No abstract

Leakages often occur in the copper tube – finned heat exchangers of cooling and air conditioning units, the dimensions of which are on the microscopic scale thus making the location and cause very difficult to determine. In the case presented here the leakages found were located using a Helium leak detector. After removing the fins of the heat exchanger, because the points of failure were not visible to the naked eye, a dye penetrant test was carried out on the affected area (Penetrations test or PT), which led to the discovery of three very small indications on one section of the tube. Subsequent microscopic laboratory examination revealed the cause to be microscopically small holes in the wall of the tube caused by corrosion through from the internal surface of the coolant side of the tube. The corrosion mechanism was identified as Formicary Corrosion otherwise referred to as Ant-Nest Corrosion. For this type of corrosion to occur, which according to the specialist literature is often found in Cooling or Air Conditioning units, an aqueous electrolyte or electrolytic film containing Carbonic acid and Oxygen is required which can lead to damage occurring within weeks to months. In the case described in this article, the unintentional ingress of an aqueous oxygenated acidic solution of Glycol into the heat exchanger was most probably responsible for the damage.

Negative temperature coefficient (NTC) and positive temperature coefficient (PTC) thick fhn thermistor pastes were developed. Using these materials, cost effective Temperature Variable Attenuators (TVA) for use up to 18 GHz are described. These devices maintain the signal level in amplifiers, switches, delay lines, and other microwave and RF signal processing devices. By combining (NTC) and (PTC) thennistors in "T" and "n'' configurations, the attenuation changes to compensate for temperature induced signal level changes. Judicious choice of resistance and TCR yields constant impedance over the operating range thus avoiding unwanted signal reflections.While the temperature sensitivity of NTC and PTC chip thennistors is large, their use in TVAs requires a large and expensive inventory to provide the required resistance and TCR values. Thick f b i thermistor systems were developed to overcome these limitations. These include an NTC materials series with TCR values as high as -500,000 ppm/"C and resistance range coverage h m 100 Rlsq. to 1M Rlsq. The complimentary PTC system features linear TCR values of 3000 ppm/OC and greater with resistance values of 5 to 50OORlsq.Process variables for these new thick fhn thermistor materials are discussed. Attenuation and frequency curves are presented. Impedance match, as characterized by Voltage Standing Wave Ratio (VSWR) versus frequency curves are presented. Current applications include Thennopad@ type TVA's with improved high frequency perfonnance appropriate for mobile communications and similar uses.

Granulocytic colonies grown in culture from marrow and peripheral blood from five patients with Ph1-positive CML and heterozygous at the G-6-PD locus were analyzed for G-6-PD in order to identify CFU-C that do not arise from the CML clone. The patients had both B and A enzymes in normal tissues, but their CML clones typed as B. Whereas about 50% of colonies from normal subjects heterozygous as the G-6-PD locus show type-A G-6-PD and 50% type B, only two of the 1308 colonies from the CML patients had type-A G-6-PD. These data provide little evidence for persistence of normal committed stem cells in CML, a finding in contrast to that made previously in polycythemia vera, another clonal stem cell myeloproliferative disorder.

This paper discusses the status of thick conductive films formed by firing in air. The metals under study include nickel, chromium, aluminum and copper. The properties of the films are presented for firing temperatures ranging from 580 to 850 degrees C. Compatibility with various dielectrics and glasses have been studied. Multilayer wiring seems promising. Potential uses on porcelain enamelled steel, window glass and alumina substrates seem feasible.Initial studies have been made of compatibility with thick film ruthenium-based resistors. Some preliminary information on termination resistance and other properties observed in such combinations is presented.Possible applications in gas discharge displays for cathodes and conductor runs are discussed. Potential uses may include economical ground planes, solar cell electrodes, varistor and thermistor terminations, and resistance thermometers.