My K40 (and various other CO2 lasers) does not have a water flow sensor switch on it. There are folks who put a water flow meter on the outside of their lasers which gives you a visual representation that the water is indeed flowing. That’s great. Until you turn your back and the pump fails causing water to stop flowing and your laser tube and power supply go “POOF”.
You would have to watch that flow meter 100% of the time to be safe.
With a water flow sensor switch it kills power to the laser firing circuit if no water flow is detected thus saving your precious CO2 laser tube and power supply.
The switch in the flow sensor is in the normally open position. That means the circuit is BROKEN until water flows through the sensor and then the switch closes which ENERGIZES the laser firing circuit.
In fact laser power supplies have a protection circuit ready to roll.
BUT LOOK AT HOW THEY BYPASS THIS CIRCUIT IF THERE IS NO WATER FLOW SENSOR SWITCH INSTALLED!

Bypassing the Water Protection Circuit On A CO2 Laser
Yikes. The jumper, if it could talk would say……”there is a protection circuit here, and it is working just fine”. But if your pump fails it would also say…….”there is a protection circuit here, and it is working just fine”. NO PROTECTION. THIS IS UNSAFE!