[ELaser]

Been asked this so often, how do you test, without special expensive equipment..

From An (approximate) CO2 laser power test | Exeter Laser

I get asked quite regularly how to determine the actual power a CO² laser is putting out, the stock answer is buy an expensive and calibrated power meter.
But, there is another way…


Take a small glass beaker, fill it with as near as you can 50 milliliters of water.


Measure the temperature of this water as accurately as you can, let's say it is 12 degrees C.


Set up a job on the laser to raster engrave a square 20 mm a side at 10 mm / sec and a chosen power P and 0.1 mm line resolution / spacing.


The laser will be "on" for 20 x (20 / 0.1) = 20 x 200 = 4,000 mm of travel, and 4,000 / 10 = 400 seconds of "light on" time.

The only caveat is make sure the area the laser is trying to engrave is 100% above the water in your beaker, and try to get the head within 10 mm or so of the water surface, and turn the air assist almost completely off, that will just cool things down.


Measure the temperature of the water again, let's say it is 36 degrees C


We know that water is basically 100% opaque to CO² laser energy, so we can fairly safely assume that all the beam energy is going to be absorbed by the water, therefore it is just maths from now on.


The specific heat capacity of water is approximately 4.186 Joules per gramme per degree C


We know 50 ml of water = 50 grammes, and we know 36 – 12 = 24 degree temperature rise, so we can say it took 50 x 24 x 4.186 Joules to do this which = a shade over 5 kilo joules.


1 Joule per Second = 1 Watt, and we know we got 5,000 Joules in 400 seconds, so 5,000 / 400 = 12.5 Watts, therefore our chosen power P was actually 12.5 Watts.
In absolute accuracy terms I'd say this was, dependent on the accuracy of your measurements, good for plus or minus 5%, so for a value of P of 12.5 Watts that would be plus or minus 0.625 watts, for 25 Watts plus or minus 1.2 Watts, and so on.


Now, if for your experiment you had a 40 watt tube, and you set the power to 31% then you can say for sure that your laser settings are pretty much bang on.


If however you set it to 50% then there are two possibilities, one is that the control software is not accurate and linear in the relationship between what it says on the display and the actual PWM power factor being sent to the laser, and the other is that the laser tube simply is not as good as it once was.


Of course, it could well be a combination of both of these, and the only way to determine that is over time, repeat the experiment once a month and plot your findings over time.


In due course I will do a video to demonstrate this.

[buddydog]

What a great idea!

[TehPepper]

You might want to account for the heat capacity of the glass beaker as well. Perhaps put on something to insulate it from your bed to prevent losses there.

[ELaser]

Hence my 5% proviso.. >

[TehPepper]

I'm having trouble finding a typical weight for a given size of beaker but I think with a 100ml beaker it will be more like 20-25%. Thermal capacity of glass is a bit less than 1/4 of water. A 100ml beaker would weigh about 50g.

I'm tempted to try it with a polystyrene cup to avoid that loss.

[ELaser]

my 50 ml borosilicate lab beaker weighs 34g, my plastic glass one that I used weighs 5g, admittedly a bigger factor than I thought

[buddydog]

You know a bigger factor given a working machine is a reference mark to compare to over time when needed, so any standard that can be rechecked later when things are not working properly would be useful although actual power is interesting. I have been meaning to do this with an aluminum block and my water temperature probe but I haven't got around to it. I'll be kicking myself for being so lazy if the power seems to drop and and the usual measures don't work.

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