HOT CAPILLARY ACTION!
Q The mechanical temperature gauge on my Saab 96 has failed. It showed lower and lower temperature even though the engine gets hot, so I suspect the ether has leaked out. I looked at instructions on how to refill it, but finding and fixing the leak must be the first step. I cut the pipe to prepare for refill and sent compressed air to the bulb and the gauge (including the connecting pipe) but can’t find any leak.
The needle moves when I do this, but I don’t see any bubbles when submerged into water. Any advice? Martin Nilson, Västerås, Sweden
Matt says:
A The fact that the gauge has failed gradually indicates that something has become slightly porous. As you say, it’s most likely in the capillary pipe near the bulb because this is subject to the vibration of the engine. It would be best to start again using a fresh length of copper pipe. You’ll need to find a short length of copper tube that will slip snugly over the old and new pipes to join them together. Place the new bulb into a bowl of ice to liquefy the ether. Cut the old and new tubes at the gauge end (leave a stub on the old gauge). Slip the ‘joiner’ over both tubes and quickly solder them both together. Heat the joint and feed solder under the joiner at both ends. To make the process quick and easy, clean both tubes to be joined with steel wool and coat with soldering flux; soak the joiner in a dilute acid such as white vinegar to remove oxides from inside it. Dry it and try to get a tiny amount of flux inside it too – not so much that it will cause a blockage. Use either a pocket gas-powered soldering iron or a powerful (80W or larger) electric iron to complete the heating process as quickly as possible, as the tube will conduct heat away from the joint. When the solder flows out, rather than sitting in a blob, the process is complete. The ice minimises the loss of ether during this process. To test, place the bulb in boiling water. It may be necessary to adjust the gauge – reposition the pointer on its shaft using a watch or clock hand remover, or bend the wire link between the curved Bourdon Tube of the gauge and the pointer. Once adjusted for the correct 100ºc position, the rest of the gauge’s scale should be sufficiently accurate.