4.1 Procedure I – Blowing Dust.
4.2 Procedure II – Blowing Sand
4.2.1 Test Levels and Conditions
4.2.1.1 Temperature
Unless otherwise specified, conduct the blowing sand tests with the test item at the high operating or storage temperature. In the absence of this information, perform the tests at the maximum ambient air temperature for the A1 climatic category induced or ambient, as required.
4.2.1.2 Air Velocity
Winds of 18 m/s (40 mph) capable of blowing the large particle sand are common, while gusts up to 29 m/s (65 mph) are not unusual. Recommend using an air velocity of 18 m/s (40 mph) or greater to ensure the blowing sand particles remain suspended in the air stream. If the induced flow velocity around the materiel in its field application is known to be outside of this range, use the known velocity.
NOTE: Ensure the sand particles impact the test item at velocities ranging from 18-29 m/s (40-65 mph). In order for the particles to attain these velocities, maintain an approximate distance of 3 m (10 ft) from the sand injection point to the test item. Use shorter distances if it can be proven the particles achieve the necessary velocity at impact.
MIL-STD-810H
METHOD 510.7
For typical testing, uniform wind speeds are provided across the test area. If the test item is large and at ground level some consideration should be given to account for the wind profile from the ground to the test item height.
The sampling rate for wind speed measurements will be a minimum of 4 samples per second. The steady state wind speeds will be verified by averaging the wind speeds over 10 seconds with the wind generation equipment controls held constant. A gust is defined as a 3 second period at the level of increased wind speed. When performing a test with gusts, a minimum of four gusts per hour is recommended. The accuracy of wind measurement devices, such as cup, propeller and hot wire anemometers, are negatively affected by the sand and dust environment, therefore wind speed measurements should be taken in clean air with these devices. Variation in wind direction, or high turbulence, can also influence cup and propeller measurement accuracy.
4.2.1.3 Sand Composition. WARNING: Refer to the supplier's Safety Data Sheet (SDS) or equivalent for health hazard data. Exposure to crystalline silica can cause silicosis; other material may cause adverse health effects
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Unless otherwise specified, for the sand test, use quartz sand (at least 95 percent by weight SiO2). Use sand of sub- angular structure, a mean Krumbein number range of 0.5 to 0.7 for both roundness and sphericity, and a hardness factor of 7 Mohs. Due to the loss of subangular structure and contamination, re-use of test sand is normally not possible. If possible, determine the particle size distribution from the geographical region in which the materiel will be deployed. There are 90 deserts in the world, each with different particle size distributions. Therefore, it is impossible to specify a particle size distribution that encompasses all areas. The recommended particle size distribution for the sand test is from 150 µm to 850 µm, with a mean of 90 ±5 percent by weight smaller than 600 µm and larger than or equal to 150 µm, and at least 5 percent by weight 600 µm and larger. When materiel is designed for use in a region that is known to have an unusual or special sand requirement, analyze a sample of such sand to determine the distribution of the material used in the test. Specify the details of its composition in the requirements documents.
4.2.1.4 Sand Concentrations
Unless otherwise specified, maintain the sand concentrations as follows (references 6.1a & b):
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- a. For materiel likely to be used close to helicopters and other aircraft operating over unpaved surfaces: 2.2 ±0.5 g/m3.
- b. For material never used or exposed in the vicinity of operating aircraft, but that may be used or stored unprotected near operating surface vehicles: 1.1 ±0.3 g/m3.
- c. For material that will be subjected only to natural conditions: 0.18 g/m3, -0.0/+0.2 g/m3. (This large tolerance is due to the difficulties of measuring concentrations at low levels.)
NOTE: If the wind velocity is increased intermittently to simulate a gust it is permissible to allow the sand concentration to be reduced for this period of time of intermittent increase in wind speed.
4.2.1.5 Orientation
Orient the test item with respect to the direction of the blowing sand such that the test item will experience maximum erosion effects. The test item may be re-oriented at 90-minute intervals. Consider the incident angle of sand particle impact on the severity of erosion in selecting orientations. When possible, evaluate of the airflow around the test item in-service to determine required chamber boundary conditions to create similar airflow and cooling conditions.
4.2.1.6 Duration
Perform blowing sand tests for a minimum of 90 minutes per each vulnerable face.
MIL-STD-810H
METHOD 510.7
4.2.1.7 Operation During Test
Determine the need to operate the test item during exposure to sand from the anticipated in-service operational requirements. For example, in addition to items that are exposed directly to natural conditions, consider items inside environmentally controlled enclosures that should be operated while the enclosure is exposed to the blowing sand environment. This should include operation of ECUs to ensure the adverse environment does not result in a failure of the test item to meet performance requirements. If the test item must be operated during the test, specify the time and periods of operation in the test plan. Include at least one 10-minute period of operation of the test item during the last hour of the test, with the test item's most vulnerable surface facing the blowing sand. Ensure the period of operation includes the essential operational requirements.
A build-up of sand, especially in internal compartments, will reduce the ability of the test item to shed the thermal load generated by electronics. If sand accumulations are expected in the field, perform the operational test without removing the sand. To fully evaluate the performance of the sand laden materiel in low wind speed and high temperature conditions, consider performing a high temperature operational test on the sand laden materiel using the procedures provided in Methods 501 or 505.
Consider manipulation of the test item as part of the operational test. This may uncover issues with binding or wear of moving parts. The test item should be manipulated in the same manner it would be utilized in the field in a desert environment.
4.2.2 Information Required – Refer to Paragraphs 3.1 to 3.3
4.2.3 Test Details
4.2.3.1 Test Facility
- a. Ground the test item and facility to avoid buildup of an electrostatic charge. Verify resistance/continuity in accordance with applicable safety requirements for the materiel. Employ a data collection system to measure the test volume conditions (see Part One, paragraph 5.18). Except for gaseous nitrogen (GN2), achieve dehumidification, heating and cooling of the air envelope surrounding the test item by methods that do not change the chemical composition of the air, dust, sand, and water vapor within the chamber test volume air. The following information is also appropriate.
- b. Test facility design considerations.
- (1) In order to provide adequate circulation of the sand-laden air, use a test chamber of sufficient size that no more than 50 percent of the test chamber's cross-sectional area (normal to airflow) and 30 percent of the volume of the test chamber is occupied by the test item(s).
- (2) Control the sand feeder to emit the sand at the specified concentrations. To simulate the effects produced in the field, locate the feeder to ensure the sand is approximately uniformly suspended in the air stream when it strikes the test item.
- (3) Because of the abrasive characteristics of blowing sand, do not re-circulate the sand through the fan or air conditioning equipment.
4.2.3.2 Controls
Record chamber temperature and humidity in accordance with Part One, paragraphs 5.2 and 5.18 at a sufficient rate to satisfy the post-test analysis (see Part One, paragraph 5.18), and provide sand rate calculations for each test interval. Verify chamber air velocity and sand concentration prior to test. Calculate the sand feed rate and verify it by measuring the sand quantity delivered over unit time using the following formula:
Feed Rate = (Concentration)(Area)(Velocity)
where:
Feed Rate = mass of sand introduced into the test chamber per set time interval
Concentration = sand concentration required by the test plan
Area = cross-sectional area of the sand laden wind stream at the test item location.
Velocity = average velocity of air across the cross-sectional area at the test item location
MIL-STD-810H METHOD 510.7
4.2.3.3 Test Interruption
Test interruptions can result from two or more situations, one being from failure or malfunction of test chambers or associated test laboratory equipment. The second type of test interruption results from failure or malfunction of the test item itself during operational checks.
4.2.3.3.1 Interruption Due To Chamber Malfunction
- a. General. See Part One, paragraph 5.11 of this Standard.
- b. Specific to this Method.
- (1) Undertest interruption. Follow any undertest interruption by reestablishing the prescribed test conditions and continue from the point of interruption.
- (2) Overtest interruption. Following exposure to excessive sand concentrations, remove as much of the accumulation as possible (as would be done in service) and continue from the point of interruption. If abrasion is of concern, either restart the test with a new test item or reduce the exposure period by using the concentration-time equivalency (assuming the overtest concentration rate is known).
4.2.3.3.2 Interruption Due To Test Item Operation Failure
Failure of the test item(s) to function as required during operational checks presents a situation with several possible options.
- a. The preferable option is to replace the test item with a “new” one and restart from Step 1.
- b. A second option is to replace / repair the failed or non-functioning component or assembly with one that functions as intended, and restart the entire test from Step 1.
NOTE: When evaluating failure interruptions, consider prior testing on the same test item and consequences of such.
4.2.4 Test Execution
The following steps, alone or in combination, provide the basis for collecting necessary information concerning the test item in sand environments.
4.2.4.1 Preparation For Test
WARNING: The relatively dry test environment combined with the moving air, sand particles may cause a buildup of electrostatic energy that could affect operation of the test item. Be aware of potential anomalies caused by electrostatic discharge during test item checkout.
4.2.4.1.1 Preliminary Steps
Before starting the test, review pretest information in the currently approved test plan to determine test details (e.g., procedures, item configuration, cycles, durations, parameter levels for storage/operation, etc.). (See paragraph 4.2.1, above.)
- a. Determine from the test plan specific test variables to be used.
- b. Operate the test chamber without the test item to confirm proper operation.
- (1) Calibrate the sand dispensing system for the sand concentration specified in the test plan.
- (2) Operate the test chamber without the test item to confirm proper operation. Adjust the air system or test item position to obtain the specified air velocity for the test item.
MIL-STD-810H
METHOD 510.7
4.2.4.1.2 Pretest Standard Ambient Checkout
All items require a pretest standard ambient checkout to provide baseline data. Conduct the pretest checkout as follows:
Step 1. Conduct a complete visual examination of the test item with special attention to sealed areas and small/minute openings, and document the results.
Step 2. Prepare the test item in its operating configuration or as specified in the test plan. Install test item instrumentation as required by the test plan. When applying surface mount thermocouples, minimize the coverage of the test item surface to the greatest extent possible.
Step 3. Position the test item at the required distance from the sand injection point. Orient the test item to expose the first face to the sand stream.
Step 4. Ensure the test item is grounded (either through direct contact with the test chamber or with a grounding strap).
NOTE: If required to change the orientation during the test, ensure that the instrumentation and fixtures will allow this to occur
Step 5. Stabilize the test item temperature at standard ambient conditions.
Step 6. Conduct an operational checkout in accordance with the test plan and record the results.
Step 7. If the test item operates satisfactorily, proceed to Step 1 of the test procedure. If not, resolve the problem and restart at Step 1 of pretest checkout.
4.2.4.2 Test Procedure II. Blowing Sand WARNING:
- Refer to the supplier's Safety Data Sheet (SDS) or equivalent for health hazard data.
- The relatively dry test environment combined with the moving air and sand particles may cause a buildup of electrostatic energy that could affect operation of the test item.
NOTE: Unless the requirements documents indicate otherwise, if the following test procedure is interrupted because of work schedules, etc., maintaining the test item at the test temperature for the time required will facilitate completion of the test when resumed. If the temperature is changed, before continuing the test, re-stabilize the test item at the temperature of the last successfully completed period before the interruption.
CAUTION: When temperature conditioning, ensure the total test time at elevated temperatures do not exceed the life expectancy of any safety critical materials. This is particularly applicable to energetic materials (see Part One, paragraph 5.19).
Step 1. Increase the chamber temperature (at a rate not to exceed 3 °C/min (5 °F/min)) and stabilize the test item at the required high temperature (see paragraph 4.2.1.1).
Step 2. Adjust the air velocity according to test plan (see paragraph 4.2.1.2).
Step 3. Adjust the sand feeder to obtain the sand mass flow rate determined from the pretest calibration.
Step 4. Maintain the conditions of Steps 1 through 3 for the duration specified in the test plan. If required, interrupt the blowing sand and re-orient the test item at 90-minute intervals to expose all vulnerable faces to blowing sand, and repeat Steps 1-3.
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Step 5. If operation of the test item during the test is required, perform an operational test with the most vulnerable face exposed during the last hour of the test, and document the results. If the test item fails to operate as intended, follow the guidance in paragraph 4.2.3.3. Otherwise proceed to Step 6. SEE THE ABOVE WARNING REGARDING HEALTH HAZARDS.
MIL-STD-810H
METHOD 510.7
Step 6. Stop the sand feed. Allow the chamber air temperature to return to standard ambient conditions at a rate not to exceed 3 °C/min (5 °F/min). Stabilize the test item temperature. Stop any air flow through the chamber.
Step 7. Visually inspect the item looking for clogging effects, abrasion, and sand accumulation that may impede operation of the test item.
Step 8. Photograph the sand accumulation on the test item.
Step 9. Remove accumulated sand from the test item by using the methods anticipated to be used in service such as brushing, wiping, etc., taking care to avoid introduction of additional sand into the test item.
Step 10.Visually inspect the test item looking for abrasion and clogging effects, and any evidence of sand penetration. Document the results.
Step 11. Conduct an operational check of the test item in accordance with the approved test plan, and record results for comparison with pretest data. See paragraph 5.2 for analysis of results.