Today Richard M. tested the new (borrowed) SCR that Kyle installed last night on CP4's regen control circuit. It tested fine and then shorted after about an hour with GN2 flow. I was conservative in driving it while ramping temperature. We are not sure of the exact cause of failure, but are convinced it's an overheating issue. We might install one of the larger HVAC SCRs or buy a 16A variable transformer to control the heater.