MIL-STD-810 Method 506.6 — Rain Test Procedures I, II and III Compared.

MIL-STD-810 Method 506.6 evaluates how equipment withstands rain and water ingress — how well seals, covers and enclosures keep water out, whether the item still performs during and after exposure, and whether rain causes any physical deterioration. The method defines three procedures, each reproducing a different real-world water exposure. The right choice depends on how the equipment is deployed and how large it is.

Procedure I — Rain and Blowing Rain

Procedure I reproduces wind-driven natural rain on outdoor equipment that is unprotected from the weather. A minimum rainfall rate of 1.7 mm/min (about 4 in/hr) is recommended, with droplets near 500 μm, accompanied by a wind velocity around 18 m/s (about 64 km/h); the rate can be tailored to the expected deployment region and duration. The specimen is usually conditioned about 10 °C warmer than the water — as the rain cools it, the internal pressure drops slightly and draws water toward any weak seal, revealing ingress paths that a static test would miss.

Use it when: equipment is deployed outdoors and exposed to rain or blowing rain, and is small enough for a blowing-rain facility.

Procedure II — Exaggerated

Procedure II is not intended to simulate natural rain. It applies water deliberately hard — through overlapping pressure nozzles at 276 kPa (50 psig) with a flow of about 20.8 L/min (5.5 gal/min), with droplets driven at roughly 64 km/h — to build a high degree of confidence in watertightness.

Use it when: the item is too large (shelter-size) for a blowing-rain facility, or such a facility isn't available or practical. It is the pragmatic substitute for Procedure I on large equipment.

Procedure III — Drip

Procedure III reproduces water falling from above — condensation or leakage onto equipment that is normally protected from direct rain. Water drips from a defined hole pattern at a rate greater than 280 L/m² per hour (7 gal/ft²/hr). An alternative covers items exposed to 140 L/m² per hour (3.5 gal/ft²/hr): the drip rate is reduced while the test is extended to 30 minutes, so the same total volume of water reaches the item.

Use it when: equipment is normally sheltered but may face dripping or heavy condensation — inside shelters, cargo holds, or covered-but-humid spaces.

Choosing — and combining

A quick guide: Procedure I for outdoor, exposed, smaller equipment; Procedure II for large or shelter-size equipment, or where a blowing-rain facility isn't practical; Procedure III for sheltered equipment facing only dripping or condensation. Select the procedure that represents the most severe realistic exposure for the item's size. The procedures are not mutually exclusive — they can be run individually, in pairs, or all three, so a single test program can cover several exposure modes at once.

Testing 506.6 with ULMEKA

ULMEKA's MIL-STD-810 506.6 rain systems run Procedures I, II and III — individually or in any combination — under PLC + HMI control with real-time monitoring of pressure, flow and temperature. For self-contained specimens, see the Rainfall Test Cabinet; for vehicle- or shelter-class items, the Walk-in Rain Test Room. Tell us your standard, procedures and specimen dimensions and we will propose a matched system.