The answer depends on the height of the stack" which involves the weld-o-let n ipple and ball valve. Refer to Chart #1 below for the following example: If the "stack" height from the top of the pipe to the top of the hot-tap assembly is 152mm (6 in.) and the pipe size is DN500 (20 in.) the chart below indicates that a sensor with the part number 3-2552-2 should be used. On the other hand if the "stack" height exceeds 152mm (6 in) a long sensor length needs to be used (and will be available in late 2007). Note that a typical "stack' height is 5.5 inches or larger.