On Thursday, I was setting up to do a project for a friend and my computer that I use for the laser decided to go on the fritz. I told her to come back this morning so we could finish the project. Lightburn let me know that there was an update so I went ahead and updated. Well I was able to frame out the project, but the system would beep and not run the project. I was getting an error message that the system was busy. So now I am trying to troubleshoot the computer, Windows and Lightburn to see why things are not working. After awhile, I decided to look at the Ruida controller display a little more thoroughly and noticed a tiny error message that there was a water flow error. First time in over the 4 years that I have owned the laser did I ever see such an error message. Well I checked my pump and it was not on! Due to the way the hoses are run, I never looked to see if the system included a flow switch. It saved me a tube! I have been playing with fire by not hooking up the flow switch from the chiller to the controller. A BIG reminder that a few minutes to make sure all fail safes are hooked up and working can save you money. I am very lucky that the system had a flow switch and it actually works. I was relived that it was all that was stopping me from completing her project since it was imperative to be completed today after the computer failure on Thursday. Disaster averted!
Glad to hear you avoided a crises. I have flow sensors on my lasers but I have been meaning to try and add visual indicators for the flow. Like those impeller type flow indicators I’ve seen other people use. I can usually confirm the flow by checking to see if the tube temp starts dropping but a visual indicator would be nice. 
One of my eventual projects is to add red/green lights for easy error notifications.
Good save but let me tell you that it is a VERY good idea to have a visual indicator of the water flow.
Why? Because one of the guys at the shop who had only the water flow sensor blew his tube when his pump stopped working but the switch had some time earlier failed in the OK position. There is nothing in the circuits to detect the switch is changing state and only looks for the operating/A-OK state.
Some like this one also have a 3rd port for a temp sensor and come with a display.
What is oxymoronic is that I did do this proof of concept for when flow does stop but never did interface it with my lasers.
I have had my wp coming from the chiller. It detects more than just flow.
However that input failed on the Ruida, so I moved it to the ‘door protect’ input. Seems to work OK there, halts the machine anyway.
I also have a flow meter. Which looks similar to @HalfNormal device. I 3d printed a mount and put it in the laser.
Had an Arduino initially reading/displaying the liters/min flow rate. I’m currently moving it over to the Pi 4 along with the other controls.
Cloudray has this switch that just wires up to the controller directly… Looks a tab more simple to implement than the method used by @HalfNormal
Hard to say if it’s a vane or pressure type. Does not look like it takes any power. Bottom line is to be diligent and hope you get lucky if there is a problem.
I don’t know how it works internally. I’ve seen a couple articles on this switch as a replacement. Cloudray is selling them with the ‘schematic’, so I’d guess it’s a vane type … ?
I installed one of those “whirlygig” fish tank flow meters in my return line back to the chiller.
The one I got has a temp sensor in it and has a display:
Display is in the upper left:
If you install a UV diode in that it will glow in the dark. 
No kidding, the one I have really glows. I was doing a garage scorpion inspection with a black light and man!!
Beevo
Looks great… I have a flow meter hooked to an 328p to monitor flow rate only… the lasers safeties are controlled by the CW-5200 chiller…
Definitely higher tech than my setup. I have the alarm output on the
chiller connected to the Smoothie controller.
Beevo
The only advantage to mine, is I know the current flow rate… for what it’s worth… I also know the anode voltage… 
Like yours, mine is dependent on the chiller raising the alarm via the Ruida for any kind of coolant issues…
Having a CNC milling machine crash badly when I was out of the room, I
am seldom out of eyesight from the laser, I do not fully trust it.
Beevo
Pretty much the same feeling I have now about autonomous vehicles…
Good idea. You would not believe how many laser fires stories I’ve read which had “and I only stepped away from the laser for a moment to do X”.
Autonomous vehicles without a driver can be stopped from what I read by
placing a traffic cone or something of similar size on the hood.
roflmao
We have gobs of those here and I live a few miles from the Waymo staging
lot. Not unusual to see a car traipsing up teh street with nobody in
it. I am still not used to that…
Beevo
I do not trust the laser to be away from it for mor than a minute. I
can get my wife to baby sit it if I have to hit teh ehad or something,
but it is never left running unattended.
Closest I came to a fire was when I grabbed a piece of acryllic and it
turned out to be Makrolon. (
Makrolon® | Polycarbonates | Covestro AG) Like lasing
gasoline.
Beevo