by canadianavenger » Wed Feb 22, 2012 11:23 pm
The problem is thermal shock. Sudden changes in temperature from hot to cold, or cold to hot can cause the glass to crack.
5 degree water should be okay, as long as the tube glass and mirrors are near 5 degrees when the water is added, or the tube is turned on. If the tube is at or above room temperature and turned on before the water can bring the tube down in temperature, the tube is likely to crack with the sudden rise [and consequent fall] in temperature from the laser inefficiencies and the cold water. If the water is run through the tube for several minutes before the tube is turned on, it should be okay, as the rise in temperature of the glass will be more gradual then. The tube may still crack if the water and tube temperature are too far apart, even if the tube itself is never turned on. Glass is fragile, and does not like thermal stress.
Bottom line, the water and tube should be close to the same starting temperature. If very cold water is to be used [relative to the tubes starting temperature], it should be introduced slowly to bring the tube down to the water temp. Once the tube and water are at, or near, the same temperature, then the tube can be fired up.
Note that a poorly designed tube may still crack, if the cooling is uneven along the tube, due to uneven expansion of the glass.