Some improvements have been made to enhance the role of water as a test solvent for determining the amount of hydrocarbon nonvolatile residue (NVR) present on an item of hardware that is required to be totally or nearly devoid of such residue. Water is now used as an NVR-testing solvent because (1) even highmolecular- weight hydrocarbon greases are at least slightly soluble in water, (2) water is safer and less expensive than are chlorofluorocarbon solvents [in particular, 1,1,2- trichloro-1,2,2-trifluoroethane (also known as CFC-113), which was used previously in testing for hydrocarbon NVR], and (3) CFC solvents are expensive and are now recognized as environmentally harmful.
The present improvements address two major aspects of the NVR-testing problem: (1) increasing the efficiency of removal of NVR from hardware by use of water and (2) increasing the sensitivity of measuring the concentration of NVR dissolved in the water. Prior to these improvements, it was known that when small parts are subjected to ultrasound in a water bath and NVR is present, the amount of NVR dispersed into the water is sufficient to enable measurement of the concentration of the dissolved NVR by the total-organic-carbon (TOC) method.
The present improvements extend the applicability of water-wash NVR testing to parts too large to fit in an ultrasonic bath. The efficiency of removal of NVR from a large part can be increased somewhat by use of steam or high-pressure spray of water, with or without a gas mixed in. However, the technique found to be most efficient (and least dependent on the size of the part) is spraying water on the surface of the part from an ultrasonic nozzle.
The increase in the sensitivity of measurement is achieved by concentrating the NVR from the wash water before attempting the measurement. For this purpose, all of the wash water is made to flow through a bed of silica gel. During this flow, a significant portion of the dissolved NVR becomes adsorbed onto the silica gel particles. Then part or all of the silica gel is analyzed by the TOC method. Tests have shown that by use of both ultrasonic spraying of water and measurement of the TOC of silica gel through which the wash water has passed, it is possible to measure areal concentrations of NVR as low as 0.05 mg/ft² (≈0.5 mg/m²) on the surfaces of parts under test.
This work was done by Christian Clausen III of the University of Central Florida for Kennedy Space Center.