution and contamination behavior; use it only as part of a validated electromagnetic and environmental design.
Before commissioning, record the approved fittings and seals, regulator and relief configuration, purge result, stabilized pressure and temperature, leakage result and RF baseline in the acceptance report. Include the selected inlet drawing, required pressure, gas, dew point and testing scope when requesting a custom pressurized assembly.
