Note that web-based summaries of standard operating
procedures are not exhaustive.
Not all of the relevant safety information is included in this document. Further training is required for safe operation and will be provided in person. |
The DC welder in P160 is used for forming and sealing sample capsules. Electrodes must be sharp - use a pencil sharpener and, for the best finish, polish with paper to remove loose graphite dust. The electrode degrades quickly with use, so resharpen frequently. Wear eye protection - the spark is extremely bright (it emits UV and IR radiation, as well as visible) and, in order to protect your eyesight, welding goggles with a minimum DIN rating of 10 should be worn. Never look at the arc with the naked eye. Do not touch live electrical parts. Electric shocks are hazardous and potentially fatal. The welder has a short duty cycle. Using it for more than a few seconds may produce fumes of ozone and nitrogen oxides.
The normal technique for welding noble metal capsules and thermocouple junctions is to hold the tip of the electrode very close to the grounded surface of the vice to produce an arc, and then adjust the height of the electrode until a 'smooth' arc is formed (no spluttering or crackling). Once the arc is clean, move the electrode over the surface to be welded in one smooth motion. Avoid touching the metal with the electrode. Release the foot switch as soon as you have finished welding, and touch the electrode to the grounded surface to discharge the capacitor completely. Always check the quality of the weld carefully under the microscope. Reform and reweld as neccessary to form the perfect weld. The power level required to melt the metal depends largely on how far the metal extends from the body of the vice. If very little material extends above the surface, heat will be removed efficiently by conduction, so a higher power level will be required than in the case of a capsule that protrudes further. Increase power level incrementally if you are not sure of the correct setting, in order to avoid fusing the entire workpiece.
Fine welding of noble metals is a skill that takes practice. Types of welding done in the lab include coned capsule tip welds, seam welds for crimped capsules, 'trash-can' lids, cooled capsule welding (for sealing water in a capsule), and Pt-Rh and W-Re thermocouple bead welding. Learn from people in the lab who have experience in many different welding techniques.
Training is provided by: MMH, ACW.
ACW 2005
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