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Transformer-rectifiers have moveable parts that/end to wear out. However these scooter machines can produce some of the best arc characteristics.A silicon-coated-rectifier (SCR) power source, electronically voltage-controlled, has no real moving parts other than the primary current contactor and cooling--fan the only parts that will wear out. If fabricator requires pulsed welding, the SCR power source starts to show its limitations and inverters become the power supply of choice.AC-DC inverters, which convert AC in put frequency to very high frequencies--up to 1,000 times that of input frequency--allow transformers and other components to be small in size and the power supply to be relatively lightweight. This high operating frequency enables the inverter to precisely control output power and to deliver smooth arc initiation with minimal or no spatter. Current ripple is low for a very stable arc:, even at low amperages. And the improved arc control can, in some cases, minimize electrode overheating and fume emissions. Finally, the smallish transformer allows for more efficient use of primary power compared to conventional power supplies.

Wildfire perimetersGPS is also a new asset for mapping the perimeters of wildfires, but there are problems. Walking the fire line with a GPS unit is too sale dangerous, and flying the perimeter in a helicopter equipped with GPS or mapping by hand from the air at a safe distance aren''t often feasible. During the initial stages of a wildland fire, helicopters are usually busy with the more critical task of dropping water.The Los Angeles County Fire Department has found a third alternative. It has equipped bulldozers with built-in GPS units, a scheme that''s proved to be valuable because it takes the guesswork out of where a bulldozer line exists. The GPS receivers in these bulldozers have removable PC Type II cards that can be processed with associated mapping software. However, this bulldozer line typically makes up only a part of the fire line, the rest of which must still be mapped by other means.

3. Welding position. The first performance characteristic to consider. High cellulose coatings, like those on E6010 and E6011 electrodes, generate a light slag that makes for rapid solidifying of the weld metal good for-out-of-position welds. Also good are rutile-fluxed electrodes-E6012 and E6013--that hold out-of-position weld metal in place as it solidifies.4. Welding current. Covered electrodes run on AC, DC, or both. When welding DC, the positive lead typically connects to the electrode (DCEP, or reverse polarity). Some suppliers design electrodes that weld with the electrode negative (DCEN, or straight polarity). Assure the electrode will perform with the available current.