The fault I saw in the code is just the pin (0) mentioned in the attachInterrupt function.. It should be as same as the FLOW SENSOR PIN. its hard coded as 0 ( and there is a FLOWSENSORPIN defined as 2.

Also for what Juan asked. SF800 sensor needs 2.2K resistor from the sensor pin to be pulled up to the power (5V). the sensor grounds it when there is data from the sensor.

Hope this works.