2.1 Selecting the Rain Method
After examining the requirements documents and applying the tailoring process in Part One of this Standard to determine where rain is foreseen in the life cycle of the materiel, use the following to aid in selecting this Method and placing it in sequence with other methods. The term "rain" encompasses the full range of "free water" (blowing, steady state, drip) tests included in this Method.
2.1.1 Effects of Rain Environments
Rain (when falling, upon impact, and as deposited as pooled water) has a variety of effects on materiel. Consider the following typical problems to help determine if this Method is appropriate for the materiel being tested. This list is not intended to be all-inclusive, and some of the examples may overlap the categories.
2.1.1.1 In the Atmosphere
In the atmosphere the effects resulting from exposure to these environments include:
- a. Interference with or degradation of radio communication.
- b. Limited radar effectiveness.
- c. Limited aircraft operations due to restricted visibility and decreased lift from wing surfaces (excessive rain rates only).
- d. Damage to aircraft in flight.
- e. Affect on munitions launch and flight.
- f. Degradation or negation of optical surveillance.
- g. Decreased effectiveness of personnel in exposed activities.
- h. Premature functioning of some fuses.
- i. Inhibited visibility through optical devices.
2.1.1.2 On Impact
On impact it erodes surfaces.
2.1.1.3 After Deposition and/or Penetration
After deposition and/or penetration, the effects resulting from exposure to these environments include:
- a. Degraded strength/swelling of some materials.
- b. Increased corrosion potential, erosion, or even fungal growth.
- c. Increased weight.
- d. Electrical or electronic apparatus become inoperative or unsafe.
- e. Malfunction of electrical materiel.
- f. Freezing inside materiel that may cause delayed deterioration and malfunction by swelling or cracking of parts.
- g. Modified thermal exchange.
- h. Slower burning of propellants.
2.1.2 Sequence Among Other Methods
- a. General. Use the anticipated life cycle sequence of events as a general sequence guide (see Part One, paragraph 5.5).
- b. Unique to this Method. This Method is applicable at any stage in the test program, but its effectiveness in determining the integrity of an enclosure is maximized if it is performed after the dynamic tests.
MIL-STD-810H
METHOD 506.6
2.2 Selecting Procedures
This Method includes three rain-related test procedures: Procedure I (Rain and Blowing Rain), Procedure II (Exaggerated), and Procedure III (Drip). Before conducting the test, determine which test procedure(s) and test conditions are appropriate.
2.2.1 Procedure Selection Considerations
Differences among rain test procedures are explained below. Select the procedure that represents the most severe exposure anticipated for the materiel commensurate with materiel size. When selecting a procedure, consider:
- a. The materiel configuration.
- b. The logistical and operational requirements (purpose) of the materiel.
- c. The operational purpose of the materiel and data to verify it has been met.
- d. The natural exposure circumstances.
- e. Procedure sequence.
2.2.2 Difference Among Procedures
- a. Procedure I - Rain and Blowing Rain. Procedure I is applicable for materiel that will be deployed out-of-doors and that will be unprotected from rain or blowing rain. The accompanying wind velocity can vary from almost calm to extremely high. Consider using Procedure II for materiel that cannot be adequately tested with this procedure because of its (large) size.
- b. Procedure II - Exaggerated. Consider Procedure II when large (shelter-size) materiel is to be tested and a blowing-rain facility is not available or practical. This procedure is not intended to simulate natural rainfall but will provide a high degree of confidence in the watertightness of materiel.
- c. Procedure III - Drip. Procedure III is appropriate when materiel is normally protected from rain but may be exposed to falling water from condensation or leakage from upper surfaces. There are two variations to the drip test:
- (1) for materiel that may experience falling water (generally from condensation), and
- (2) for materiel that may be subjected to heavy condensation or leaks from above.
2.3 Determine Test Levels and Conditions
Having selected this Method and relevant procedures (based on the materiel's requirements documents and the tailoring process), it is necessary to complete the tailoring process by selecting specific parameter levels and special test conditions/techniques for these procedures based on requirements documents or Life Cycle Environmental Profile (LCEP) (see Part One, Figure 1-1), and information provided with this procedure. From these sources of information, determine the functions to be performed by the materiel in rain environments or following storage in rain environments. Then determine the rainfall levels of the geographical areas and micro-environments in which the materiel is designed to be employed. Variables under each test procedure include the test item configuration, rainfall rate, wind velocity, test item exposure surfaces, water pressure, and any additional appropriate guidelines in accordance with the requirements document.
2.3.1 Test Item Configuration
Perform the test using all the configurations in which the materiel may be placed during its life cycle. As a minimum, consider the following configurations:
- a. In a shipping/storage container or transit case.
- b. Protected or not protected.
- c. In its operational configuration.
- d. Modified with kits for special applications.
MIL-STD-810H
METHOD 506.6
NOTE: Do not use any sealing, taping, caulking, etc., except as required by the design specification for the materiel. Unless otherwise specified, do not use test items that have surface contamination such as oil, grease, or dirt that could prevent wetting.
2.3.2 Rainfall / Drip Rate
- a. Procedure I – Rain and Blowing Rain: The rainfall rate used in Procedure I may be tailored to the anticipated deployment locale and duration. Although various rainfall intensities have been measured in areas of heavy rainfall, recommend a minimum rate of 1.7 mm/min (4 in/hr) since it is not an uncommon occurrence, and would provide a reasonable degree of confidence in the materiel. MIL-HDBK-310 (paragraph 6.1, reference a) contains further information. During the pretest set-up, rain fall measurements should be taken at a minimum of 5 random locations. The average of these rain rate measurements should be within 10% or +/- 0.1 mm/min (0.25 in/hr) of the specified value, whichever is less. To ensure a uniform distribution of simulated rain on the test item each measurement should be within 25% or +/- 0.2 mm/min (0.5 in/hr) of the specified rain rate whichever is less.
- b. Procedure II - Exaggerated: This procedure uses (as a guideline) a 276 kPa (40 psig) nozzle pressure with a flow rate of 20.8 L/min (5.5 gal/min) that should produce water droplets traveling at approximately 64 km/h (40 mph) when using a nozzle such as specified in paragraph 4.1.2.
- c. Procedure III - Drip: The drip test has a requirement for a volume of water greater than 280 L/m²/hr (7 gal/ft²/hr) dripping through a pre-determined hole pattern. An alternative requirement is for items exposed only to 140 L/m²/hr (3.5 gal/ ft²/hr): Appropriately reduce the drip rate as long as the duration of the test is extended to 30 minutes to ensure the equivalent volume of water falls on the test item.
2.3.3 Droplet Size
Nominal drop-size spectra exist for instantaneous rainfall rates but for the long-term rainfall rates they are meaningless since rates are made up of many different instantaneous rates possessing different spectra (paragraph 6.1, reference a). For Procedures I and II, use droplet sizes predominantly in the range of approximately 500 μm in diameter (that is considered to be mist or drizzle rather than rain (paragraph 6.1, reference b), to 4500 μm in diameter (paragraph 6.1, reference c). For lower rain rates, it may be difficult to achieve specified droplet size. For drip tests using dispensing tubes (Figure 506.6-1), polyethylene tubing sleeves added to the dispensing tubes will increase the droplet size to its maximum. Procedure III is not meant to simulate rain but rather droplets of condensation or overhead leakage, and therefore droplets may be larger than 4500 μm in diameter. Since the drip test is not to simulate rain, the droplets do not need to reach terminal velocity. It is possible to achieve larger droplet sizes, since the air resistance may not be sufficient to cause them to break up. The largest drop size that can be achieved without coalescence is recommended.
NOTE: Observations have shown that water droplets introduced into a high velocity air stream tend to break up over distance (paragraph 6.1, references d and e). Accordingly, recommend introducing the droplets as close as possible to the test item while assuring the droplets achieve the required velocity prior to impact with the test item.
2.3.4 Wind Velocity
High rainfall intensities accompanied by winds of 18 m/s (40 mph) are not uncommon during storms. Unless otherwise specified or when steady state conditions are specified, recommend this velocity. Where facility limitations preclude the use of wind, use Procedure II.
NOTE: Without straightening vanes, fans may not produce the required wind velocity near the center of the wind stream.
MIL-STD-810H
METHOD 506.6
2.3.5 Test Item Exposure Surface (Orientation)
Wind-blown rain will usually have more of an effect on vertical surfaces than on horizontal surfaces, and vice versa for vertical or near-vertical rain. Expose all surfaces onto which the rain could fall or be driven to the test conditions. Rotate the item as required to expose all vulnerable surfaces.
2.3.6 Water Pressure
Procedure II relies on pressurized water. Vary the pressure as necessary to comply with the requirements documents, but a minimum value of 276 kPa (40 psig) nozzle pressure is given as a guideline based on past experience. This value will produce water droplets traveling at approximately 64 km/h (40 mph) when using a nozzle as specified in paragraph 4.1.2.
2.3.7 Preheat Temperature
Experience has shown that a temperature differential between the test item and the rainwater can affect the results of a rain test. When specified for nominally sealed items, increasing the test item temperature to about 10 °C (18 °F) higher than the rain temperature at the beginning of each exposure period to subsequently produce a negative pressure inside the test item will provide a more reliable verification of its watertightness. Ensure the heating time is the minimum required to stabilize the test item temperature, and not sufficient to dry the test item when not opened between exposures.
2.3.8 Exposure Duration
Determine the exposure duration from the life cycle profile, but do not use a duration less than that specified in the individual procedures. For items made of material that may absorb moisture, the duration may have to be significantly extended to reflect real life cycle circumstances and, for drip tests, the drip rate appropriately reduced. With certain materials, the water penetration and thus the degradation is more a function of time (length of exposure) than the volume or rain/drip rate exposure.