Climatic chambers for standard RTCA DO-160G Environmental conditions and test procedures for airborne equipment Section 14 Salt Fog
This test determines the effects on the equipment of prolonged exposure to a salt atmosphere or to salt fog experienced in normal operations. This standard needs a climatic chamber--salt spray test chamber.
The main adverse effects to be anticipated are:
a. Corrosion of metals.
b. Clogging or binding of moving parts as a result of salt deposits.
c. Insulation fault.
d. Damage to contacts and uncoated wiring.
When the equipment is installed in locations where it is subjected to a corrosive atmosphere in the course of normal aircraft operations, the equipment is identified as Category S and the salt fog test is applicable.
When the equipment is installed in locations where it is subjected to a severe salt atmosphere, such as equipment exposed directly to external unfiltered air on hovering aircraft that may operate or be parked near the sea, the equipment is identified as category T and the severe salt fog test is applicable.
The apparatus used in the salt fog shall include the following:
a. Exposure chamber with racks for supporting test items.
b. A salt solution reservoir with means for maintaining an adequate level of solution.
c. A means for atomizing the salt solution, including suitable nozzles and compressed air supply.
d. A means of heating and controlling chamber temperature.
e. A means for humidifying the air at temperature above the chamber temperature.
The chamber and accessories shall be constructed and arranged so that there is no direct impingement of the fog or dripping of the condensate on the test items; the fog circulates freely about all test items to the same degree, and no liquid that has come in contact with the test item returns to the salt-solution reservoir. The chamber shall be properly vented to prevent pressure build-up and allow uniform distribution of the salt fog. The discharge end of the vent shall be protected from strong drafts to prevent strong air currents in the test chamber.
The atomizers shall be designed and constructed to produce a finely divided, wet, dense fog. Atomizing nozzles shall be made of materials that are non-reactive to the salt solution.
The compressed air entering the atomizes shall be essentially free from all impurities, such as oil and dirt. Means shall be provided to humidify and warm the compressed air as required to meet the operating conditions. The air pressure shall be suitable to produce a finely divided dense fog with the atomizer used. To avoid clogging the atomizers with salt deposition, the air should have a relative humidity of at least 85 percent at the point of release from the nozzle.
A satisfactory method is to pass the air in very fine bubbles through a tower containing heated water that is automatically maintained at a constant level. The temperature of the water should be at least 35 degrees. The permissible water temperature increases with the increasing volume of air and with the decreasing heat insulation of the chamber and the chamber’s surroundings.
However, the temperature should not exceed a value above which excessive moisture is introduced into the chamber ( for example 43 degrees C at an air pressure of 84 kPa) or a value that makes it impossible to meet the requirements for operating temperature.
The salt shall be sodium chloride containing on the dry basis not more than 0.1 percent sodium iodide and not more than 0.5 percent of total impurities. Unless otherwise specified, a five ±1percent solution shall be prepared by dissolving five parts by weight of salt in 95 parts by weight of distilled or demineralized water. The solution shall be adjusted to and maintained at a relative density between the limits shown on Figure 14-1 by utilizing the measured temperature and density of the salt solution.
The PH of the salt solution shall be maintained and measured daily so that the solution atomized at 35℃ and collected by the method specified in subparagraph 14.3.6.3 will be in the PH range of 6.5 to 7.2. Only diluted chemically pure hydrochloric acid or chemically pure sodium hydroxide shall be used to adjust the PH.
The PH measurement shall be made electrometrically, using a glass electrode with a saturated potassium chloride bridge, by a colorimetric method such as bromothymol blue or other measuring instruments or litmus paper, provided the results are equivalent to those obtained with the electrometric method. The PH shall be measured when preparing each new batch of solution and as specified in subparagraph 14.3.6.4.
A filter fabricated of noncorrosive materials similar to that shown in Figure 14-2 shall be provided in the supply line and immersed in the salt solution reservoir as illustrated in Figure 14-2.
The test shall be conducted with a temperature in the exposure zone maintained at 35 degrees C. Satisfactory methods for controlling the temperature accurately are by housing the apparatus in a properly controlled constant temperature room, by thoroughly insulating the apparatus and preheating the air to the proper temperature prior to atomization or by jacketing the apparatus and controlling the temperature of the water or of the air used in the jacket. The use of immersion heaters within the chamber for the purpose of maintaining the temperature within the exposure zone is prohibited.
Suitable atomization has been obtained in chambers having a volume of less than 0.34m³ with the following conditions:
a. Nozzle pressure shall be as low as practical to produce fog at the required rate.
b. Orifices shall be between 0.5 mm and 0.8 mm in diameter.
c. Atomization of approximately three liters of salt solution per 0.28m³ of chamber volume per 24 hours.
When using large size chambers having a volume considerably in excess of 0.34m³, the condition specified may require modification to meet the requirements for operating conditions.
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