Aqueous Alternatives for Cleaning Measurement Instrumentation
Rosemount Aerospace Inc. is a large manufacturer of air data instrumentation used on both commercial and military aircraft. The instrumentation measures parameters such as air speed and altitude. This intern project consisted of testing aqueous alternatives to the chlorinated solvents that Rosemount Aerospace currently uses for cleaning two key components of the instrumentation: temperature sensors and pitot static tubes.
Sensors are lightweight components that come in various sizes, ranging from one-half to two inches in diameter. Each sensor is constructed with metal and glass. To fabricate this part, glass is cleaned with CFC-113 (TMS) and then melted onto the metal. After a sensor is assembled, it comes in contact with silicone oil during testing. 1,1,1-trichloroethane (TCA) is used to remove silicone oil before a soldering process.
Pitot static tubes deliver air pressures from outside an aircraft to the sensors to help determine an aircraft’s velocity, altitude and angle of attack. Rosemount Aerospace manufactures hundreds of different pitot static tube models. The majority of the metal tubes are four to 15 inches long, with a few models 2.5 feet in length. TCA is used in a vapor degreaser to remove coolants and cutting oils that accumulate on the tubes during turning and drilling operations.
Incentive for Change
Rosemount Aerospace wanted to find alternative cleaning systems to TCA and CFC-113 degreasing due to the 1990 Clean Air Act’s production phase-out of ozone-depleting chemicals scheduled for January 1, 1996. TCA and CFC-113 are two of the many chlorinated solvents on the list to be eliminated.
Prior to this intern project, Rosemount Aerospace had already started reducing the amount of TCA used. Between 1989 and 1992, the company’s annual TCA use dropped from 23,000 pounds to 15,000 pounds, primarily through conservation. To further reduce use of chlorinated solvents, the company assembled a Degreaser Replacement Team to identify objectives for replacing their current degreasing practices in their machining and assembly areas.
A MnTAP intern worked closely with the Degreaser Replacement Team in testing various cleaning methods. The primary focus of the testing was on aqueous immersion methods. The MnTAP intern evaluated different cleaning chemistries and different types of agitation. Aqueous cleaners were targeted for research because of their low toxicity and nonflammability. In addition, three types of agitation were tested: simple mechanical, flushing solution through tube, and ultrasonics.
Pitot Tube Project
Samples of pitot static tubes were taken to the machining area, filled with either a waterbased coolant or a cutting oil, then cleaned in an aqueous solution. A number of tubes were cleaned with various aqueous solutions and agitation methods, followed by “smoke” cleanliness testing.
Since each pitot static tube is fitted with an internal heating coil, these sample parts were tested for cleanliness by heating the tube assembly, and observing any smoke. The presence of smoke indicated uncleaned residue inside the tube.
Results: Pitot Tube Project
Most aqueous cleaners, even with simple agitation, yielded smoke-free tubes when the soil was a waterbased coolant. However, when the soil was cutting oil, the aqueous cleaners did not clean the inside of the tubes as well as the outside of the tubes. This was especially true when testing simple mechanical and flush-through agitation. These tests did not yield smoke-free parts.
Results with ultrasonic agitation were highly dependent on the equipment and method used. For these tests, the ultrasonic power density and the tube orientation were critical variables. Also, better results were achieved when the tubes were allowed to drain and refill multiple times throughout the ultrasonic immersion cycle.
The intern also tested aqueous and semi-aqueous cleaners for removing light soils and finger oils from sample sensors prior to glass fusing, and for removing silicon oil prior to soldering. The cleanliness test for the former was observing the quality of the resulting glass, and for the latter, a water-break test followed by soldering and inspection of the solder joint.
Results: Sensor Project
All eight aqueous and semi-aqueous cleaners tested on the sensors removed light oils and fingerprints at least as well as the existing vapor degreasing system. Three aqueous cleaners, Fremont Industries 410 (Shakopee, MN, 952/445-4121), LPS Precision Clean (Tucker, GA, 800/241-8334), and Micro International Products Corporation (Burlington, NJ, 609/386-8770), also effectively removed silicon oil from sensors. This was demonstrated by favorable results in soldering tests.