Faqs

Did You Know? MIL-STD-810

Did you know that single axis vibration testing requires cross axis control?

Discussion:  MIL-STD-810 establishes a requirement to limit cross axis acceleration to 0.45 times the axis under test acceleration.  This calls for installing accelerometers in both axis orthogonal to the primary axis or using a multi-axis accelerometer.  If the cross axis vibration is greater than 3.5dB below the primary axis acceleration, then the cause needs to be determined.  If the cause is related to the vibration test fixture inducing the cross axis issue, then the fixture should be corrected to eliminate or move the resonant frequency outside the test range.  If the test article is the source, you may need to limit the drive spectrum or consider multi-axis testing.

Article Reference ID: DYK00006

Did you know that IPX7 and MIL-STD-810 Immersion are “almost” the same?

In general procedure, both tests are performed the same way; immersion in 1-meter of fresh water for 30-minutes. The primary differences lie in smaller details; MIL-STD 810G testing is typically performed with the unit to be tested either 10C or 27C above the water temperature depending on deployment. This allows for contraction of air in any internal voids during testing, which results in a slight negative pressure inside the unit. IEC 60529, the IPX7 test, requires that the test unit is within 5K of the water temperature. The immersion depth is also measured slightly differently between the methods; 810G measures the 1 meter immersion depth from the highest surface of the test unit, while the IPX7 test is measured from the bottom of the test unit.

Pass/fail criteria between the two standards is also similar; both allow water ingress, by requiring the manufacturer determine what an acceptable level of water ingress is, but also require that the test unit function following (and in some cases, during) the immersion test and not result in a hazardous condition due to water ingress (electrical shorts, etc.). For small pieces of equipment, and units that cannot be disassembled (epoxied clamshells), we typically measure the weight before and following testing, to determine if any water ingress occurred. For some test units, where it cannot be disassembled and where it is difficult to remove all water by patting dry following the test due to the physical shape of the unit, the pass/fail criteria is typically limited to confirming the equipment operates following testing, and doesn’t pose a hazard (water ingress to transformers, batteries, etc).

Of the two tests, the 810G procedure is slightly more difficult to pass due to the temperature differential requirement, but as a general statement both tests are very similar. I would also note that IPX8 testing is essentially the same as IPX7 testing, but with either a longer duration, or a deeper depth requirement; a 45-minute test at 1 meter, or a 30-minute test at 1.3 meters, would both qualify as IPX8.

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Article Reference ID: DYK00014