Check out the new MoBeam posts! To find K40 conversion information use the "INDEX", "SEARCH" or "LABELS" sections in the sidebar.

Wednesday, May 17, 2017

K40 Coolant Flow and Termperature Sensing


Note: much of this approach has been replaced by: Improved k40 cooling circuit

The K40 laser needs coolant that is maintained at the correct temperature to prevent damage to the tube. It is not uncommon to forget to turn on a coolant pump or to have a pump or water system failure while running the K40. 
Sensors are easy to install in a converted K40 and the protection of the laser tuber certainly warrants the installation annoyance and cost of a flow sensor.

It is also desirable to know the temperature of the water and the tube. This post also outlines the installation of an inexpensive water temp sensor and control.


Please consider donating (button to the right of this post).
Your donations help fund additional research, tools and parts that I will return to the community as information.
For other information on the K40-S build use the  K40-S BUILD INDEX with schematics

Flow Sensor


The loss of cooling water will certainly cause damage to the laser and the laser power system. Every system should have a flow sensor plumbed in series with the pump and the lasers cooling jacket.
The sensor that I use is:

This sensor needs to be installed on the output side of the laser. I made a hanger to hold it upright on the side of a 5 gal bucket. This way it insures that water is flowing out of the laser and it can detect any leaks from the pumps output to the sensors input.

Electrical connections;

The flow sensor is connected in series with the interlock circuit and in effect stops the laser from firing if there is no flow. 
See Build Schematics  for full machine details

If you for some reason do not want to add a sensor at least insure that the pump comes on with the machine. You can simply plug the pump and machine into the same power strip and turn them both on at the same time.

The End of My Tube Fell Off!

If you did not install a water sensor then at some point the pump will not be on, due to failure or  simply forgetting to turn it on, and the tube will overheat.

If the tube overheats the water jacket on the end of the tube can de-laminate and fall off.

Apparently if you are careful to keep it off the optical output area of the jacket you can use EPOXY it back on. I would surmise that high temp epoxy would be best.


Temperature Monitoring

The laser must stay within its coolant operating range if it is to operate consistently and reliably. The cooler the laser is kept the more power it will be capable of. The power capacity of the laser will change with temperature therefore it is important to monitor the water temperature and prevent the laser from operating is the temp gets to high.
Install temperature monitoring electronics such as:

This device's relay contacts (NC) is also wired in series with the interlock circuit and will disable the laser from firing if the temperature is to high or low. The probe is put into the bucket near the output or the flow sensor.
See Build Schematics  for details.

This controller can be set up to produce an audible alarm outside of its set-points. (See the manual). I mounted it on thr front of the machine but plans are to move it up to the control panel later. The unit requires 12VDC so an additional supply is needed. Before I installed the 12V supply in my conversion I used a 12V brick plugged into a power strip. 

Enjoy and comment
Maker Don


  1. What is a safe operating temperature range for a normal k40 laser?

  2. Just a though, i just got one and my first though was to install the flow switch on inlet side, only because the sensor has some resistance, and if you apply it on output would it not increse pressure on where hose attach to tube? I would think you want the least resistance path so the hoses would not push off the tube? Does that make sense? And also incorporate a simple water level switch in tank, so if a leak developed the return supply would be less and shut down? This could be in series with flow switch. SO the flow switch would assure the pump was working and float switch assures the water is returning, temp switch keeps you in range. Also to add a twist these 3 could be in series, to power a relay which then the NO would turn on laser and a dual color led indicator light could also be wired to the NO and NC to light green when all water is good and red when is a problem with one of three?