While they both perform the same function, measuring the level of fluid in your tank the can report this information differently. The FSA is strictly a visual gauge. You can add an internal thermometer in the sight glass to show temperature or and an external dial thermometer to the FSA. The FSK is can also be used visually for fluid level and can have a dial thermometer also, but the FSK is also fitted with an electrical switch to detect fluid level and output a signal. Also, it can be fitted with thermal switches to measure and output fluid temperature to a monitoring system or safeguard.